LEARNING PACKAGE FOR HYDROLOGY
Home Contents Go Back
LEARNING HYDROLOGY
Introduction
Hydrology means the
science of water. It is the science that deals with the occurrence, circulation
and distribution of water of the earth and earth's atmosphere. As a branch of
earth science, it is concerned with the water in
Hydrology is basically an applied science. To further emphasis the degree of applicability, the subject is sometimes classified as
• Scientific
hydrology - the study which is concerned chiefly with academic
aspects.
• Engineering or
In a general sense
engineering hydrology deals with •
Estimation of water resources,
•
The study of processes such as precipitation,
runoff, evapotranspiration and
their
interaction and
•
The study of problems such as floods and droughts and strategies to
combat them.
HYDROLOGIC CYCLE
Water occurs
on the earth in all its three states, viz. liquid, solid and gaseous, and in
various degrees of motion. Evaporation of water from
water bodies such as oceans and lakes, formation and movement of clouds, rain
and snowfall, streamflow and groundwater
movement are some examples of the dynamic aspects of water. The various aspects
of water related to the earth can be explained in terms of a cycle known as the
hydrologic cycle.
A convenient starting point to describe the cycle is in the oceans. Water in the oceans evaporates due to the heat energy provided by solar radiation. The water vapour moves upward and form clouds. While much of the clouds condense and fall back to the oceans as rain, a part of the clouds is driven to the land areas by winds. There they condense and precipitate onto the landmass as rain, snow, hail, sleet, etc. A part of the precipitation may evaporate back to the atmosphere even while falling. Another part may be intercepted by vegetation, structures and other such surface modifications from which it may be either evaporated back to atmosphere or move down to the ground surface.
A portion of the water that reaches the ground enters the earth's surface through infiltration, enhance the moisture content of the soil and reach the groundwater body. Vegetation sends a portion of the water from under the ground surface back to the atmosphere through the process of transpiration. The precipitation reaching the ground surface after meeting the needs of infiltration and evaporation moves down the natural slope over the surface and through a network of gullies, streams and rivers to reach the ocean. The groundwater may come to the surface through springs and other outlets after spending a considerably longer time than the surface flow. The portion of the precipitation which by a variety of paths above and below the surface of the earth reaches the stream channel is called runoff. Once it enters a stream channel, runoff becomes stream flow.
The sequence of events as above is a simplistic picture of a very complex cycle that has been taking place since the formation of the earth. It is seen that the hydrologic cycle is a very vast and complicated cycle in which there are a large number of paths of varying time scales. Further, it is a continuous re-circulating cycle in the sense that there is neither a beginning nor an end or a pause. Each path of the hydrologic cycle involves one or more of the following, aspects:
• Transportation of water,
• Temporary storage and
• Change of state.
For example,
(a) the process of rainfall has the change of
state and transportation and
(b) the groundwater path has storage and
transportation aspects.
The quantities of water going through various individual paths of the hydrological cycle can be described by the continuity equation known as water-budget equation or hydrologic equation.
For a given problem area, say a catchment, in an interval of
time At,
Mass inflow-mass outflow = change in mass storage
if the density
of the inflow, outflow and storage volumes are same.
Vi - Vo = D S 9; 9; (1.1)
Where Vi inflow volume of water into the problem area during the time period, Vo outflow volume of water from the problem area during the time period, and D S = change in the storage of the water volume over and under the given area during the given period. In applying this continuity equation to the paths of the hydrologic cycle involving change of state, the volumes considered are the equivalent volumes of water at a reference temperature.
It is important to note that the total water resources of the earth are constant and the sun is the source of energy for the hydrologic cycle. A recognition of the various processes such as evaporation, precipitation and groundwater flow helps one to study the science, of hydrology in a systematic way. Also, one realises that man can interfere with virtually any part of the hydrologic cycle, e.g. through artificial rain, evaporation suppression, change of vegetal cover and land use, extraction of ground. water, etc. Interference at one stage can cause serious repercussions at some other stage of the cycle.
Back to TopThe hydrological cycle has important influences in a variety of fields including agriculture, forestry, geography, economics, sociology and political science. Engineering applications of the knowledge of the hydro-logic cycle, and hence of the subjects of hydrology, are found in the design and operation of projects dealing with water supply, irrigation and
drainage, water power, flood control, navigation, coastal works, salinity control and recreational uses of water.APPLICATIONS IN
ENGINEERING
Hydrology finds its greatest application in the design and
operation of introduction water-resources engineering projects, such as those
for irrigation, water supply, flood control, water power and navigation. In all
these projects hydrological investigations for the proper assessment of the
following factors are necessary.
• The capacity of storage structure such as
reservoir.The hydrological study of a project should of necessity precede structural and other detailed design studies. It involves the collection of relevant data and analysis of the data by applying the principles and theories of hydrology to seek solutions to practical problems.
Many important projects in the past have failed due to improper assessment of the hydrological factors. Some typical failures of hydraulic structures are:
• Overtopping and consequent failure of an earthen
dam due to an inadequateVarious phases of the hydrological cycle, such as rainfall, runoff, evaporation and transpiration are all non-uniformly distributed both in time and space. Further, practically all hydrologic phenomena are complex and at the present level of knowledge, they can at best be interpreted with the aid of
probability concepts. Hydrological events are treated as random processes and the historical data relating to the event are analysed by statistical methods to obtain information on probabilities of occurrence of various events. The probability analysis of hydrologic data is an important component of present-day hydrological studies and enables the engineer to take suitable design decisions consistent with economic and other criteria to be taken in a given project.Precipitation
The term
"precipitation" denotes all forms of water that reach the earth from the
atmosphere. The usual forms are rainfall, snowfall,
The study of precipitation forms a major portion of the subject of
hydrometeorology. In this chapter, a brief introduction is given to familiarize the engineer with important aspects of rainfall and, in particular, with the collection and analysis of rainfall data. For precipitation to form: •
The atmosphere must have moisture,
• There must be sufficient nuclei present to aid
condensation,
• Weather
conditions must be good for condensation of water vapour to take place,
• The products of
condensation must reach the earth.
Under proper weather conditions, the water vapour condenses over nuclei to form tiny water droplets of sizes less than 0.1 mm in diameter. The nuclei are usually salt particles or products of combustion and are normally available in plenty. Wind speed facilitates the movement of clouds while its turbulence retains the water droplets in suspension. Water droplets in a cloud are somewhat similar to the particles in a colloidal suspension. Precipitation results when water droplets come together and coalesce to form larger drops that can drop down. A considerable part of this precipitation gets evaporated back to the atmosphere. The net Precipitation at a place and its form depend upon a number of meteorological factors, such as the weather elements like wind, temperature, humidity and pressure in the volume region enclosing the clouds and the ground surface at the given place.
FORMS OF PRECIPITATION
Some of the common forms of
precipitation are rain, snow, drizzle, glaze, sleet and
hail.
Rain
It is the principal form of precipitation in India.
The term "rainfall" is used to describe precipitation in the form of water drops
of sizes larger than 0.5 mm. The maximum size of a raindrop is about 6 mm. Any
drop larger in size than this trends to break up into
drops of smaller sizes during its fall from the clouds. On the basis of its
intensity, rainfall is classified as:
Type |
Intensity |
Light Rain |
Trace to 2.5 mm/h |
Moderate rain |
2.5 mm/h to 7.5 mm/h |
Heavy Rain |
> 7.5 mm/h |
Snow
Snow is another important form of precipitation.
Snow consists of ice crystals which usually combine to form flakes. When new,
snow has an initial density varying from 0.06 to 0.15 g/cm3 and it is
usual to assume an average density of 0. 1 g/cm3. In India, snow
occurs only in the Himalayan regions.
Drizzle
A fine sprinkle of numerous water droplets of
size less than 0.5 mm and intensity less than 1 mm/h is known as drizzle. In
this the drops are so small that they appear to float in the air.
Glaze
When rain or
Sleet
It is frozen raindrops of transparent grains which
form when rain falls through air at subfreezing temperature. In Britain, sleet
denotes precipitation of snow and rain simultaneously.
Hail
It is a showery precipitation in the form of
irregular pellets or lumps of ice of size more than 8 mm. Hails occur in violent
thunderstorms in which vertical currents are very strong.
WEATHER SYSTEMS
FOR PRECIPITATION
For the formation of clouds and subsequent
Front
A front is the interface between two distinct air
masses. Under certain favourable conditions when a warm air mass and cold air
mass meet, the warmer air mass is lifted over the colder one with the formation
of a front. The ascending warmer air cools adiabatically with the consequent
formation of clouds and precipitation.
Cyclone
A cyclone is a large low-pressure region with
circular wind motion. Two types of cyclones are recognized:
Tropical cyclone
A tropical cyclone, also called cyclone
in India,
During summer months, tropical cyclones originate in the open ocean at around 5-10° Latitude and move at speeds of about 10-30 kmph to higher latitudes in an irregular path.
Fig.2.1 Schematic section of a tropical cyclone
They derive their energy from the latent heat of condensation of ocean water vapour and increase in size as they move on oceans. When they move on land the source of energy is cut off and the cyclone dissipates its energy very fast. Hence, the intensity of the storm decreases rapidly. Tropical cyclones cause heavy damage to life and property on their land path and intense rainfall and heavy floods in streams are its usual consequences. Tropical cyclones give moderate to excessive precipitation over very large areas, of the order of 10³ km² for several days.
Extratropical cyclone
These are cyclones formed in
locations outside the tropical zone. Associated with a frontal system, they
possess a strong counter-clockwise wind circulation in the northern hemisphere.
The magnitude of precipitation and wind velocities are relatively lower than
those of a tropical cyclone. However, the duration of precipitation is usually
longer and the areal extent also is longer.
Orographic Precipitation
The moist air masses may get
lifted-up to higher altitudes due to the presence of mountain barriers and
consequently undergo cooling, condensation and precipitation. Such a
precipitation is known as
CHARACTERISTIC OF
PRECIPITATION ON INDIA
From the point of view of
South-west
Transition-1, post-monsoon (October-November)
Winter season (December-February)
Transition-11, Summer, (March-May)
South-West Monsoon (June-September)
The
The former sets in at the extreme southern part of Kerala and the latter at Assam, almost simultaneously in the first week of June. The Bay branch first covers the north-eastern regions of the country and turns westwards to advance into Bihar and UP. The Arabian Sea branch moves northwards over Karnataka, Maharashtra and Gujarat. Both the branches reach Delhi around the same time by about the fourth week of June. A low-pressure region known as monsoon trough is formed between the two branches. The trough extends from the Bay of Bengal to Rajasthan and the precipitation pattern over the country is generally determined by its position. The monsoon winds increase from June to July and begin to weaken in September. The withdrawal of the monsoon, marked by a substantial rainfall activity starts in September in the northern part of the country. The onset and withdrawal of the monsoon at various parts of the country are shown in Fig. 2.2.
Fig.2.2 (a) Normal dates of onset of monsoon
Fig.2.2 (b) Normal dates of withdrawal of monsoon
The monsoon is not a period of continuous rainfall. The weather is generally cloudy with frequent spells of rainfall. Heavy rainfall activity in various parts of the country owing to the passage of low pressure region is common. Depressions formed in the Bay of Bengal at a frequency of 2-3 per month move along the trough causing excessive precipitation of a 100-200 mm per day. Breaks of about a week in which the rainfall activity is the least is another feature of the monsoon. The south-west monsoon rainfall over the country is indicated in Fig. 2.3.
As seen from this figure the heavy rainfall areas are Assam and the north-eastern region with 200-400 cm; west coast and western ghats with 200-300 cm; West Bengal with 120-160 cm, UP, Haryana and the Punjab with 100-120 cm.
Back to TopPost-Monsoon (October-November)
As the south-west
monsoon retreats, low-pressure areas form in the Bay of Bengal and a
north-easterly flow of air that picks UP moisture in the Bay of Bengal is
formed. This air mass strikes the East Coast of the southern peninsula
(Tamilnadu) and causes rainfall. Also, in this period, especially November,
severe
Winter Season (December-February)
By about mid-December,
disturbances of extra tropical origin travel; wards across Afghanistan and
Pakistan. Known as western disturbances, they cause moderate to heavy rain and
snowfall (about 25 cm) in Himalayas and Jammu and Kashmir. Some light
Summer (Pre-monsoon) (March-May)
There is very little
rainfall in India in this season. Convective cells cause some thunderstorms
mainly in Kerala, West Bengal and Assam. Some cyclone activity, dominantly on
the cast coast, also occurs.
Annual Rainfall
The annual rainfall over the country is
shown in Fig. 2.4. Considerable areal variation exists for the annual rainfall
in India with high rainfall the magnitude of 200 cm in Assam and north-eastern
parts and the western ghats, and scanty rainfall in eastern Rajasthan and parts
of Gujarat, Maharashtra and Karnataka. The
of the annual rainfall varies between 15 to 70, from place to place with an average value of about 30. Variability is least in regions of high rainfall and largest in regions of scanty rainfall. Gujarat, Haryana, the Punjab and Rajasthan have large variability of rainfall.
MEASUREMENT
Precipitation is
expressed in terms of the depth to which rainfall water would stand on an area
if all the rain were collected on it. Thus 1 cm of rainfall over a
The ground must be level and in the open and the instrument must present a horizontal catch surface.
The
The instrument must be surrounded by an open fenced area of at least 5.5 m X 5.5 m. No object should be nearer to the instrument than 30 m or twice the height of the obstruction.
Raingauges can be broadly classified into two categories as non-recording raingauges and recording gauges.
Nonrecording Gauges
The nonrecording gauge extensively
used in India is the Symons' gauge. It essentially consists of a circular
collecting area of 12.7 cm (5.0 inch) diameter connected to a funnel. The rim of
the collector is set in a horizontal plane at a height of 30.5 cm above the
ground level. The funnel discharges the rainfall catch into a receiving vessel.
The funnel and receiving vessel are housed in a metallic container. Figure 2.5
shows the details of the installation.
Water contained in the receiving vessel is measured by a suitably graduated measuring glass, with an accuracy up to 0.1 mm. Recently, the India Meteorological Department (IMD) has changed over to the use of fibreglass reinforced polyster raingauges, which is an improvement over the Symons' gauge. These come in different combinations of collector and bottle. The collector is in two sizes having areas of 200 and 100 cm² respectively. Indian Standard (IS : 5225-1969) gives details of these new raingauges.
For uniformity, the rainfall is measured every day at 8.30 AM (IST) and is recorded as the rainfall of that day. The receiving bottle normally does not hold more than 10 cm of rain and as such in the case of heavy rainfall the measurements must be done more frequently and entered. However, the last reading must be taken at 8.30 Am and the sum of the previous readings in the past 24 h entered as total of that day. Proper care' maintenance and inspection of raingauges, especially during dry weather to keep the instrument free from dust and dirt is very necessary. The details of installation of non-recording raingauges and measurement of rain are specified in Indian Standard (IS : 4986-1968). This raingauge can also be used to measure snowfall. When snow is expected, the funnel and receiving bottle are removed and the snow is allowed to collect in the outer metal container. The snow is then melted and the depth of resulting water measured. Antifreeze agents are some times used to facilitate melting of snow. In areas where considerable snowfall is expected, special snowgauges with shields (for minimizing the wind effect) and storage pipes (to collect snow over longer durations) are used.
Recording Gauges
Recording gauges produce a continuous
Plot of rainfall against time and provide valuable data of intensity and
duration of rainfall for hydrological analysis of storms. The following are some
of the commonly used
Tipping-Bucket Type
This is a 30.5 cm size raingauge
adopted for use by the US Weather Bureau. The catch from the funnel falls onto
one of a pair of small buckets. These buckets are so balanced that when 0.25 mm
of rainfall collects in one bucket, it tips and brings the other one in
position. The water from the tipped bucket is collected in a storage can. The
tipping actuates an electrically driven pen to trace a record on
clockwork-driven chart. The water collected in the storage can is measured at
regular intervals to provide the total rainfall and also serve as a check. It
may be noted that the record from the tipping bucket gives data on the intensity
of rainfall. Further, the instrument is ideally suited for digitalizing of the
output signal.
Weighing-Bucket Type
In this raingauge the catch from
the funnel empties into a bucket mounted on a weighing scale. The weight of the
bucket and its contents are recorded on a clockwork-driven chart. The clockwork
mechanism has the capacity to run for as long as one week. This instrument gives
a plot of the accumulated rainfall against the elapsed time, i.e. the
Natural-Syphon Type
This type of recording raingauge is
also known as float-type gauge. Here the rainfall collected by a funnel-shaped
collector is led into a float chamber causing a float to rise. As the float
rises, a pen attached to the float through a lever system record the elevation
of the float on a rotating drum driven by a clockwork mechanism. A syphon
arrangement empties the float chamber when the float has reached a pre-set
maximum level. This type of raingauge is adopted as the standard recording-type
raingauge in India and its details are described in Indian Standard (IS :
5235-1969). A typical chart from this type of raingauge is shown in Figure 2.6.
This chart shows a rainfall of 53.8 mm in 30 h. The vertical lines in the pen trace correspond to the sudden emptying of the float chamber by syphon action which resets the pen to zero level. It is obvious that the natural syphon-type recording raingauge gives a plot of the mass curve of rainfall.
Telemetering Raingauges
These raingauges are of the
recording type and contain electronic transmit the data on rainfall to a base
station both at regular inter on interrogation. The tipping-bucket type
raingauge, being ideally suited is usually adopted for this purpose. Any of the
other types of recording raingauges can also be used equally effectively.
Telemetering gauges are utmost use in gathering rainfall data from mountainous
and genera inaccessible places.
Radar Measurement of Rainfall
The meteorological radar
is a powerful instrument for measuring the are extent, location and movement of
rainstorms. Further, the amount rainfall over large areas can be determined
through the radar with a go degree of accuracy. The radar emits a regular
succession of pulses of electromagnetic radiation in a narrow beam. When
raindrops intercept a radar beam, it has be shown that
9; 9; 9; (2.1)
where Pr = average echo power, Z = radar-echo factor, r = distance target volume and C = a constant. Generally the factor Z is related to the intensity of rainfall as
(2.2)
Where, a and b are coefficients and I = intensity or rainfall in mm/h. The values a and b for a given radar station have to be determined by calibration with the help of recording raingauges. A typical equation for Z is
Z = 200 I 1.60
Meteorological radars operate with wavelengths ranging from 3 to 10 cm, the common values being 5 and 10 cm. For observing details of heavy flood-producing rains, 10 cm radar is used while for light rain and snow a 5-em radar is used. The hydrological range of the radar is about 200 km. Thus a radar can be considered to be a remote-sensing super
gauge covering an areal extent of as much as 100,000 km². Radar measurement is continuous in time and space. Present-day developments in the field include (i) On-line processing of radar data on a computer and (ii) Doppler-type radars for measuring the velocity and distribution of raindrops.RAINGAUGE NETWORK
Since the
catching area of a raingauge is very small compared to the areal extent of a
storm, it is obvious that to get a representative picture of a storm over a
catchment the number of raingauges should be as large as possible, i.e. the
catchment area per gauge should be small. On the other hand, economic
considerations to a large extent and other considerations, such as topography,
accessibility, etc. to some extent restrict the number of gauges to be
maintained. Hence one aims at an optimum density of gauges from which reasonably
accurate information about the storms can be obtained. Towards this the World
Meteorological Organisation (WMO) recommends the following densities.
In flat regions of temperate, Mediterranean and tropical zones:
ideal-1 station for 600-900 km², acceptable-1 station for 900-3000
km²;
In mountainous regions of temperate, Mediterranean and tropical
zones: ideal-1 station for 100-250 km² acceptable-1 station for 250-1000 km²;
and
In arid and polar zones: 1 station for 1500-10,000 km² depending on the feasibility.
Ten per cent of raingauge stations should be equipped with self-recording gauges to know the intensities of rainfall.
Back to TopPREPARATION OF DATA
Before
using the rainfall records of a station, it is necessary to first check the data
for continuity and consistency. The continuity of a record may be broken with
missing data due to many reasons such as damage or fault in a raingauge during a
period. The missing data can be estimated by using the data of the neighboring
stations. In these calculations the nor-mal rainfall is used as a standard of
comparison. The
Estimation of Missing Data
Given the annual
precipitation values, P1, P2, P3, ...
Pm at neighbouring M stations 1, 2, 3,..., M respectively, it is
required to find the missing annual precipitation Px at a station X
not included in the above M stations. Further, the normal annual precipitations
NI, N2, ..., Ni... at each of the above (M + 1)
stations including station X are known.
If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station X, then a simple arithmetic average procedure is followed to estimate Px. Thus
; ; ; (2.4)
If the normal precipitation vary considerably, then Px is estimated by weighing the precipitation at the various stations by the ratios of normal annual precipitation. This method, known as the
normal ratio method gives Px as(2.5)
PRESENTATION OF
RAINFALL DATA
A few commonly used methods of presentation of rainfall data
which have been found to be useful in interpretation and analysis of such data
are given below:
Mass Curve of Rainfall
The
Hyetograph
A
Point Rainfall
MEAN PRECIPITATION OVER AN
AREA
As indicated earlier, raingauges represent only point sampling of the
areal distribution of a storm. In practice, however, hydrological analysis
requires distribution of f the rainfall over an area, such as over a catchment.
To convert the point rainfall values at various stations into an average value
over a catchment the following three methods are in use:
•
Arithmetical-mean method
•
Thiessen-polygon method and
• Isohyetal method.
Arithmetical-Mean Method
When the rainfall measured at
various stations in a catchment show little variation, the average precipitation
over the catchment area is taken as the arithmetic mean of the station values.
Thus if P1, P2,........., Pi......Pn
are the rainfall values in a given period in N stations within a catchment, then
the value of the mean precipitation ` P over the
catchment by the arithmetic mean method is
(2.7)
In practice, this method is used very rarely.
Thiessen-Mean Method
In this method the rainfall
recorded at each station is given a weightage on the basis of an area closest to
the station. The procedure of determining the weighing area is as follows:
Consider a catchment area as in Fig. 2.10 containing three raingauge stations.
There are three stations outside the catchment but in its neighbourhood. The
The boundary of the catchment, if it cuts the bisectors taken as the outer limit of the polygon. Thus for station 1, the bounding polygon is abcd. For station 2, kade is taken as the bounding polygon. These bounding polygons are called
Thiessen polygons. The areas of these six Thiessen polygons are determined either with a planimeter or by using an overlay grid. If P1, P2, ..., P6, are the rainfall magnitudes recorded by the stations 1, 2, ..., 6 respectively, and A1, A2,.. .... A6, are the respective areas of the Thiessen polygons, then the average rainfall over the catchment ` P is given byThus in general for M stations,
(2.8)
Back to TopThe ratio Ai /A is called the weightage factor for each station.
The Thiessen-polygon method of calculating the average precipitation over an area is superior to the arithmetic-average method as some weightage is given to the various stations on a rational basis. Further, the raingauge stations outside the catchment are also used effectively. Once the weightage factors are determined, the calculation of ` P is relatively easy for a fixed network of stations.
lsohyetal Method
An isohyetal is a line joining points
of equal rainfall magnitude. In the isohyetal method, the catchment area is
drawn to scale and the raingauge stations are marked. The recorded values for
which area] average ` P is to be determined are then
marked on the plot at appropriate stations. Neighbouring stations outside the
catchment are also considered. The
The area between two adjacent isohyets are then determined with planimeter. If the isohyets go out of catchment, the catchment boundary is used as the bounding line. The average value of the rainfall indicated by two isohyets is assumed to be acting over the inter-isohyet area. Thus P1, P2, .... Pn, are the values of isohyets and if a1, a2, ..., a n-1, are the inter-isohyet areas respectively, then the mean precipitation over the catchment of area A is given by
9; (2.9)
The
isohyet method is superior to the other two methods especially when the stations are large in number.DEPTH-AREA-DURATION
RELATIONSHIPS
The areal distribution characteristics of a storm of given
duration is reflected in its depth-area-relationship.
Depth-Area Relation
For a rainfall of a given duration,
the average depth decreases with the area in an exponential fashion given
by
&#` P = Po exp (- KAn) (2.10)
where ` P = average depth in cms over an area A km², Po= highest amount of rainfall in cm at the storm centre and K and n are constant for a given region. On the basis of 42 severe most storms in north India, Dhar and Bhattacharya (1975) have obtained the following values for K and n for storms of different duration.
S.No. | Duration | K | n |
1 | Day | 0.0008526 | 0.6614 |
2 | Day | 0.0009877 | 0.6306 |
3 | Day | 0.001745 | 0.5961 |
Since it is very unlikely that the storm centre coincides over a raingauge station, the exact determination of Po is not possible. Hence in the analysis of large area storms the highest station rainfall is taken as the average depth over an area of 25 km².
Maximum
Depth-Area-Duration CurvesFREQUENCY OF POINT
RAINFALL
In many hydraulic-engineering applications such as those concerned
with floods, the
First, it is necessary to correctly understand the terminology used in frequency analysis. The probability of occurrence of an event (e.g. rainfall) whose magnitude is equal to or in excess of a specified magnitude X is denoted by P. The
recurrence interval (also known as return period) is defined asT = 1/P
Back to Top Plotting PositionA simple empirical technique is to arrange the given annual extreme series in descending order of magnitude and to assign an order number m. Thus for the first entry m = 1, for the second entry m = 2 and so on till the last event for which m = N = Number of years of record. The probability P of an event equalled to or exceeded is given by the Weibull formula
; ; (2.14)
The recurrence interval T = 1/P = (N + 1)/m.
Before the rainfall reaches the outlet of a
basin as runoff, certain demands of the catchment such as interception, depression storage and infiltration have to be met. If the precipitation not available for surface runoff is defined as "loss", then these processes are also "losses". In terms of groundwater the infiltration process is a "gain". Aspects of interception, depression storage and infiltration that are important from the point of view of engineering hydrology.EVAPORATION
PROCESS
Evaporation is the process in which a liquid changes to the gaseous
state at the free surface, below the boiling point through the transfer of heat
energy. Consider a body of water in a pond. The molecules of water are in
constant motion with a wide range of instantaneous velocities. An addition of
heat causes this range and average speed to increase. When they cross over the
water surface. Similarly, the atmosphere in the immediate neighborhood or the
water surface contains water molecules within the water vapour in motion and
some of them penetrate the water surface. The net escape of water molecules from
the liquid state to the gaseous state constitute evaporation. Evaporation is a
cooling process in that the latent heat of vaporization (at about 585 cal/g of
evaporated water) must be provided by the water. The rate of evaporation is
dependent on
• Vapour pressure at the water surface
• Air and water temperatures,
• Wind speed,
• Atmospheric pressure,
• Quality of water and
• Size of the
Vapour Pressure
The rate of evaporation is proportional
to the difference between the
Where, EL = rate of evaporation (mm/day) and C = a constant; ew and ea, are in mm of mercury. This Equation is known as Dalton's law of evaporation after John Dalton (1802) who first recognised this law. Evaporation continue till ew = ea. If ew > ea, condensation takes place.
Temperature
Other factors remaining same, the rate of
evaporation increases with an increase in the water temperature. Regarding air
temperature, although there is a general increase in the evaporation rate with
increasing temperature, a high correlation between evaporation rate and air
temperature does not exist. Thus for the same mean monthly temperature it is
possible to have evaporation to different degrees in a
Wind
Wind aids in removing the evaporated water vapour
from the zone of evaporation and consequently creates greater scope for
evaporation. However, if the wind velocity is large enough to remove all the
evaporated water vapour, any further increase in wind velocity does not
influence the evaporation. Thus the rate of evaporation increases with the
wind-speed up to a critical speed beyond which any further increase in the wind
speed has no influence on the evaporation rate. This critical wind-speed value
is a function of the size of the water surface. For large water bodies,
high-speed turbulent winds are needed to cause maximum rate of
evaporation.
Atmospheric Pressure
Other factors remaining same, a
decrease in the barometric pressure, as in high altitudes, increases
evaporation.
Soluble Salts
When a solute is dissolved in water, the
vapour pressure of the solution is less than that of pure water and hence causes
reduction in the rate of evaporation. The per cent reduction in evaporation
approximately corresponds to the percentage increase in the specific gravity.
Thus, for example, under identical conditions evaporation from sea water is
about 2-3% less than that from
Heat Storage in Water Bodies
Deep water bodies have more
heat storage than shallow ones. A deep lake may store radiation energy received
in summer and release it in winter causing less evaporation in summer and more
evaporation in winter com-pared to a shallow lake exposed to a similar
situation. However, the effect of heat storage is essentially to change the
seasonal evaporation rates and the annual evaporation rate is seldom
affected.
EVAPORIMETERS
Estimation of
evaporation is of utmost importance in many hydrologic problems associated with
planning and operation of reservoirs and irrigation systems. In and zones, this
estimation is particularly important to conserve the scarce
• Using evaporimeter
data,
•
Empirical evaporation equations and
•
Analytical methods.
Types of Evaporimeters
Evaporimeters are
water-containing pans which are exposed to the atmosphere and the loss of water
by evaporation measured in them at regular intervals. Meteorological data, such
as humidity, wind movement, air and water temperatures and precipitation are
also noted along with evaporation measurement.
Many types of evaporimeters are in use and a few commonly used pans are :
Class A Evaporation
Pan
It is a standard pan of 1210 mm diameter and
255 mm depth used by theUS Weather Bureau and is known as Class A Land Pan. The depth of
water is maintained between 18 em and 20 em (Fig. 3.1). The pan is normally made
of unpainted galvanised iron sheet. Monel metal is used where corrosion is a
problem. The pan is placed on a wooden platform of 15 cm height above the ground
to allow free circulation of air below the pan. Evaporation measurements are
made by measuring the depth of water with a hook gauge in a stilling
well.
The evaporation from this pan is found to be less by about 14% compared to that from unscreened pan. The pan is placed over a square wooden platform of 1225 mm width and 100 mm height to enable circulation of air underneath the pan.
Colorado Sunken PanUS Geological Survey
Floating PanPan Coefficient, Cp
Evaporation pans are not
exact
They differ in the heat-storing capacity and heat transfer from the sides and bottom. The sunken pan and
The height of the rim in an evaporation pan affects the wind action over the surface. Also, it casts a shadow of variable magnitude over the water surface.
The heat-transfer characteristics of the pan material is different from that of the reservoir.
In view of the above, the evaporation observed from a pan has to be corrected to get the evaporation from a lake under similar climatic and exposure conditions. Thus a coefficient is introduced as
Lake evaporation = Cp X pan evaporation
in which Cp =
pan coefficient. The values of Cp in use for different pan are given in Table 3. I.TABLE 3.1 VALUES OF PAN COEFFICIFNT Cp
S. No. |
Type of pan | Average value | Range |
1 | Class A Land Pan | 0.70 | 0.60-0.80 |
2 | ISI Pan (modified Class A) | 0.80 | 0.65-1.10 |
3 | Colorado Sunken Pan | 0.78 | 0.75-0.86 |
4 | USGS Floating Pan | 0.80 | 0.70-0.82 |
Evaporation Stations
It is usual to install
• Arid zones-One station for every 30,000 km²,
• Humid temperate
Currently India has about 200 pan-evaporimeter stations maintained by the India Meteorological Department.
TRANSPIRATION
Transpiration is the
process by which water leaves the body of a living plant and
exists between transpiration and
evaporation. Transpiration is essentially confined to daylight hours and the rate of transpiration depends upon the growth periods of the plant. Evaporation, on the other hand, continues all through the day and night although the rates are different.EVAPOTRANSPIRATION
While
transpiration takes place, the land area in which plants stand also lose
moisture by the evaporation of water from soil and water bodies. In
P – Rs – Go - Eact = D S ; ; (3.12)
Where, P = precipitation, Rs = surface runoff, Go = subsurface outflow, Eact = actual evapotranspiration (AET) and D S = change in the moisture storage. This water budgeting can be used to calculate Eact by knowing or estimating other elements of above equation. The sum of Rs and Go can be taken as the
stream flow R at the basin outlet without much error.Except in a few specialised studies, all applied studies in hydrology use PET for various estimation purposes. It is generally agreed that PET is a good approximation for lake evaporation.
MEASUREMENT OF
EVAPOTRANSPIRATION
The measurement of evapotranspiration for a given
vegetation type can be carried out in two ways: either by using
Lysimeters
A lysimeter is a special watertight tank
containing a block of soil and set in a field of growing plants. The plants
grown in the lysimeter are the same as in the surrounding field.
Evapotranspiration is estimated in terms of the amount of water required to
maintain constant moisture conditions within the tank measured either
volumetrically or gravimetrically through an arrangement made in the
Field Plots
In special plots all the elements of the
water budget in a known interval of time are measured and the
Evapotranspiration = [precipitation + irrigation input - runoff
- increase in soil
storage
- groundwater loss]
Measurements are usually confined to precipitation, irrigation input, surface runoff and soil moisture. Groundwater loss due to deep
percolation is difficult to measure and can be minimised by keeping the moisture condition of the plot at the field capacity. This method provides fairly reliable results.POTENTIAL
EVAPOTRANSPIRATION OVER INDIA
Using Penman's equation and the available
climatalogical data, PET estimated for the country has been made. The mean
annual PET (in cm) over various parts of the country is shown in the form of
INITIAL LOSS
In the precipitation reaching the surface of a
catchment the major abstraction is from the infiltration process. However, two
other processes, though small in magnitude, operate to reduce the water volume
available for runoff and thus act as abstractions. These are the
INTERCEPTION
When it rains over a
catchment not all the precipitation falls directly onto the ground. Before it
reaches the ground, a part of it may be caught by the vegetation and
subsequently evaporated. The volume of water so caught is called interception.
The intercepted precipitation may follow one of the three possible routes:
It may be retained by the vegetation as surface storage and returned to the atmosphere by evaporation; a process termed interception loss;
It can drip off the plant leaves to join the ground surface or. the surface flow; this is known as
The rainwater may run along the leaves and branches and down the stem to reach the ground surface. This part is called stemflow.
Interception loss is solely due to evaporation and does not include transpiration, through fall or stemfiow.
The amount of water intercepted in a given area is extremely difficult to measure. It depends on the species composition density and also on the storm characteristics. It is estimated that of the total rainfall in an area during a plant-growing season the interception loss is about 10 to 20%.
Interception is satisfied during the first part of a storm and if an area experiences a large number of small storms, the annual interception loss due to forests in such cases will be high, amounting to greater than 25% of the annual precipitation. Quantitatively, the variation of interception loss with the rainfall magnitude per storm for small storms is as shown in Fig. 3.7. It is seen that the interception loss is large for a small rainfall and levels off to a constant value for larger storms.
For a given storm, the interception loss is estimated as
Ii = Si + Ki Et ; ; (3.18)
Where Ii = interception loss in mm, Si = interception storage whose value varies from 0.25 to 1.25 mm depending on the nature of vegetation, Ki = ratio of vegetal surface area to its projected area, E = evaporation rate in mm/h during the precipitation and t = duration of rainfall in hours.
It is found that coniferous trees have more interception loss deciduous ones. Also, dense grasses have nearly same interception losses as full grown trees and can account for nearly 20% of the total rainfall in a season. Agricultural crops in their growing season also contribute high interception losses. In view of these the interception process has a very significant impact on the ecology of the area related to silvicultural aspects and in the water balance of a region. However, in hydrological studies dealing with floods interception loss is rarely significant and is not separately considered, The common practice is to allow a lump sum value as the initial loss to be deducted from the initial period of the storm.
DEPRESSION STORAGE
When the
precipitation of a storm reaches the ground, it must first fill up all
depressions before it can flow over the surface. The volume of water trapped in
these depressions is called depression storage. This amount is eventually lost
to runoff through processes of infiltration and evaporation and thus form a part
of the initial loss. Depression storage depends on a vast number of factors the
chief of which are :
• The type of
soil,
• The condition of the
surface reflecting the amount and nature of depression,
• The slope of the catchment and
• The
INFILTRATION PROCESS
It is
well-known that when water is applied to the surface of a soil, a part of it
seeps into the soil. This movement of water through the soil surface is known as
infiltration and plays a very significant role in the runoff process by
affecting the timing, distribution and magnitude of the surface runoff. Further,
infiltration is the primary step in the natural
Infiltration is the flow of water into the ground through the soil surface and the process can be easily understood through a simple analogy. Consider a small container covered with wire gauze as in Fig. 3.8. If water is poured over the gauze, a part of it will go tainer and a part overflows. Further, the container can hold only a fixed quantity and when it is full no more flow into the container can take place. This analogy, though a highly simplified one, underscores two important aspects, viz., the maximum rate at which the ground can absorb water, the
infiltration capacity and the volume of water that it can hold, the field capacity.Since the infiltered water may contribute to groundwater discharge in addition to increasing the soil moisture, the process can be schematically modelled as in Fig. 3.9(a) and (b). This figure considers two situations, viz. low-intensity rainfall and high intensity rainfall, and is self explanatory.
Fig. 3.9 An infiltration model
INFILTRATION CAPACITY
The
maximum rate at which a given soil at a given time can absorb water is defined
as the infiltration capacity. It is designated as fc and is expressed
in units of cm/h. The actual rate of infiltration f can be expressed as
f = fc when i >
fc
; ; (3.19)
f = i when i < fc
where i = intensity of rainfall. The infiltration capacity of a soil is high at the beginning of a storm and has an exponential decay as the time elapses. The infiltration process is affected by a large number of factor and a few important ones affecting fc are described below.
Characteristics of Soil
The type of soil, viz. sand,
silt or clay, its texture, structure,
Surface of Entry
At the soil surface, the impact of
raindrops causes the fines in the soils to be displaced and these in turn can
clog the pore spaces in the upper layers. This is an important factor affecting
the
Fluid Characteristics
Water infiltrating into the soil
will have many impurities, both in solution and in suspension. The turbidity of
the water, especially the clay and colloid content is an important factor as
such suspended particles block the fine pores in the soil and reduce its
infiltration capacity. The temperature of the water is a factor in the sense
that it affects the viscosity of the water which in turn affects the
MEASUREMENT OF
INFILTRATION
Information about the infiltration characteristics of the soil
at a given location can be obtained by conducting controlled experiments on
small areas. The experimental set-up is called an
Flooding-type infiltrometer
Rainfall simulator.
Flooding-Type lnfiltrometer
This is a simple instrument
consisting essentially of a metal cylinder, 30 cm diameter and 60 cm long, open
at both ends. This cylinder is driven into the ground to a depth of 50 cm
(Fig.3.10). Water is poured into the top part to a depth of 5 cm and a pointer
is set to mark the water level. As infiltration proceeds, The volume is made up
by adding water from a burette to keep the water level at the tip of the
pointer. Knowing the volume of water added at different time intervals, the plot
of the infiltration capacity vs lime is obtained. The experiments are continued
till a uniform rate of infiltration is obtained and this may take 2-3 h. The
surface of the soil is usually protected by a perforated disk to prevent
formation capacity vs lime is obtained. The experiments re continued till a
uniform rate of infiltration is obtained and this may take 2-3 h.
Fig.3.10 Simple infiltrometer
The surface of the soil is usually protected by a perforated disk to prevent formation of turbidity and its settling on the soil surface.
A major objection to the simple infiltrometer as above is that the infiltered water spreads at the outlet from the tube (as shown by dotted lines in Fig. 3.10) and as such the tube area is not representative of the area in which infiltration takes place. To overcome this a ring infiltrometer consisting of a set of two concentric rings (Fig.3.11) is used. In this two rings are inserted into the ground and water is maintained on the soil surface, in both the rings, to a common fixed level. The outer ring provides a water jacket to the infiltering water of the inner ring and hence prevents the spreading out of the infiltering water of the inner tube. The measurements of water volume is done on the inner ring only.
Fig.3.11 Ring infiltrameter
The main disadvantages of flooding-type
infiltrometer are : • Raindrop-impact
effect is not simulated;
• Driving
of the tube or rings disturbs the soil structure;
• Results of the infiltrometer depend to some extent on
their size with the larger
meters giving less
rates than the smaller ones; this is due to the border effect.
Rainfall Simulator
In this a small plot of land, of about 2
m X 4 m size, is provided with a size of nozzles on the longer side with
arrangements to collect and measure the surface runoff rate. The specially
designed nozzles produce raindrops falling from a height of 2 m and are capable
of producing various intensities of rainfall. Experiments are conducted under
controlled conditions with various combinations of intensities and durations and
the surface runoff is measured in each case. Using the water-budget equation
involving the volume of
INFILTRATION-CAPACITY
VALUES
The typical variation of the infiltration capacity for two soils and
for two initial conditions is shown in Fig. 3.12. It is clear from the figure
that the infiltration capacity for a given soil decreases with time from the
start of rainfall; it decreases with the degree of saturation and depends upon
the type of soil. Horton (1930) expressed the decay of the infiltration capacity
with time as
(3.20)
Where,
fct = infiltration capacity at any time t from start
of the rainfall
fco = initial infiltration capacity at t =
0
fcf = final steady state value
td = duration of
the rainfall and
Kh = constant depending upon the soil
characteristics and vegetation cover.
The difficulty of finding the variation of the three parameters fco, fcf and Kh with soil characteristics and antecedent moisture conditions precludes the general use of Eq. (3.20).
Fig. 3.12 Variation of infiltration capacity
It is apparent that infiltration-capacity values of soils are subjected to wide variations depending upon a large number of factors. Typically, a bare, sandy area will have fc » 1.2 cm/h and a bare, clay soil will have fs » 0.15 cm/h. A good grass cover or vegetation cover increases these values by as much as 10 times.
INFILTRATION INDICES
In
hydrological calculations involving
The F index is the average rainfall above which the rainfall volume is equal to the runoff volume. The F index is derived from the rainfall
hyetograph with the knowledge of the resulting runoff volume. The initial loss is also considered as infiltration. The F value is found by treating it Back to Topas a constant infiltration capacity. If the
rainfall intensity is less than 0, then the infiltration rate is equal to the rainfall intensity; however, if the rainfall intensity is larger than F the difference between rainfall and infiltration in an interval of time represents the runoff volume (Fig. 3.13). The amount of rainfall in excess of the F index is called rainfall excess. The F index thus accounts for the total abstraction and enables runoff magnitudes to be estimated for a given rainfall hyetograph.Fig.3.13 f Index
STREAMFLOW
MEASUREMENT
Streamflow representing the runoff phase of the
A stream can be defined as a flow
channel into which the surface runoff from a specified basin drains. Generally, there is considerable exchange of water between a stream and the underground water. Streamflow is measured in units of discharge (m³/s) occurring at a specified time and constitutes historical data. The measurement of discharge in a stream forms an important branch of Hydrometry, the science and practice of water measurement. This chapter deals with only the salient streamflow measurement techniques to provide an appreciation of this important aspect of engineering hydrology.Streamflow measurement techniques can be broadly classified into two categories as (i) direct determination and (ii) indirect determination.
1. Direct determination of stream
discharge:
(a)
2. Indirect determination of stream
flow:
(a)
Hydraulic structures, such as weirs, flumes and gated
structures
(b) Slope-area method.
Barring a few exceptional cases, continuous measurement of stream discharge is very difficult to obtain. As a rule, direct measurement of discharge is a very time-consuming and costly procedure. Hence, a two step procedure is followed. First, the discharge in a given stream is related to the elevation of the water surface (stage) through a series of careful measurements. In the next step the stage of the steam is observed routinely in a relatively inexpensive manner and the discharge is estimated by using the previously determined
stage-discharge relationship. The observation of the stage is easy, inexpensive, and if desired, continuous readings can also be obtained. This method of discharge determination of streams is adopted universally.MEASUREMENT OF STAGE
The
stage of a river is defined as its water-surface elevation measured above a
datum. This datum can be the mean-sea level (MSL) or an arbitrary datum
connected independently to the MSL.
Staff Gauge
The simplest of stage measurements are made by
noting the water surface in contact with a fixed graduated staff. The staff is
made of a durable material with a low coefficient of expansion with respect both
temperature and moisture. It is fixed rigidly to a structure, such an abutment,
pier, wall, etc. The staff may be vertical or inclined with clearly and
accurately graduated permanent markings. The markings a distinctive, easy to
read from a distance and are similar to those or surveying staff. Sometimes, it
may not be possible to read the entire range of water-surface elevations of a
stream by a single
Wire Gauge
If is a gauge used to measure the water-surface
elevation from above the surface such as from a bridge or similar structure. In
this a weight is lowered by a reel to touch the water surface. A mechanical
counter measures the rotation of the wheel which is proportional to the length
of
Fig.4.1 (a) Vertical staff gauge (b) Sectional staff
the wire paid out. The operating range of this kind of gauge is about 25 m.
Back to TopAutomatic Stage Recorders
The staff gauge and
Float-Gauge Recorder
The Float-operated stage recorder is
the most common type of automatic stage recorder in use. In this a float
operating in a stifling
To protect the float from debris and to reduce the water surface wave effects on the recording, stifling wells are provided in all float-type stage-recorder installations. Figure 4.2 shows a typical stifling well installation. Note the intake pipes that communicate with the river and flushing arrangement to flush these intake pipes off the sediment and debris occasionally. The water-stage recorder has to be located above the highest water level expected in the stream to prevent it from getting inundated
Fig. 4.2 Stilling well installation
during floods. Further, the instrument must be properly housed in a suitable enclosure to protect it from weather elements and vandalism. On account of these, the water-stage-recorder installations prove to costly in most instances. A water-depth recorder is shown in Fig.4.3 (Plate 1).
Bubble GaugeFig.4.4 Bubble Gauge
The bubble gauge has certain specific advantages over a float operated water stage recorder and these can be listed as under :
• There is no need for costly stifling wells;
• A large change in the stage, as much as 30 m, can be
measured;
• The recorder assembly can be quite far away from the
sensing point; and
• Due to constant bleeding action there is less
likelihood of the inlet
getting
blocked
or choked.
Stage Data
The stage data is often presented in the form of
a plot of stage against chronological time (Figure 4.7) known as
Fig. 4.7 Stage hydrograph
MEASUREMENT OF
VELOCITY
The measurement of velocity is an important aspect of many direct
stream-flow measurement techniques. A mechanical device, called
Current Meters
The most commonly used instrument in
Vertical-axis meters, and
Horizontal-axis meters.
Vertical-Axis Meters
These instruments consist of a series
of conical cups mounted around a vertical axis [Figs. 4.8 and 4.9 (Plate 111)].
The cups rotate in a horizontal plane and a cam attached to the vertical axial
spindle records generated signals proportional to the revolutions of the cup
assembly. The Price current meter and Gurley current meter are typical
instruments under this category. The normal range of velocities is from 0.15 to
0.40 m/s. The accuracy of these instruments is about 1.50% at the threshold
value and improves to about 0.30% at speeds in excess of 1.0 m/s.
Fig. 4.8 Vertical-axis current meter
Vertical-axis instruments have the disadvantage that they cannot be used where there are appreciable vertical components of velocities. For example, the instrument shows a positive velocity when it is lifted vertically in still water.
Horizontal-Axis Meters
These meters consist of a propeller
mounted at the end of a horizontal shaft [Fig. 4.10 (Plate 111) and 4.11]. These
come in a wide variety of sizes with propeller diameters in the range 6 to 12 cm
and can register velocities in the range of 0.15 to 4.0 m/s. Ott-, Neyrtec=
[Fig. 4.12 (Plate IV)] and Watt-type meters are typical instruments under this
kind.
Fig. 4.11 Propeller,type current meter
These meters are fairly rugged and are not affected by oblique flows of as much as 15°. The accuracy of the instrument is about 1% at the threshold value and is about 0.25% at a velocity of 0.3 m/s and above.
Back to TopA
current meter is so designed that its rotation speed varies linearly with the stream velocity v at the location of the instrument. A typical relationship isv = a Ns+ b (4.1)
where v = stream velocity at the instrument location in m/s,
Ns = revolutions per
second of the meter and
a, b = constants of the meter. Typical values of a and b for a standard size 12.5 cm dia Price meter (cup-type) is a = 0.65 and b = 0.03. Smaller meters of 5 cm. diameter cup assembly called pigmy meters run faster and are useful in measuring small velocities. The values of the meter constants for them are of the order of a = 0.30 and b = 0.003. Further, each instrument has a threshold velocity below which Eq. (4.1) is not applicable. The instruments have a provision to count the number of revolutions in a known interval of time. This is usually accomplished by the making and breaking of an electric
circuit either mechanically or electro-magnetically at each revolution of the shaft. In older model instruments the breaking of the circuit would be counted through an audible sharp signal ("tick") heard on a headphone. The revolutions per second is calculated by counting the number of such signals in a known interval of time, usually about 100 s. Present-day models employ electromagnetic counters with digital or analogue displays.
Calibration
The relation between the stream velocity and
revolutions per second of the meter as in Eq. (4.1) is called the calibration
equation. The calibration equation is unique to each instrument and is
determined by towing the instrument in a special tank. A towing tank is a long
channel containing still water with arrangements for moving a carriage
longitudinally over its surface at constant speed. The instrument to be
calibrated is mounted on the carriage with the rotating element immersed to a
specified depth in the
AREA-VELOCITY METHOD
This
method of discharge measurement consists essentially of measuring the area of
cross-section of the river at a selected section called the
• The
stream should have a well-defined cross-section which does not change
in
various
seasons.
•
It should be easily accessible all through the
year.
•
The site should be in a straight, stable
At the selected site the section line is marked off by permanent survey markings and the cross-section determined. Towards this the depth at various locations are measured by sounding rods or
sounding weights. When the stream depth is large or when quick and accurate depth measurements are needed, an electro-acoustic instrument called echo-depth recorder is used. In this a high frequency sound wave is sent down by a transducer kept immersed at the water surface and the echo reflected by the bed is also picked up by the same transducer. By comparing the interval between the transmission of the signal and the receipt of its the distance to the bed is obtained and is indicated or recorded in the instrument. Echo-depth recorders are particularly advantageous in high-velocity streams, deep streams and in streams with Oft or mobile beds. For purposes of discharge estimation, the cross-section is considered to be divided into a large number of subsections by verticals (Fig. 4.14). The average velocity in these subsections are measured by current meters or floats. It is quite obvious that the accuracy of discharge estimation increases with the number of subsections used.Fig. 4.14 Stream section for area-velocity method
However, the larger the number of segments, the larger is the effort, time and expenditure involved. The following are some of the guidelines to select the number of segments:
The segment width should not be greater than 1/15 to 1/20 of the width of the river.
The discharge in each segment should be less than 10% of the total discharge.
The difference of velocities in adjacent segments should not be more than 20%.
It should be noted that in natural rivers the verticals for velocity measurement are not necessarily equally spaced. The
area-velocity method as above using the current meter is often called as the standard current meter method. Back to TopMoving-Boat Method
Discharge measurement of large alluvial
rivers, such as the Ganga, by the standard current meter method is very
time-consuming even when the flow is low or moderate. When the river is in
spate, it is almost impossible to use the standard current meter technique due
to the difficulty of keeping the boat stationary on the fast-moving surface of
the stream for observation purposes. It is in such circumstance that the newly
developed moving-boat techniques prove very helpful.
In this method a special propeller-type current meter which is free to move about a vertical axis is towed in a boat at a velocity vb at right angles to the stream flow. If the flow velocity is vf the meter will align itself in the direction of the resultant velocity vR making an angle q with the direction of the boat (Fig. 4.15). Further, the meter will register the velocity vR. If Vb is normal to vf,
&#vb = vR cos q and vf = vR sin q
If the time of transit between two verticals is D t, then the width between the two verticals (Figure 4.8) is
W = vb D t
The flow in the sub-area between two verticals i and i+1 where the depths are yi and yi+1. respectively, by assuming the current meter to measure the average velocity in the vertical, is
Fig. 4.15 Moving-boat method
(4.11)
Thus by measuring the depths yi, velocity vR and q in a reach and the time taken to cross the reach D t, the discharge in the sub-area can be determined. The summation of the partial discharges D Qi over the whole width of the stream gives the stream discharge
Q = å D Qi (4.12)
In field applications a good stretch of the river with no shoals, islands, bars, etc. is selected. The cross-sectional line is defined by permanent land marks so that the boat can be aligned along this line. A motor boat with different sizes of outboard motors for use in different river stages is selected. A special current meter of the propeller-type, in which the velocity and inclination of the meter to the boat director 0 in the horizontal plane can be measured, is selected. The current meter is usually immersed at a depth of 0.5 m from the water surface to record surface velocities. To mark the various vertical sections and know the depths at these points, an
echo-depth recorder is used.In a typical run, the boat is started from the water edge and aligned to go across the cross-sectional line. When the boat is in sufficient depth of water, the instruments are lowered. The echo-depth recorder and current meter are commissioned. A button on the signal processor when pressed marks a distinctive mark line on the depth vs time chart of the echo-depth recorder. Further, it gives simultaneously a sharp audio signal to enable the measuring party to take simultaneous readings of the velocity vR and the inclination q . A large number of such measurements are taken during the traverse of the boat to the other bank of the river. The operation is repeated in the return journey of the boat. It is important that the boat is kept aligned along the cross-sectional line and this requires considerable skill on the part of the pilot. Typically, a river of about 2km stretch takes about 15 min for one crossing. A number of crossings are made to get the average value of the discharge. The surface velocities are converted to average velocities across the vertical by applying a coefficient [Eq. (4.5)]. The depths yi and time intervals D t are read from the echo-depth recorder chart. The discharge is calculated by Eqs. (4.11) and (4.12). In practical use additional coefficients may be needed to account for deviations from the ideal case and these depend upon the actual field conditions.
Back to TopDILUTION TECHNIQUE OF STREAMFLOW
MEASUREMENT
The dilution method of flow measurement, also known as the
chemical method depends upon the continuity principle applied to a tracer which
is allowed to mix completely with the flow. Consider a tracer which does not
react with the fluid or boundary.
Let Co be the small initial concentration of the tracer in the streamflow. At section 1 a small quantity (volume V1) of high concentration C1 of this tracer is added (Fig. 4.16). Let section 2 be sufficiently far away on the
downstream of section 1 so that the tracer mixes thoroughly with the fluid due to the turbulent mixing process while passing through the reach. The concentration profile taken at section 2 is schematically shown in Fig. 4.16. The concentration will have a base value of CO, increases from time t1 to a peak value and gradually reaches the base value of Co at time t2. The streamflow is assumed to be steady.Fig. 4.16 Sudden-injection method
By continuity of the tracer material M1 = mass of tracer added at section 1 = V1 Cl
Back to TopNeglecting the second term on the right-hand side as insignificantly small,
(4.13)
Thus the discharge Q in the stream can be estimated if for a known M1 the variation of C2 with time at section 2 and Co are determined. This method is known as sudden injection or gulp or integration method.
Another way of using the dilution principle is to inject the tracer of concentration C1 at a constant rate Qt at section 1. At section 2, the concentration gradually rises from the background value of Co at time t1 to a constant value C2 (Fig. 4.17). At the stready state, the continuity equation for the tracer is
i.e. (4.14)
This technique in which Q is estimated by knowing C1, C2, Co and Q is known as constant rate injection method or plateau gauging.
Fig.4.17 Constant rate injection method
It is necessary to emphasise here that the dilution method of gauging is based on the assumption of steady flow. If the flow is unsteady and the flow rate changes appreciably during gauging, there will be a change in the storage volume in the reach and the steady-state continuity equation used to develop Eqs. (4.13) and (4.14) is not valid. Systematic errors can be expected in such cases.
Tracers
The tracer used should have ideally the following
properties:
• It should not
be absorbed by the sediment, channel boundary and vegetation.
It
should not chemically react with any
of the above surfaces and also should not
be
lost by evaporation.
• It should be
non-toxic.
• It should be
capable of being detected in a distinctive manner in
small
concentrations.
• It should not be very expensive.
The tracers used are of three main types:
• Chemicals (common salt and sodium dischromate are
typical);
• Fluorescent dyes
(Rhodamine-WT and Sulpho-Rhodamine B Extra are typical);
• Radioactive materials (such as Bromine-82,
Sodium-24 and Iodine-132).
Common salt can be detected with an error of ± 1% up to a concentration of 10 ppm. Sodium dichromate can be detected up to 0.2 ppm concentrations. Fluorescent dyes have the advantage that they can be detected at levels of tens of nanograms per litre (~1 in 1011) and hence require very small amounts of solution for injections. Radioactive traces are detectable up to accuracies of tens of picocuries per litre (~1 in 1014) and therefore permit large-scale dilutions. However, they involve the use of very sophisticated instruments and handling by trained personnel only. The availability of detection instrumentation, environmental effects- of the tracer and overall cost of the operation are chief factors that decide the tracer to be used.
Back to TopLength of Reach
The length of the reach between the dosing
section and sampling section should be adequate to have complete mixing of the
tracer with the flow. This length depends upon the geometric dimensions of the
channel cross-section, discharge and turbulence levels. An empirical formula
suggested by Rimmar (1960) for estimation of mixing length for point injection
of a tracer in a straight reach is
(4.15)
where L = mixing length (m), B = average width of the stream (m), d = average depth of the stream (m), C = Chezy coefficient of roughness which varies from 15 to 50 for smooth to rough bed conditions and g = acceleration due to gravity. The value of L varies from about 2 km for a mountain stream carrying a discharge of about 1.0 m³/s to about 100 km for river in a plain with a discharge of about 300 m³/s. The mixing length becomes very large for large rivers and is one of the major constraints of the dilution method. Artificial mixing of the tracer at the dosing station may prove beneficial for small streams in reducing the mixing length of the reach.
Use
The dilution method has the major advantage that the
discharge is estimated directly in an absolute way. It is a particularly
attractive method for small turbulent streams, such as those in mountainous
areas. Where suitable, it can be used as an occasional method for checking the
calibration, stage-discharge curves, etc. obtained by other methods.
ELECTROMAGNETIC
METHOD
The electromagnetic method is based on the Faraday's principle that an
emf is induced in the conductor (water in the present case) when it cuts a
normal magnetic field. Large coils buried at the bottom of the channel carry a
current I to produce a controlled vertical magnetic field, (Fig.4.18).
Electrodes provided at the sides of the channel section measure the small
voltage produced due to flow of water in the channel.
Fig. 4.18 Electromagnetic method
It has been found that the signal output E will be of the order of millivolts and is related to the discharge Q as
&# (4.16)
where d = depth of flow, I = current in the coil, and n, K1 and K2 are system constants.
Back to TopThe method involves sophisticated and expensive instrumentation and has been successfully tried in a number of installations. The fact that this kind of set-up gives the total discharge when once it has been calibrated, makes it specially suited for field situations where the cross-sectional properties can change with time due to weed growth, sedimentation, etc. Another specific application is in tidal channels where the flow undergoes rapid changes both in magnitude as well as in direction. Present-day commercially available
electromagnetic flowmeters can measure the discharge to an accuracy of ±3%, the maximum channel width that can be accommodated being 100 m. The minimum detectable velocity is 0.005 m/s.ULTRASONIC METHOD
This is
essentially an
Consider a channel carrying a flow with two transducers A and B fixed at the same level h above the bed and on either sides of the channel (Fig. 4.19). These transducers can receive as well as send ultrasonic signals. Let A send an ultrasonic signal to be received at B after an elapse time t1.
Fig. 4.19 Ultrasonic method
Similarly, let B send a signal to be received at A after an elapse time t2.
If C = velocity of sound in water,
(4.17)
where L = length of path from A to B and vp = component of the flow velocity in the sound path = v cos q . Similarly, from Fig.4.19 it is easy to see that
(4.18)
Thus
Or (4.19)
Thus for a given L and q , by knowing t1 and t2, the average velocity along the path AB, i.e. v can be determined. It may be noted that v is the average velocity at a height h above the bed and is not the average velocity V for the whole cross-section. However, for a given channel cross-section v can be related to V and by calibration a relation between v/V and h can be obtained. For a given set-up, as the area of cross-section is fixed, the discharge is obtained as a product of area and mean velocity V. Estimation of discharge by using one signal path as above is called single-path gauging. Alternatively, for a given depth of flow, multiple single paths can be used to obtain v for different h values. Mean velocity of flow through the cross-section is obtained by averaging these v values. This technique is known as multi-path gauging.
Ultrasonic flowmeters using the above principal have frequencies of the order of 500 kHz. Sophisticated electronics are needed to transmit, detect and evaluate the mean velocity of flow along the path. In a given installation a calibration (usually performed by the current-meter method) is needed to determine the system constants. Currently available commercial systems have been installed successfully at many places and accuracies of about 2% for the single-path method and 1% for the multipath method are reported. The systems are currently available for rivers up to 500 m width.The specific advantages of the ultrasonic system of river gauging are:
• It is rapid
and gives high accuracy;
• It
is suitable for automatic recording of data;
• It can handle rapid changes in the magnitude and
direction of flow as in tidal
rivers;
• The cost of installation
is independent of the size of rivers.
The accuracy of this method is limited by the factors that affect the signal velocity and averaging of flow velocity, such as
• Unstable cross-section í fluctuating weed growth,
•
High loads of suspended solids, í air
entertainment and
•
INDIRECT METHODS
Under this
category are included those methods which make use of relationship between the
flow discharge and the depths at specified locations. The field measurement is
restricted to the measurement of t depths only. Two broad classifications of
these indirect methods are:
• Flow measuring
structures, and
• Slope area methods.
Flow-Measuring Structures
Use of structures like notches,
weirs flumes and sluice gates for flow measurement in hydraulic laboratories is
well known. These conventional structures are used in field conditions also but
their use is limited by the ranges of head, debris or sediment load of the
stream and the back-water effects produced by the installations. To overcome
many of these limitations a wide variety of flow measuring structures with
specific advantages are in use.
The basic principle governing the use of a weir, flume or similar flow measuring structure is that these structures produce a unique
control section in the flow. At these structures, the discharge Q is a function of the water-surface elevation measured at a specified upstream location,Q =f (H) (4.20)
where H = water surface elevation measured from a specified datum.
Back to TopThus, for example, for weirs, Eq. (4.20) takes the form
&#Q = K Hn (4.21)
where H = head over the weir and K, n = system constants. Equation (4.20) is applicable so long as the
downstream water level is below a curtain limiting water level known as the modular limit. Such flows which are independent of the downstream water level are known as free flows. If the tailwater conditions do affect the flow, then the flow is known as drowned or submerged flow. Discharges under drowned condition are obtained by applying a reduction factor to the free flow discharges. For example, the sumberged flow over a weir (Fig. 4.20) is estimated by the Villemonte formula,(4.22)
Where, Qs = submerged discharge, Q1 = free flow discharge under head H1, H1 = upstream water surface elevation measured above the weir crest, H2 = downstream water surface elevation measured above the weir crest, n exponent of head in the free flow head discharge relationship [ Eq. (4.21)]. For a rectangular weir n = 1.5.
The various flow measuring structures can be broadly considered under three categories:
Thin-plate structures are usually made from a vertically set metal plate. The V-notch, rectangular full width and contracted notches are typical examples under this category.
Long-base weirs, also known as broad-crested weirs are made of concrete or masonry and are used for large discharge values.
Flumes are made of concrete, masonry or metal sheets depending on their use and location. They depend primarily on the width constriction to produce a control section.
Fig. 4.20 Flow over a weir: (a) Free flow (b) Submerged flow
Back to TopSlope-Area Method
The resistance equation for uniform flow
in an open
Fig. 4.21 Slope-area method
STAGE-DISCHARGE
RELATIONSHIP
As indicated earlier the measurement of discharge by the direct
method involves a two step procedure; the development of the stage-discharge
relationship which forms the first step is of utmost importance. Once the
stage-discharge (G-Q) relationship is established, the subsequent procedure
consists of measuring the stage (G) and read the discharge (Q) from the (G-Q)
relationship. This second part is a routine operation. Thus the aim of all
current-meter and other direct-discharge measurements is to prepare a stage
discharge relationship for the given channel gauging section. The
stage-discharge relationship is also known as the
The measured value of discharges when plotted against the corresponding stage give a relationship that represents the integrated effect of a wide range of channel and flow parameters. The combined effect of these parameters is termed control. If the (G-Q) relationship for a gauging section is constant and does not change with time, the control is said to be Permanent. If it changes with time, it is called shifting control.
Permanent Control
A majority of streams and rivers,
especially nonalluvial rivers exhibit
Q = Cr (G - a)b (4.26)
in which Q = stream discharge, G = gauge height (stage), a = a constant which represent the gauge reading corresponding to zero discharge, Cr and b are rating curve constants. This relationship can be expressed graphically by plotting the observed stage against the corresponding discharge values in an arithmetic or logarithmic plot [Fig. 4.22 (a) and (b)] . Logarithmic plotting is advantageous as Eq. (4.26) plots as a straight line in logarithmic coordinates. In Fig. 4.22(b) the straight line is drawn to best represent the data plotted as Q vs (G-a). Coefficients Cr and b need not be the same for the full range of stages.
Fig. 4.22 (a) Stage-discharge curve: arithmetic plot
Fig. 4.22 (b) Stage-discharge curve: logarithmic plot
The best values of Cr and b in Eq. (4.26) for a given range of stage are obtained by the least-square-error method. Thus by taking logarithms,
log Q = b log (G - a) + log
Cr
(4.27)
or Y = b X +
b
(4.27a)
in which Y = log Q, X = log (G - a) and b = log Cr.
For the best-fit straight line of N observations of X and Y,
(4.28)
and
In the above it should be noted that a is an unknown and its determination poses some difficulties. The following alternative methods are available for its determination:
• Plot Q vs G on an arithmetic graph paper and draw a best-fit curve.
By extrapolating the curve by eye judgement find a as the value of G corresponding to Q = 0. Using this value of a, plot log Q vs log (G-a) and verify whether the data plots as a straight line. If not, select another value in the neighbourhood of previously assumed value and by trial and error find an acceptable value of a which gives a straight line plot of log Q vs log (G-a).
A graphical method due to Running' is as follows: The Q vs G data are plotted to an arithmetic scale and a smooth curve through the plotted points are drawn. Three points A, B and C on the curve are selected such that their discharges are in geometric progression, (Fig. 4.23) i.e.
At A and B vertical lines are drawn and then horizontal lines are drawn at B and C to get D and E as intersection points with the verticals. Two straight lines ED and BA are drawn to intersect at F.
Fig. 4.23 Running's method for estimation of the constant
The ordinate at F is the required value of a, the gauge height corresponding to zero discharge. This method assumes the lower part of the
Plot Q vs G to an arithmetic scale and draw a smooth good-fitting curve by eye-judgement. Select three discharges Q1, Q2 and Q3, such that Q1/Q2 = Q2/Q3 and note from the curve the corresponding values of gauge readings G1, G2 and G3. From Eq. (4.27)
i.e. (4.29)
A number of optimization procedures that are based on the use of computers are available to estimate the best value of a. A trial-and-error search for a which gives the best value of the
Shifting Control
The control that exists at a gauging
section giving rise to a unique stage discharge relationship can change due to:
• Changing
characteristics caused by weed growth, dredging or
channel
encroachment, aggradation or
degradation phenomenon in an alluvial channel,
• Variable backwater effects affecting the gauging
section and
•
Backwater Effect
If the shifting control is due to variable
&#Q = f (G, F) (4.30)
Schematically, this functional relationship is shown in Fig. 4.24. Instead of having a three-parameter plot, the observed data is normalized with respect to a constant fall value. Let Fo be a normalizing value of the fall
Fig. 4.24 Backwater effect on a rating curve-normalized curve
taken to be constant at all stages, F the actual fall at a given stage when the actual discharge is Q. These two fall values are related as
(4.31)
in which Qo = normalized discharge at the given stage when the fall is equal to Fo and m = an exponent with a value close to 0.5. From the observed data, a convenient value of Fo is selected. An approximate Qo vs G curve for a constant Fo called constant fall curve is drawn. For each observed data, Q/Qo and F/Fo values are calculated and plotted as Q/Qo vs F/Fo (Fig.4.25). This is called the adjustment curve. Both the constant fall curve and the adjustment curve are refined, by trial and error to get
Fig. 4.25 Backwater effect on a rating curve-adjustment curve
the best-fit curves. When finalized, these two curves provide the stage-discharge information for gauging purposes. For example, if the observed stage is G1 and fall F1, first by using the adjustment curve the value of Q1/Qo is read for a known value of F1/Fo. Using the constant fall-
rating curve, Qo is read for the given stage G1 and the actual discharge calculated as (Q1/Qo) x Qo. Back to TopUnsteady-Flow Effect
When a flood wave passes a gauging
station in the advancing portion of the wave the approach velocities are larger
than in the steady flow at corresponding stages. Thus for the same stage, more
discharge than in a steady uniform flow occurs. In the retreating phase of the
flood wave the converse situation occurs with reduced approach velocities giving
lower discharges than in an equivalent steady flow case. Thus the
stage-discharge relationship for an unsteady flow will not be a single-valued
relationship as in steady flow but it will be a looped curve as in Fig. 4.26. It
may be noted that at the same stage, more discharge passes through the river
during rising stages than in falling ones. Since the conditions for each flood
may be different, different floods may give different loops.
Fig. 4.26 Loop rating curve
If Qn is the normal discharge at a given stage under steady uniform flow and Qm is the measured (actual) unsteady flow the two are related as
(4.32)
where So = channel slope = water surface slope at uniform flow, dh/dt = rate of change of stage and Vw = velocity of the flood wave. For natural channels, Vw is usually assumed equal to 1.4 V, where V = average velocity for a given
stage estimated by applying Manning's formula and the energy slope Sf. Also, the energy slope is used in place of So in the denominator of Eq. (4.32). If enough field data about the flood magnitude and dh/dt are available, the term (1/Vw So) can be calculated and plotted against the stage for use in Eq. (4.32). For estimating the actual discharge at an observed stage, QM/Qn is calculated by using the observed data of dh/dt. Here Qn is the discharge corresponding to the observed stage relationship for steady flow in the channel reach.EXTRAPOLATION OF
RATING CURVE
Most hydrological designs consider extreme flood flows. As an
example, in the design of hydraulic structures, such as barrages,
Before attempting extrapolation, it is necessary to examine the site and collect relevant data on changes in the river cross-section due to
flood plains, roughness and backwater effects. The reliability of the extrapolated value depends on the stability of the gauging section control. A stable control at all stages leads, to reliable results. Extrapolation of the rating curve in an alluvial river subjected to aggradations and degradation is un-reliable and the results should always be confirmed by alternate methods, There are many techniques of extending the rating curve and two well-known methods are described here. Back to TopConveyance Method
The conveyance of a channel in nonuniform
flow is defined by the relation
Q = K Ö St #9; #9; #9; (4.33)
where Q = discharge in the channel, St = slope of the energy line and K = conveyance. If Manning's formula is used,
(4.34)
where n = Manning's roughness, A = area of cross-section and, R hydraulic radius. Since A and R are functions of the stage, the values of K for various values of stage are calculated by using Eq. (4.34) and plotted against the stage. The range of the stage should include values beyond the level up to which extrapolation is, desired. Then a smooth curve is fitted to the plotted points [Fig. 4.27(a)). Using the available discharge and stage data, values of St are calculated by using Eq. (4.33) as St = Q²/K² and are plotted against the stage. A smooth curve is fitted through the plotted points [Fig. 4.27(b)]. This curve is then extrapolated keeping in mind that Sf approaches a constant value at high stages.
Fig.4.27 Conveyance method rating curve extension: (a) K vs stage (b) Sf vs stage
Logarithmic-plot Method
In this technique the
stage-discharge relationship by Eq.(4.26) is made use of. The stage is plotted
against the discharge on a log-log paper. A best-fit linear relationship is
obtained for data points lying in the high-stage range and the line is extended
to cover the range of extrapolation. Alternatively, coefficients of Eq. (4.26)
are obtained by the least-square-error method by regressing X on Y in Eq.
(4.27a). For this Eq. (4.27a) is written as
X = a Y + C (4.35)
where X = log (G-a) and Y = log Q. The coefficients a and C are obtained as,
(4.35a)
and (4.35b)
The relationship governing the stage and discharge is now
(G - a) = Cl Qa (4.36)
where Cl= antilog C.
By the use of Eq. (4.36) the value of the stage corresponding to a design
flood discharge is estimated. Back to TopRUNOFF
Runoff means the draining or
flowing off of
Consider a catchment area receiving precipitation. For a given precipitation, the evapotranspiration, initial loss, infiltration and detention-storage requirements will have to be first satisfied before the commencement of
runoff. When these are satisfied, the excess precipitation moves over the lend surfaces to reach smaller channels. This portion of the runoff is called overland flow and involves building up of a storage over the surface and daining off of the same. Usually the lengths and depths of overland flow are small and the flow is in the larninar regime. Flows from several small channels join bigger channels and flows from these in turn combine to form a larger stream, and so on, till the flow reaches the catchment outlet. The flow in this mode where it travels all the time over the surface as over landflow and through the channels as open-channel flow and reaches the catchment outlet is called surface runoff.A part of the precipitation that infilters moves laterally through upper crusts of the soil and returns to the surface at some location away from the point of entry into the soil. This component of runoff is known variously as
interflow, through flow, storm seepage, subsurface, storm flow or quick return flow (Fig. 5.1). The amount of interflow depends on the biological conditions of the catchment. A fairly pervious soil overlying a hard impermeable surface is conducive to large interflows. Depending upon time delay between the infiltration and the outflow, the interflow is sometimes classified into prompt interflow, i.e. the interflow with the least time lag and delayed interflow.Another route for the infiltered water is to undergo deep
percolation andFig. 5.1 Different routes of runoff
reach the groundwater storage in the soil. The groundwater follows a complicated and long path of travel, and ultimately reaches the surface. The time lag, i.e. the difference in time between the entry into the soil and outflows from it is very large, being of the order of months and years. This part of runoff is called groundwater runoff or groundwater.
Ground water flow provides the dry-weather flow in perennial streams.Based on the time delay between the precipitation and the runoff, the runoff is classified into two categories; as direct runoff and
Base flow Back to TopDirect Runoff
It is that part of runoff which enters the
Runoff, representing the response of a catchment to precipitation reflects the integrated effects of a wide range of catchment,
climate and precipitation characteristics. True runoff is therefore stream flow in the natural condition, i.e. without human intervention. Such a stream flow unaffected by works of man, such as structures for storage and diversion on a stream is called virgin flow. When there exist storage or diversion works on a stream, the flow in the downstream channel is affected by structures and hence does not represent the true runoff unless corrected for storage effects and the diversion of flow and return flow.HYDROGRAPH
A plot of the discharge
in a stream plotted against time chronologically is called a
Annual hydrographs showing the variation of daily or weekly or 10 daily mean flows over a year;
Monthly hydrogaphs showing the variation of daily mean flows over a month;
Seasonal hydrograps depicting the variation of the discharge in a particular season such, as the
í Flood bydrographs or hydrographs due to a storm representing stream flow due to a storm over a catchment.
Each of these types have particular applications. Annual and seasonal hydrographs are of use in
• Calculating
the surface water potential of stream,
• Reservoir studies and
• Drought studies.
Flood hydrographs are essential in analysing stream characteristics associated with floods.
Water Year
In annual runoff studies it is advantageous to
consider a water year beginning from
the time when the precipitation exceeds the average evapotranspiration losses.
In India, June Ist is the beginning of a water year which ends on May
31st of the following calendar year. In a water year a complete cycle
of climatic changes is expected and hence the water budget will have the least
amount of carry over.
RUNOFF
CHARACTERISTICS OF STREAMS
A study of the annual hydrographs of streams
enables one to classify stream into three classes as perennial, intermittent and
ephemeral. A
Fig. 5.2 Perennial stream
Even during dry seasons the
water table will be above the bed of the stream.An
intermittent stream has limited contribution from the groundwater. During the wet season the water table is above the stream bed and there is a contribution of the base flow to the stream flow. However, during dry seasons the water table drops to a level lower than that of the stream bed and the stream dries up. Excepting for an occasional storm which can produce a short-duration flow, the stream remains dry for the most part of the dry months (Fig. 5.3).Fig. 5.3 Intermittent stream
An
ephemeral stream is one, which does not have any base-flow contribution. The annual hydrograph of such a river show series of short-duration spikes marking flash flows in response to storms (Fig. 5.4). The stream becomes dry soon after the end of the storm flow. Typically an ephemeral stream does not have any well-defined channel. Most rivers in and zones are of the ephemeral kind,Fig. 5.4 Ephemeral stream
The flow characteristics of a stream depend upon:
The ranifall characteristics, such as magnitude intensity, distribution time and space and its variability;
Catchment characteristics such as soil, vegetation, slope, geology, shape and
The interrelationship of these factors is extremely complex. However, the risk of oversimplification, the following salient points can be noted:
The seasonal variation of rainfall is clearly reflected in the High stream discharges occur during monsoon months and 1ow which is essentially due to base flow is maintained during the year.
The shape of the storm hydrograph and hence the peak flow is essentially controlled by the storm and physical characteristics the catchment. Evapotranspiration plays a minor role in this.
The annual runoff volume (
YIELD (ANNUAL RUNOFF VOLUME)
The total
quantity of water that can be expected from a stream in a given period such as a
year is called the yield of the river. It is usual for yield to be referred to
the period of a year and then it represents the annual runnoff volume. In this
book the term yield is used to mean annual runoff volume unless otherwise
specified. The calculation of yield is of fundamental importance in all
water-resources development studies. The various methods used for the estimation
of yield can be listed below:
Correlation of stream flow and rainfall,
Empirical equations, and
Rainfall-Runoff Correlation
The relationship between
rainfall and the resulting runoff is quite complex and is influenced by a host
of factors relating the catchment and climate. Further, there is the problem of
paucity of data which forces one to adopt simple correlations for the adequate
estimation of runoff. One of the most common methods is to correlate runoff, R
with rainfall, P values. Plotting of R values against P and drawing a best-fit
line can be adopted for very rough estimates. A better method is to fit a linear
regression line between R and P and to accept the result if the
R = a P + b (5.1)
and the values of the coefficients a and b are given by
(5.2)
and (5.3)
in which N = number of observation sets R and P. The coefficient of correlation r can be calculated as
(5.4)
The value of r lies between 0 to + 1 as R can have only positive correlalion with P. A value of 0.6 < r < 1.0 indicates good correlation. Further it should be noted that R ³ 0.
For large catchments, it is found advantageous to have an exponential relationship as
R= b Pm (5.5)
where b and m are constants, instead of the linear relationship given by Eq. (5. 1). In that case Eq. (5.5) is reduced to a linear form by logarithmic transformation as
1n R = m 1n p + 1n b (5.6)
and the coefficients m and ln b determined by using the method indicated earlier,
Since rainfall records of longer duration than the runoff data are normally available for a catchment, the regression equation [(Eq. (5.1) or (5.5)] can be used to generate synthetic runoff data by using rainfall data. While this may be adequate for preliminary studies, for accurate results sophisticated methods are adopted for synthetic generation the runoff data. Many improvements of the above basic rainfall-runoff correlation by consider additional parameters such as soil moisture or antecedent rainfall have been attempted. Antecedent rainfall influences the initial soil moisture and hence the
infiltration rate at the start of a storm. For calculation of the annual runoff from the annual rainfall a commonly used antecedent precipitation index, Pa is given byPa = a Pi + b Pi-1 + c Pi (5.7)
where Pi, Pi-1 and Pi-2 are the annual precipitation in the ith, (i-1)th (i-2)th year, i=current year, a, b and c are coefficients with their sum equal to unity. The coefficients are found by trial and error to produce best results.
There are many other types of
antecedent precipitation indices in us achieve good correlations of rainfall and runoff. The use of coaxial chart with a defined antecedent precipitation index is given by Linsley et al.Empirical Equations
The importance of estimating the water
availability from, the available hydrologic data for purposes of planning
water-resource projects was recognised by engineers even in the last century.
With a keen sense of observation in their region of their activity many
engineers of the past have developed empirical runoff estimation formulae.
However, these formulae are applicable only to the region in which they were
derived. These formulae are essentially rainfall-runoff relations with
additional third or fourth parameters to account for climatic or catchement
characteristics. Some of the important formulae used in various parts of India
are given below.
Binnie's Percentages
Sir Alexander Bitinic measured the
runoff from a small catchment near Nagpur (Area of 16 km2 ) during
1869 and 1872 and developed curves of cumulative runoff against cumulative
rainfall. The two curves were found to be similar. From these he established
percentages of runoff from rainfall. These percentages have been used in Madhya
Pradesh and Vidarbha region of Maharashtra for the estimation of yield.
Barlow's Tables
Barlow, the first Chief Engineer of the
Hydro-Electric Survey of India (1915) on the basis of his study in small
catchments (area~ 130Km²)in Uttar Pradesh expressed
runoff R as
R = Kb P (5.8)
Where Kb = runoff coefficient which depends upon the type of catchment and nature of monsoon rainfall.
Strange's Tables
Strange (1928) studied the then available
data on rainfall and runoff in the border areas, of present-day Maharashtra and
Karnataka and obtained the values of the runoff coefficient
&#Ks = R/P (5.9)
as a function of the catchment character. For purposes of calculating the yield from the total monsoon rainfall, the catchments were characterised as "good", "average" and "bad'. Values of Ks for these catchments are shown in Table 5.9. Strange also gave a table for calculating the daily runoff from daily rainfall. In this the runoff coefficient depends not only on the amount of rainfall but also on the state of the ground. Three categories of the original ground state as 'dry', 'damp' and 'wet' are used by him.
Iinglis and DeSouza Formula As a result of careful stream gauging in 53 sites in Western India, Inglis and DeSouza (1929) evolved two regional formulae between annual runoff R in cm and annual rainfall p in cm as follows:
• For Ghat
regions of western India R = 0.85 P - 30.5 (5.10)
• For Deccan plateau
(5.11)
Khosla's Formula
Khosla (1960) analysed the rainfall, runoff
and temperature data for various catchments in India and USA to arrive at an
empirical relationship between runoff and rainfall. The time period is taken as
a month. His relationship for Monthly runoff is
Rm = Pm - Lm (5.12)
and Lm = 0.48 Tm for Tm > 4.5°C
where Rm = Monthly runoff in cm and Rm 1
³
0
Pm =
monthly rainfall in cm
Lm = monthly losses in
cm
Tm =
mean monthly temperature of the catchment in °C
For Tm £ 4.5°C, the loss Lm may provisionally be assumed as T°c 4.5 - 1 - 6.5, Lm (cm) 2.17 1.78 1.52 and Annual runoff = S Rm
Khosla's formula is indirectly based on the water-balance concept and the mean monthly catchment temperature is used to reflect the losses due to
evapotranspiration. The formula has been tested on a number of catchments in India and is found to give fairly good results for the annual yield for use in preliminary studies. The formula can also be used to generate synthetic runoff data from historical rainfall and temperature data. Back to TopWatershed Simulation
The hydrologic water-budget equation
for the determination of runoff a given period is written as
&#R = Rs + Go = P - Eet - D S (5.13)
in which Rs = surface runoff, P = precipitation, Eet = actual evapotranspiration, Go = net groundwater outflow and D S = change in the soilmoisture storage. The sum of Rs and Go is considered to be given by the total runoff R, i.e. streamflow.
Starting from an initial set of values, one can use Eq. (5.13) to calculate R by knowing values of P and functional dependence of Eet, D S and infiltration rates with catchment and climatic conditions. For accurate results, the functional dependence of various parameters governing the runoff in the catchment and values of P at short time intervals are needed. Calculations can then be done sequentially to obtain the runoff at any time. However, the calculation effort involved is enormous if attempted manually. With the availability of digital computers the use of water budgeting as above to determine the runoff has become feasible. This technique of predicting the runoff, which is the catchment response to a given rainfall input is called deterministic watershed simulation. In this the mathematical relationships describing the interdependence of various parameters in the system are first prepared and this is called the model. The model is then calibrated i.e. the numerical values of various coefficients determined, by simulating the known rainfall-runoff records. The accuracy of the model is further checked by reproducing the results of another string of rainfall data for which runoff values are known. This phase is known as validation or verification of the model. After this, the model is ready for use.
Crawford and Linsley (1959) pioneered this technique by proposing a watershed simulation model known as the Stanford Watershed Model (SWM). This underwent successive refinements and the Stanford Watershed Model-IV (SWM-IV) suitable for use on a wide variety of conditions was proposed in 1966. The flow chart of SWM-IV is shown in Figure 5.5. The main inputs are hourly precipitation and daily evapotranspiration in addition to physical description of the catchment. The
model considers the soil in three zones with distinct properties to simulate evapotranspiration, infiltration, overland flow, channel flow, interflow and baseflow phases of the runoff phenomenon. For calibration about 5 years of data are needed. In the calibration phase, the initial guess value of parameters are adjusted on a trial-and-error basis until the simulated response matches the recorded values. Using an additional of rainfall-runoff of about 5 years duration, the model is verified for its ability to give proper response. A detailed description of the application of SWM to an Indian catchment is given in Ref. 3.SWM-IV has been tested in a number of applications and it has been found to give satisfactory results for the yield and not so satisfactory results in predicting peak values. However, it requires considerable familiarity with the model to arrive at optimal values in the calibrating stage. An improved version called Hydrocomp Simulation Program (HSP) (1966) gives a package of three simulation modules to solve a variety of water-shed-simulation problems. Another model called the SSARR model (Streamflow Synthesis and Reservoir Regulation Model) developed by Rockwood (1968) for the Columbia river
basin, USA has been successfully tested on large watersheds. The Kentucky Watershed model (KWM) (1 970) is a revised and updated version of SWM-IV. KWM is used with an optimization programme called OPSET which generates best-fit parameter estimates. The successful use of KWM to catchments up to 1200 km² size have been reported. Back to Top DURATION CURVEThe streamflow data is arranged in a descending order of discharges, using class intervals if the number of individual values is very large. The data used can be daily, weekly, ten daily or monthly values. If N number of data points are used in this listing, the
plotting position of any discharge (or class value) Q is(5.14)
where m is the order number of the discharge (or class value), Pp = percentage probability of the flow magnitude being equalled or exceeded. The plot of the discharge Q against Pp is the flow-duration curve (Fig. 5.6). Arithmetic scale paper, or semi-log or log-log paper is used depending upon the range of data and use of the plot. The flow-duration curve represents the cumulative
frequency distribution and can be considered to represent the streamflow variation of an average year. The ordinate Qp at any percentage probability Pp, represents the flow magnitude in an average year that can be expected to be equalled or exceeded Pp percent of time and is termed as Pp% dependable flow. In a perennial river Q100 = 100% dependable flow is a finite value. On the other hand in an intermittent or ephemeral river the streamflow is zero for a finite part of an year and as such Q100 is equal to zero.Fig. 5.6 Flow-duration curve
The following characteristics of the flow-duration curve are of interest:
The slope of a flow-duration curve depends upon the interval of selected. For example, a daily stream flow data gives a steeper curve than a curve based on Monthly data for the same stream. This is due to the smoothening' of small peaks in monthly data.
The presence of E.I.
Figure 5.7 shows the typical reservoir regulation effect.
Fig. 5.7 Reservoir regulation effect
The virgin-flow-
The flow-duration curve plotted on a log-log paper (Fig. 5.8) is useful in comparing the flow characteristics of different streams. A steep slope of the curve indicates a stream with a highly variable discharge. On the other hand, a flat slope indicates a slow response of the catchment to the rainfall and also indicates small variability. At the lower end of the curve, a flat portion indicates considerable
Flow-duration curves find considerable use in water-resources planning and development activities. Some of the important uses are :
In evaluating various dependable flows in the planning or water-resources engineering projects;
In evaluating the characteristics of the hydropower potential of a river;
Fig. 5.8 Flow-duration curve
• In the design of
drainage systems;(5.15)
where `to’ is the time at the beginning of the curve and Q is the discharge rate. Since the
hydrograph is a plot of Q vs t, it is easy to see that the flow-mass-curve is an integral curve (summation curse) of the hydrograph. The flow-mass curve is also known as Rippl’s mass curve after Rippl (1882) who suggested its use first. Figure 5.9 shows a typical flow-mass curve. Note that the abscissa is chronological time in months in this figure. It can also be in days, weeks or months depending on the data being analysed. The ordinate is in units of volume in million m³. Other units employed for ordinate include m³/s. day (cumec day), ha.m and cm over a catchment area.The slope of the mass curve at any point represents dV/dt = Q = rate of flow at that instant. If two points M and N are connected by a straight
Fig. 5.9 Flow mass curve
line, the slope of the line represents the average rate of flow that can be maintained between the times tm and tn if a reservoir of adequate storage is available. Thus the slope of the line AB joining the first and the last points of a mass curve represents the average discharge over the whole period of plotted record.
Back to TopHYDROGRAPHS
Consider a concentrated
storm producing a fairly uniform rainfall of duration, Tr over a
catchment. After the initial losses and infiltration losses are met, the
The hydrograph is the response of a given catchment to a rainfall input.
Fig.6.1 Elements of a flow hydrograph
It consists of flow in all the three phases of runoff, viz. surface runoff, interflow and base flow, and embodies in itself the integrated effects of a wide variety of atchnient and rainfall parameters having complex inter-actions. Thus two different storms in a given catchment Produce hydrhographs differing from each other. Similarly, identical storms in two catecments produce hydrographs that are different. The interactions of various storms and catchments are in general extremely complex. If one examines the record of a large number of flood hydrographs of a stream, it will be found that many of than will have kinks, multiple peaks, etc. resulting in shapes much different from the simple single-peaked hydrograph of Fig. 6.1. These complex hydrographs are the result of
storm and catchment Peculiarities and their complex interactions. While it is theoretically possible to resolve a complex hydrograph into a set of simple hydro-graphs for purposes of hydrograph analysis, the requisite data of accept-able quality are seldom available. Hence, simple hydrographs resulting from isolated storms are preferred for hydrograph studies. Back to TopFACTORS AFFECTING FLOOD HYDROGRAPH
The factors that affect
the shape of the hydrograph can be broadly grouped into climatic factors and
physiographic factors. Each of these two groups contain a host of factors and
the important on Table 6.1. Generally, the climatic factors control the rising
limb and the recession limb is independent of storm characteristics, being
determined by catchment characteristics only. Many of the factors listed in
Table 6.1 are interdependent. Further, their effects are very varied and
complicated. As such only important effects are listed below in qualitative
terms only.
FACTORS AFFECTING FLOOD HYDROGRAPH
Physiographic
Factors
•
Climatic Factors
•
Storm characteristics: precipitation, intensity, duration, magnitude and
movement
of Storm.
• Initial loss
• Evapotranspiration
Shape of the Basin
The shape of the basin influences the
time taken for water from the remote parts of the catchment to arrive at the
outlet. Thus the occurrence of the peak and hence the shape of the hydrograph
are affected by the basin shape. Fan-shaped, i.e. nearly semi-circular shaped
catchments give high peak and narrow hydrographs while elongated catchments give
broad-and low-peaked hydrographs. Figure 6.2 shows schematically the hydrographs
from
Fig. 6.2 Effect of catchment shape on the hydrograph
three catchments having identical infiltration characteristics due to identical rainfall over the catchment. In catchment A the hydrograph is skewed to the left, i.e. the peak occurs relatively quickly. In catchment B, the hydrograph is skewed to the right, the peak occurring with a relatively longer lag. Catchment C indicates the complex hydrography produced by a composite shape.
Back to TopSize
Small basins behave different from the large ones in
terms of the relative importance of various phases of the runoff phenomenon. In
small catchments the overland flow phase is predominant over the channel flow.
Hence the land use and intensity of rainfall have important role on the peak
good. On large basins these effects are suppressed as the channel flow phase is
more predominant. The peak discharge is found to vary as An where A is the
catchment area and n is an exponent whose value is less than unity, being about
0.5. The time base of the hydrographs from larger basins will be larger than
those of corresponding hydrographs from smaller basins. The duration of the
surface runoff from the time of occurrence of the peak is proportional to
Am, where m is an exponent less than unity and is of the order of
magnitude of 0.2.
Slope
The slope of the main stream controls the velocity of
now in the channel. As the recession limb of the hydrograph represents the
depletion of storage, the stream channel slope will have a pronounced effect on
this part of the hydrograph. Large stream slopes give rise to quicker depletion
of storage and hence result in steeper recession limbs of hydrographs. This
would obviously result in a smaller time base.
The basin slope is important in small catchments where the overland low is relatively more important. In such cases the steeper slope of the catchment results in larger peak discharges.
Drainage Density
The drainage density is defined as the
ratio of the total channel length to the total drainage area. A large drainage
density creates situation conducive for quick disposal of runoff down the
channels. This fast response is reflected in a pronounced peaked discharge. In
basins with smaller drainage densities, the overland flow is predominant and the
resulting hydrograph is squat with a slowly
Land Use
Vegetation and forests increase the infiltration
and storage capacities of the Soils. Further, they cause considerable retardance
to the overland flow. Thus the vegetal cover reduces the peak flow. This effect
is usually very Pronounced in small catchments of area less than 150 km².
Further, the effect of the vegetal cover is prominent in small storms. In
general, for two catchments of equal area, other factors being identical, the
peak discharge is higher for a catchment that has a lower density of forest
cover. Of the various factors that control the peak discharge, probably the only
factor that can be manipulated is land use and thus it represents the only
practical means of exercising long-term
Fig. 6.3 Role of drainage density on the hydrograph
Climatic Factors
Among climatic factors the intensity,
duration and direction of storm movement are the three important ones affecting
the shape of a flood hydrograph. For a given duration, the peak and volume of
the surface runoff are essentially proportional to the intensity of rainfall.
This aspect is made use of in the
The duration of a storm of given intensity also has a direct proportional effect on the volume of runoff. The effect of duration is reflected in the rising limb and peak flow. Ideally, if a rainfall of given intensity i lasts sufficiently long enough, a state of equilibrium discharge proportional to iA is reached.
If the storm moves from
upstream of the catchment to the downstream end, there will be a quicker concentration of flow at the basin outlet. This results in a peaked hydrograph. Conversely, if the storm movement is up the catchment, the resulting hydrograph will have a lower peak and longer time base. This effect is further accentuated by the shape of the catchment, with long and narrow catchments having hydrographs most sensitive to the storm-movement direction.COMPONENTS OF A
HYDROGRAPH
As indicated earlier, the essential Components of a hydrograph
are:
Rising limb,
Recession limb.
Rising Limb
The rising limb of a hydrograph, also known as
Crest Segment
The crest segment is one of the most important
parts of a hydrograph as it contains the peak flow. The peak flow occurs when
the runoff from various parts of the catchment simultaneously contribute the
maximum amount of flow at the basin outlet. Generally for large catchments, the
peak flow occurs after the cessation of
Recession Limb
The recession limb which extends from the
point of inflection at the end of the crest segment to the commencement of the
natural groundwater flow represents the withdrawal of water from the storage
built up in the basin during the earlier phases of the hydrograph. The starting
point of the recession limb, i.e. the point of inflection represents the
condition of maximum storage. Since the depletion of storage takes place after
the cessation of rainfall, the shape of this part of the hydrograph is
independent of storm characteristics and depends entirely on the basin
characteristics.
The storage of water in the basin exists as surface storage, which includes both surface detention and
channel storage, interflow storage, and groundwater storage, i.e. base-flow storage. Barnes (1940) showed that the recession of a storage can be expressed as(6.1)
in which Qo and Qt are discharges at a time interval of t days with Qo being the initial discharge; Kr is a recession constant of value less than unity. Equation (6.1) can also be expressed in an alternative form of the exponential decay as
(6.2)
Where, a = - 1n Kr
The recession constant Kr can be considered to be made up of three components to take care of the three types of storage as
Kr = Krs . Kri . Krb (6.3)
where Krs = recession constant for surface storage, Kri = recession constant for interflow and Krb = recession constant for base flow. Typically the values of these recession constants, when t is in days, are
Krs = 0.05 to 0.20
Kri = 0.50 to 0.85
Krb = 0.85 to 0.99
If the interflow is not significant Kri can be assumed to be unity. When Eq. (6.1) or (6.2) is plotted on a semilog paper with the discharge on the log-scale, it plots as a straight line and from this the value of Kr can be found.
Back to TopEFFECTIVE RAINFALL
For purposes of correlating DRH with the
rainfall which produced the flow, the hydrograph of the rainfall is also pruned
by deducting the losses. Figure 6.6 shows the
Fig.6.6 Effective rainfall hydrograph.
UNIT HYDROGRAPH
The problem of predicting the flood
hydrograph resulting from a known storm in a catchment has received considerable
attention. A large number of methods are proposed to solve this problem and of
them probably the most popular and widely used method is the unit-hydrograph
method. This method was first suggested by Sherman in 1932 and has undergone
many, refinements since then.
A unit hydrograph is defined as the hydrograph of direct runoff resulting from one unit depth (1 cm) of rainfall excess occurring uniformly over the basin and at a uniform rate for a specified duration (Dh). The term unit here refers to a unit depth of rainfall excess which is usually taken as 1 cm. The duration, being a very important characteristic, is used as a prefix to a specific unit hydrograph. Thus one has a 6-h unit hydrograph, 12-h unit hydrograph, etc. and in general a D-h unit-hydrograph applicable to a given catchment. The definition of a unit hydrograph implies the following:
The unit hydrograph represents the lumped response of the catchment to a unit rainfall excess of D-h duration to produce a direct-runoff hydrograph. It relates only the direct runoff to the rainfall excess. Hence the volume of water contained in the unit hydrograph must be equal to the rainfall excess. As 1 cm depth of rainfall excess is considered the area of the
The rainfall is considered to have an average intensity of excess rainfall (ER) of 1/D cm/h for the duration of the storm.
The distribution of the storm is considered to be uniform all over the catchment.
Fig.6.8 shows a typical 6-h unit hydrograph. Here the duration of the rainfall excess is 6 h.
Area under the unit hydrograph = 12.92 X 106m³
Hence
Catchment area of the basin = 12.92 km²Two basic assumptions constitute the foundations for the unit hydrograph theory. These are the time invariance and the linear response.
Back to TopTime Invariance
This first basic assumption is that the
direct-runoff response to a given
Fig. 6.8 Typical 6-h unit hydrograph
Linear Response
The direct-runoff response to the rainfall
excess is assumed to be linear. This is the most important assumption of the
unit-hydrograph theory. Linear response means that if an input x1(t)
causes an output y1(t) and an input x2(t) causes an output
y2(t), then an input x1(t)+x2(t) gives an
output Yl(t)+Y2(t). Consequently, if X2(t) =
rxl(t), then y2(t) = ry1(t). Thus if the
rainfall excess in a duration D is r times the unit depth, the resulting DRH
will have ordinates bearing ratio r to those of the corresponding D-h unit
hydrograph. Since the area of the resulting DRH should increase by the ratio r,
the base of the DRH will be the same as that of unit hydrograph.
The assumption of linear response in a unit hydrograph enables method of superposition to be used to derive DRHs. Accordingly, rainfall excesses of D-h duration each occur consecutively, their combined effect is obtained by superposing the respective DRHs with due cart taken to account for the Proper sequence of events. These aspects resulting from the assumption of linear response are made clearer following two illustrative examples.
DERIVATION OF UNIT
HYDROGRAPHS
A number of isolated storm hydrographs caused by short spells of
rainfall excess, each of approximately same duration [0.90 to 1.1 Dh] are
selected from a study of the continuously gauged
Flood hydrographs used in the analysis should be selected to meet the following desirable features with respect to the storms responsible for them:
Back to Topí The storms should be isolated storms occurring individually.
í The rainfall should be fairly uniform during the duration and should cover the entire catchment area.
í The duration of the rainfall should be 1/5 to 1/3 of the
basin lag.í The rainfall excess of the selected storm should be high. A range ER values of 1.0 to 4.0 cm is sometimes preferred.
A number of unit hydrographs of a given duration are derived by the above method and then plotted on a common pair of axes as shown in Fig.6.1 1. Because of rainfall variations both in space and time due to some departures from the assumptions of the unit-hydrograph theory, the various unit hydrographs thus developed will not be identical is common practice to adopt a mean of such curves as the unit graph of a given duration for the catchment. While deriving the mean curve, the average of peak flows and time to peaks are first calculated. Then a mean curve of best fit, judged by eye, is drawn through the averaged peak to close on an averaged base length. The volume of DRH is calculated and any departure from unity is corrected by adjusting the value of the peak. The averaged ERH of unit depth is customarily drawn in the plot of the unit hydrograph to indicate the type and duration of rainfall causing the unit hydrograph.
Fig. 6.11 Derivation of an average unit hydrograph
By definition the rainfall excess is assumed to occur uniformly over the catchment during duration D of a unit hydrograph. An ideal duration for a unit hydrograph is one wherein small fluctuations in the intensity of rainfall within this duration do not have any significant effects on the run-off. The catchment has a damping effect on the fluctuations of the
rainfall intensity in the runoff-producing process and this damping is a function of the catchment area. This indicates that larger durations are admissible for larger catchments. By experience it is found that the duration of the unit hydrograph should not exceed 1/5 to 1/3 basin lag. For catchments of sizes larger than 250 km² the duration of 6h is generally satisfactory. Back to TopUnit Hydrograph from a Complex Storm
When suitable simple
isolated storms are not available, data from complex storms of long duration
will have to be used in unit-hydrograph derivation. The problem is to decompose
a measured composite flood hydrograph into its component DRHs and base flow. A
common unit hydrograph of appropriate duration is assumed to exist. This problem
is thus the inverse of the derivation of flood hydrograph through use of Eq.
(6.5). Consider a rainfall excess made up of three consecutive durations of D-h
and ER values of R1, R2 and R3. Figure 6.13
shows the ERH. By base flow separation of the resulting composite flood
hydrograph a composite DRH is obtained (Fig.6.13). Let the ordinates of the
composite DRH be drawn at a time interval of D h. At various time intervals 1D,
2D, 3D, ...from the start of the ERH, let the ordinates of the unit hydrograph
be u1, u2, u3, ... and the ordinates of the
composite DRH be Ql, Q2, Q3 ...
Fig. 6.13 Unit hydrograph from a complex storm
Then
Q1 = R1u1
Q2 =
R1u2 + R2 ul
Q3 =
R1u 3 + R2 u2 + R3
ul
(6.6)
Q4 = R1u4 + R2
u3 + R3 u2
Q5 = R1u5 + R2 u4 + R3 u3
.................……………….
so on.
From the Eq.(6.6) the values of ul, u2, u3, ... can be determined. However, this method suffers from the disadvantage that the errors propagate and increase as the calculations proceed. In the presence of errors the recession limb of the derived D-h unit hydrograph can contain oscillations and even negative values. Matrix methods with optimisation schemes are available for solving Eq. (6.6) in a digital computer.
UNIT HYDROGRAPHS OF DIFFERENT DURATIONS
Ideally, unit hydrographs are derived from simple isolated storms and if the duration of the various storms do not differ very much, say within a band width of ± 20% D, they would all be grouped under one average duration of D h. If in practical applications unit hydrographs of different duration are needed they are best derived from field data. Lack of adequate data normally precludes development of unit hydrographs covering a wide range of durations for a given catchment. Under such conditions a D-h unit hydrograph is used to develop unit hydrographs of differing durations, nD. Two methods are available for this purpose i.e. Method of superposition, and S-curve.
Back to TopMethod of Superposition
If a D-h unit hydrograph is
available, and it is desired to develop a unit hydrograph of nD h, where n is an
integer, it is easily accomplished by superposing n unit hydrographs with each
graph separated from the previous one by D h. Figure 6.14 shows three 4-h
Fig.6.14 Construction of a 12-h unit hydrograph from a 4-h unit hydrograph
Unit hydrographs A, B and C, Curve B begins 4-h after B. Thus the combination of these three curves is a DRH of 3cm due to an ER of 12h duration. If the ordinates of this DRH art now divided by 3, one obtains a 12-h unit hydrograph. The calculations easy if performed in a tabular form (Table 6.6).
The S-Curve
If it is desired to develop a unit hydrograph of
duration mD, where m is a fraction, the method of superposition cannot be used.
A different technique known as the S-curve method is adopted in such cases, and
this method is applicable for rational values of m.
The S-curve, also known as S-hydrograph is a hydrograph produced by a continuous effective rainfall at a constant rate for an infinite period. It is a curve obtained by summation of an infinite series of D-h unit hydrographs spaced D-h apart. Figure 6.15 shows such a series of D-h hydrograph arranged with their starting points D-h apart. At any given time the ordinates of the various curves occurring at that time coordinate are summed up to obtain ordinates of the S-curve. A smooth curve through these ordinates result in an S-shaped curve called S-curve.
Fig. 6.15 S-Curve
This S-Curve is due to a D-h unit hydrograph. It has an initial steep portion and reaches a maximum equilibrium discharge at a time equal to the tine base of the first unit hydrograph. The average intensity of ER producing the S-curve is 1/D cm/h and the equilibrium discharge,
where A = area of the catchment in km² and D = duration in hours of ER of the unit hydrograph used in deriving the S-curve. Alternatively
(6.7)
where A is in km² and D is in h. The quantity Qs represents the maximum rate at which an ER intensity of 1/D cm/h can drain out of a catchment of area A. In actual construction of an S-curve, it is found that curve oscillates in the top portion at around the equilibrium value due t magnification and accumulation of small errors in the hydrograph. When it occurs, an average smooth curve is drawn such that it reaches a value Qs at the time base of the unit hydrograph.
Back to TopConsider two D-h S-curves A and B displaced by Th (Fig.6.16) If the ordinates of B are subtracted from that of A, the resulting curve is a DRH produced by a rainfall excess of duration T h and magnitude (1/D x T) cm. Hence if the ordinate differences of A and B, i.e. (SA – SB) are divided by T/D, the resulting ordinates denote a hydrograph due to an ER of 1 cm and of duration T h, i.e. a T-h unit hydrograph. The derivation of a T-h unit hydrograph as above can be achieved either by graphical mean arithmetic computations in a tabular form.
Fig. 6.16 Derivation of a T-h unit hydrograph by S-curve lagging method
USE AND
LIMITATIONS OF UNIT HYDROGRAPH
As the unit hydrographs establish a
relationship between the ERH and DRH for a catchment, they are of immense value
in the study of the hydrology of a catchment. They are of great use in
Unit hydrographs assume uniform distribution of rainfall over the catchment. Also, the intensity is assumed constant for the duration of the unfair excess. In practice, these two conditions are never strictly satisfied. Nonuniform areal distribution and variation in intensity within a storm are very common. Under such conditions unit hydrographs can still be used if the areal distribution is consistent between different storms. However, the size of the catchment imposes an upper limit on the applicability of the unit hydrograph. This is because in very large
basins the centre of the storm can vary from storm to storm and each can give different DRHs under otherwise identical situations. It is generally felt that about 500 km² is the upper limit for unit-hydrograph use. Flood hydrographs in very large basins can be studied by dividing then into a number of smaller subbasins and developing DRHs by the unit-hydrograph method. These DRHs can then be routed through their respective channels to obtain the composite DRH at the basin outlet.There is a lower limit also for the application of unit hydrographs. This limit is usually taken as about 200 ha. At this level of area, a number of factors affect the rainfall-runoff relationship and the unit hydrograph not accurate enough for the prediction of DRH.
Back to TopOther limitations to the use of unit hydrographs are:
Precipitation must be from rainfall only. Snow-melt runoff cannot be satisfactorily represented by
The catchment should not have unusually large storages in terms of tanks, ponds, large flood-
In the use of unit hydrographs very accurate reproduction of results should not be expected. Variations in the hydrograph base of as much as ± 200% and in the
peak discharge by ± 10% are normally considered acceptable.DURATION OF THE
UNIT HYDROGRAPH
The choice of the duration of the unit hydrograph depends on
the records. If
(i)Time (ii)
Basin lag and (iii) Time of concentration.A value of D equal to about 1/4 of the basin lag is about the best choice. Generally, for with areas more than 1200 km² a duration D = 12 hrs is preferred.
DISTRIBUTION GRAPH
The
distribution graph introduced by Bernard (1935) is a variation of unit
hydrograph. It is basically a D-h unit hydrograph with ordinates showing the
percentage of the surface runoff occurring in successive periods of equal time
intervals of Dh. The duration of the rainfall excess (Dh) is taken as the unit
interval and distribution-graph ordinates indicated at successive such unit
intervals. Figure 6.17 shows a typical distribution graph. Note the ordinates
plotted at 4-h intervals and the area under the distribution graph adds up to
100%. The use of the distribution graph to generate a DRH for a known ERH is
exactly the same that of a unit hydrograph (Example 6.11). Distribution graphs
are useful in comparing the runoff characteristics of different catchments.
SYNTHETIC UNIT
HYDROGRAPH
To develop unit hydrographs to a catchment, detailed information
about the rainfall and the resulting flood
INSTANTANEOUS UNIT
HYDROGRAPH (IUH)
The unit-hydrograph concept discussed in the preceding
sections considered a D-h unit hydrograph. For a given catchment a number of
unit hydrographs of different durations are possible. The shape of these
different unit hydrographs depend upon the value of D. Figure 6.20 shows a
typical variation of the shape of unit hydrographs for different values of D. As
D is reduced, the intensity of rainfall excess being equal to 1/D increases and
the unit hydrograph becomes more skewed. A finite unit hydrograph is indicated
as the duration DÕ 0. The limiting case of a
unit hydrograph of zero duration is known as
Fig. 6.20 Unit hydrographs of different duration
Represents the surface runoff from the catchment due to an instantaneous precipitation of the rainfall excess volume of 1 cm. IUH is designed u (t) or sometimes as u (0, t). It is a single-peaked hydrograph with a finite base width and its important properties can be listed as below:
1. 0 £ u (t) £ a positive value, for t > 0;
2. u (t) = 0 for t £ 0;
3. u (t) ® 0 as t ® a ;
4.
= unit depth over the catchment;
and
_. time to the peak < time to the centroid of the curve.
Consider an
effective rainfall I(t ) of duration to applied to a catchment as in Fig.6.21. Each infinitesimal element of this ERH will operate on the IUH to produce a DRH whose discharge at time t is given by(6.22)
Fig. 6.21 Convolution of I (t ) and IUH
where t' = t when t < to
Back to Topand t' = to when t ³ to
Equation (6.22) is called the convolution integral or Duhamel integral. The integral of Eq.(6.22) is essentially the same as the arithmetical computation of Eq. (6.5).
The main advantage of IUH is that it is independent of the duration of ERH and thus has one parameter less than a D-h unit hydrograph. This fact and the definition of IUH make it eminently suitable for theoretical analysis of rainfall excess-runoff relationship of a catchment. For a given catchment IUH, being independent of rainfall characteristics, is indicative of the catchment storage characteristics.
FLOODS
A flood is an unusually high stage
in a river-normally the level at which the river overflows its banks and
inundates the adjoining area. The damages caused by floods in terms of loss of
life, property and economic loss due to disruption of economic activity are all
too well-known. Crores of rupees are spent every year in
• Rational method,
• Empirical method,
•
Unit-hydrograph technique
• Flood-frequency studies.
The use of a particular method depends upon (i) the desired objective, (ii) the available data and (iii) the importance of the project. Further the rational formula is only applicable to small-size (< 50km²) catchments and the unit-hydrograph method is normally restricted to moderate-size catchments with areas less than 5000 km².
Back to TopRATIONAL METHOD
Consider a
rainfall of uniform intensity and very long duration occurring over a basin. The
runoff rate gradually increases from zero to a constant value as indicated in
Fig.7.1. The runoff increases as more and more flow from remote areas of the
catchment reach the outlet. Designating the time taken for a drop of water from
the farthest part of the catchment to
Fig. 7.1 Runoff hydrograph due to uniform rainfall
reach the outlet as the time of concentration, it is obvious that if the rainfall continues beyond tc, the runoit will be constant and at the peak value. The peak value of the runoff is given by Qp = C A i; for t ³ tc (7.1)
where C = coefficient of runoff = (runoff/rainfall), A = area of the catchment and i = intensity of rainfall. This is the basic equation of the rational method. Using the commonly used units, Eq. (7.1) is written for field application as
(7.2)
Where, Qp = peak discharge
(m³/s)
C = coefficient of
runoff
itcp = the mean intensity of precipitation (mm/h) for a duration
equal to tc
and
an exceedence probability
P
A = drainage area in km²
The use of this method to compute Qp requires three parameters: tc, (ict p) and C.
EMPIRICAL FORMULAE
The
empirical formulae u3ed for the estimation of the flood peak are essentially
regional formulae based on statistical correlation of the observed peak and
important catchment properties. To simplify the form of the equation, only a few
of the many parameters affecting the flood peak are used. For example, almost
all formulae use the catchment area as a parameter affecting the flood peak and
most of them neglect the
Flood-Peak-Area Relationships
By far the simplest of the
empirical relationships are those which relate the flood Peak to the drainage
area. The maximum good discharge Qp from a catchment area A is given
by these formulae as Qp = f(A)
UNIT-HYDROGRAPH
The
unit-hydrograph technique described in the previous chapter used to predict the
peak-flood hydrograph if the rainfall producing flood, infiltration
characteristics of the catchment and the appropriate hydrograph are available.
For design purposes, extreme rainfall situations are used to obtain the design
storm, viz., the hyetograph of the rainfall excess causing extreme floods. The
known or derived unit hydrograph of the catchment is then operated upon by the
design storm to generate the desired flood hydrograph.
FLOOD-FREQUENCY
STUDIES
Hydrologic processes such as floods are exceedingly complex natural
events. They are resultants of a number of component parameters and are
therefore very difficult to
(7.11)
where m = order number of the event and N = total number of events in the data. The
recurrence interval, T (also called the return period or frequency) is calculated as(7.12)
The relationship between T and the probability of occurrence of various events is the same as described in Sec. 2.10. Thus, for example, the probability of occurrence of the event r times in n successive years is given by
where q = 1-p
Consider, for example, a list of flood magnitudes of a river arranged in blending order is shown in Table 7.2. The length of the record is 50 years.
The last column shows the
return Period T of various flood magnitude, Q. A plot of Q Vs T yields the probability distribution, for small return periods (i.e. for interpolation) or where limited extrapolation is required, a, simple best-fitting curve through plotted points can be used as the probability distribution. A logarithmic scale for T is often advantageous. However, when larger extrapolations of Tare involved, theoretical probability distributions have to be used. In frequency analysis of floods the usual problem is to predict extreme flood events. Towards this, specific extreme-value distributions are assumed and the required statistical parameters calculated from the available data. Using these the flood magnitude for a specific return period is estimated. Chow (1951) has shown that most frequency-distribution functions applicable in hydrologic studies can be expressed by the following equation known as the general equation of hydrologic frequency analysis:XT, = ` x + K s #9; #9; #9; (7.13)
Where, XT = value of the variegate X of a random hydrologic series with a return period T, ` x = mean of the variegate, s =
standard deviation of the variegate, K = frequency factor which depends upon the return period, T and the assumed frequency distribution. Some of the commonly used frequency distribution functions for the predication of extreme flood values are Gumbel's extreme-value distribution, Log-Pearson Type III distribution, and Log normal distribution. Back to TopPARTIAL-DURATION
SERIES
In the annual hydrologic data series of floods, only one maximum value
flood per year is selected as the data point. It is likely that in s catchments
there are more than one independent floods in a year many of these may be of
appreciably high magnitude. To enable all the large flood peaks to be considered
for analysis, a flood magnitude than an arbitrary selected base value are
included in the analysis. Such a data series is called partial-duration
series.
In using the partial-duration series, it is necessary to establish that all events considered are independent. Hence the partial-duration adopted mostly for rainfall analysis where the conditions of independency of events are easy to establish. Its use in flood studies is rather rare. The recurrence interval of an event obtained by annual series (TA) an
partial duration series (Tp) are related by(7.28)
From this it can be seen that the difference between TA and Tp is significant for TA < 10 years and that for TA > 20, the difference is negligibly small.
REGIONAL FLOOD
FREQUENCY ANALYSIS
When the available data at a catchment is too short to
conduct
LIMITATIONS OF
FREQUENCY STUDIES
The flood-frequency analysis described in the previous
sections is a direct means of estimating the desired flood based upon the
available flood-flow data of the catchment. The results of the frequency
analysis depend upon the length of data. The minimum number of years of record
required to obtain satisfactory estimates depends upon the variability of data
and hence on the physical and climatological characteristics of the basin.
Generally a minimun of 30 years of data is considered as essential. Smaller
lengths of records are also used when it is unavoidable. However, frequency
analysis should not be adopted if the length of records is less than 10
years.
Flood-frequency studies are most reliable in
climates that are uniform from year to year. In such cases a relatively short record gives a reliable picture of the frequency distribution. With increasing lengths of flood records, it affords a viable alternative method of flood-flow estimation in most cases.DESIGN FLOOD
In the design of
hydraulic structures it is not practical from economic considerations to provide
for the safety of the structure and the system It the maximum-possible flood in
the catchment. Small structures such as culverts and storm drainages can be
designed for less severe floods as the consequences of a higher-than-design
flood may not be very serious. It can cause temporary inconvenience like the
disruption of traffic and very rarely severe property damage and loss of life.
On the other hand, storage structures such as dams demand greater attention to
the magnitude of floods used in the design. The failure of these structures
causes large loss of life and great property damage on the down-stream of the
structure.
From this it is apparent that the type, importance of the structure and economic development of the surrounding area dictate the design criteria for choosing the flood magnitude. This section highlights the procedures adopted in selecting the flood magnitude for the design of some hydraulic structures.
Design Flood
Flood adopted for the design of a
structure.
Spillway Design Flood
Design flood used for the specific
purpose of designing. the spillway of a storage structure. This term is
frequently used to denote the maximum discharge that can be passed in a
hydraulic structure without any damage or serious threat to the stability of the
structure.
Standard Project Flood (SPF)
The flood that would result
from a severe combination of meteorological and hydrological factors that are
reasonably applicable to the region. Extremely rare combinations of factors are
excluded.
Probable Maximum Flood (PMF)
The extreme flood that is
Physically possible in a region as a result of severest combinations, including
rare combinations of meteorological and hydrological factors.
The PMF is used in situations where the failure of the structure would result in loss of life and catastrophic damage and as such complete security from potential floods is sought. On the other hand, SPF is often used where the failure of a structure would cause less severe damages. Typically, the SPF is about 40 to 60% of the PMF for the same
drainage basin. The criteria used for selecting the design flood for various hydraulic structures vary from one country to another.DESIGN STORM
To estimate the design
flood for a project by the use of a unit hydrograph, one needs the design storm.
This can be the storm-producing
The following is a brief outline of a procedure followed in India:
The duration of the critical rainfall is first selected. This will be the
basin lag if the flood peak is of interest. If the flood volume is of prime interest, the duration of the longest storm experienced in the basin is selected. Past major storms in the region which conceivably could have occurred in the basin under study are selected. DAD analysis is performed and the enveloping curve representing maximum depth-duration relation for the study basin obtained.Rainfall depths for convenient time intervals (e.g.6h) are scaled from the enveloping curve. These increments are to be arranged to get a critical sequence which produces the maximum flood peak when applied to the relevant unit hydrograph of the basin.
In critical sequence of rainfall increments can be obtained by trial and error. Alternatively, increments of precipitation are first arranged in a of relevant unit hydrograph ordinates such that
The design storm is then combined with hydrologic abstractions most conducive to high runoff, viz. low initial loss and lowest
infiltration rate to get the hyetograph of rainfall excess to operate upon the unit hydrograph. Back to TopFLOOD ROUTING
The flood hydrograph
discussed in Chap.6 is infact a wave. The stage and discharge hydrographs
represent the passage of waves of river depth and discharge respectively. As
this wave moves down the river, the shape of the wave gets modified due to
various factors, such as
In reservoir routing the effect of a flood wave entering a reservoir is studied. Knowing the volume-elevation characteristic of the reservoir and the outflow-elevation relationship for the spillways and other outlet structures in the reservoir, the effect of a flood wave entering the reservoir is studied to predict the variations of reservoir elevation and outflow discharge with time. This form of reservoir routing is essential in the design of the capacity of spillways and other reservoir outlet structures and in the location and sizing of the capacity of reservoirs to meet specific requirements.
In channel routing the changes in the shape of a hydrograph as it travels down a channel is studied. By considering a channel reach and an input hydrograph at the upstream end, this form of routing aims to predict the flood hydrograph at various sections of the reach. Information on the flood-peak attenuation and the duration of high-water levels obtained by channel routing is of utmost importance in flood-forecasting operations and flood-protection works.
A variety of routing methods are available and they can be broadly classified into two categories as: (i) hydrologic routing and (ii) hydraulic routing. Hydrologic-routing methods employ essentially the equation of continuity. Hydraulic methods, on the other hand, employ the continuity equation together with the equation of motion of
unsteady flow. The basic differential equations used in the hydraulic routing, known as St. Venant equations altord a better description of unsteady flow than hydrologic methods.HYDROLOGIC CHANNEL
ROUTING
In reservoir routing presented in the Previous sections, the storage
was a unique function of the Outflow discharge S=f(Q). However, in
Prism Storage
It is the volume that would exist if uniform
flow occurred at the
Wedge Storage
It is the wedge-like volume formed between the
actual water surface profile and the top surface of the
Fig. 8.7 Storage in a channel reach
The prism storage Sp is similar to a reservoir and can be expressed as a function of the outflow discharge, Sp =f(Q). The wedge storage Sw can be accounted for by expressing it as Sw = f(I). The total storage in the channel reach can then be expressed as
(8.8)
where K and x are coefficients and m = a constant exponent. It has been found that the value of m varies from 0.6 for rectangular channels to a value of about 1.0 for natural channels.
Muskingum Equation
Using m = 1.0, Eq. (8.8) reduces to a
linear relationship for S in terms of I and Q as
S= K [x I + (1-x) Q] (8.9)
and this relationship is known as the Muskingum equation. In this the parameter x is known as weighting factor. When x = 0, obviously the storage is a function discharge only and the Eq. (8.9) reduces to
S = K Q (8.10)
Such a storage is known as linear storage or
linear reservoir. When x = 0.5 both the inflow and outflow are equally important in determining the storage. The coefficient K is known as storage-time constant and has the dimensions of time. It is approximately equal to the time of travel of a flood wave through the channel reach.HYDRAULIC METHOD
OF FLOOD ROUTING
The hydraulic method of
Approximate Methods
These are based on the equation of
continuity only or on a drastically curtailed equation of motion. The
hydrological method of storage routing and Muskingum channel routing discussed
earlier belong to this category. Other methods in this category are diffusion
analogy and kinematic Wave models.
Complete Numerical Methods
These are the essence of the
hydraulic method of routing. In the direct method, the partial derivatives are
replaced by finite differences and the resulting algebraic equations are then
solved. In the method of characteristics (MOC) St Venant equations are converted
into a pair of ordinary differential equations (i.e. characteristic forms) and
then solved by finite difference techniques. In the finite element method (FEM)
the system is divided into a number of elements and partial differential
equations are integrated at the nodal points of the elements.
The numerical schemes are further classified into explicit and implicit methods. In the explicit method the algebraic equations are linear and the dependent variables are extracted explicitly at the end of each time step. In the implicit method the dependent variables occur implicitly and the equations are nonlinear. Each of these two methods has a host of finite-differencing schemes to choose from.
Back to TopFLOOD CONTROL
The term "flood
control" is commonly used to denote all the measures adopted to reduce damages
to life and property by floods. As there is always a possibility, however remote
it may be, of an extremely large flood occurring in a river the complete control
of the flood to a level of zero loss is neither physically possible nor
economically feasible. The flood control measures that are in use can be
classified as:
1. Structural methods:
• Storage and
2. Non-structural methods:
Flood plain zoning, #9; Flood warning and evacuation.Storage Reservoirs
Storage reservoirs offer one of the most
reliable and effective methods of control. Ideally, in this method, a part of
the storage in the reservoir kept apart to absorb the incoming flood. Further,
the stored water is deceased in a controlled way over an extended time so that
downstream channels do not get flooded. Figure 8.13 shows an ideal operating
plane of flood control reservoir. As most of the present-day storage reservoirs
aye multipurpose commitments, the manipulation of reservoir levels to satisfy
many conflicting demands is a very difficult and complicated task. It so happens
that many storage reservoirs while reducing the floods and food damages do not
always aim at achieving optimum benefits in the flood-control aspect. To achieve
complete
Fig. 8.13 Flood control operation of a reservoir
The Hirakud and Damodar Valley Corporation (DVC) reservoirs are examples of major reservoirs in the country which have specific volumes earmarked for
flood absorption.Detention Reservoirs
A
Levees
Levees, also known, as dikes or flood embankments are
earthen banks constructed parallel to the course of the river to confine it to a
fixed course and limited cross-sectional width. The heights of levees will be
higher than the design flood level with sufficient free board. The confinement
of the river to a fixed path frees large tracts land from inundation and
consequent damage (Fig. 8.14).
Levees are one of the oldest and most common methods of flood protection works adopted in the world. Also, they are probably the cheapest of structural flood-control measures. While the protection offered by a levee against
flood damage is obvious, what is not often appreciated is the potential damage in the event of a levee failure. The levees, being earth embankments require considerable care and maintenance. In the event of being overlapped, they fail and the damage caused can be enormous. In fact, the sense of protection offered by a levee encourages economic activity along the embankment and if the levee is overlapped the loss would be more than what would have been if there were no levees. Confinement of flood banks of a river by levees to a narrower space leads to higher flood levels for a given discharge. Further, if the bed level of the river also rises, as they do in agrading rivers, the top of the levees has to be raised at frequent time intervals to keep up its safety margin.Fig. 8.14 A typical levee: (a) Plan (schematic)
The design of a levee is a major task in which costs and economic benefits have to be considered. The cross-section of a levee will have to be designed like an earth dam for complete safety against all kinds of saturation and
drawdown possibilities. In many instances, especially in locations I where important structures and industries are to be protected, the water aside face of levees are protected by stone or concrete revetment. Regular maintenance and contingency arrangements to fight floods are absolutely necessary to keep the levees functional.Masonry structures used to confine the river in a manner similar to levees are known as flood nails. These are used to protect important Structures against floods, especially where the land is at a premium.
Back to TopFloodways
Floodways are natural channels into which a part
of the flood will be diverted during high stages. A
Widening or deepening of the channel to increase the cross-sectional area;
Reduction of the channel roughness, by clearing of vegetation from the channel perimeter;
Short-circuiting of meander loops by cutoff channels, leading to increased slopes.
All these three methods are essentially short-term measures and require continued maintenance.
Soil Conservation
Soil-conservation measures in the
catchment when properly planned and effected lead to an all-round improvement in
the catchment characteristics affecting abstractions. Increased infiltration,
greater evapotranspiration and reduced soil
FLOOD FORECASTING
Forecasting
of floods in advance enables a warning to be given to the people likely to be
affected and further enables civil-defense measures to be organized. It thus
forms a very important and relatively inexpensive nonstructural flood-control
measure. However, it must be realised that a flood warning is meaningful if it
is given sufficiently in advance. Also, erroneous warnings will cause the
populace to loose faith in the system. Thus the dual requirements of reliability
and advance notice are the essential ingredients of a flood-forecasting system.
The
Short-range forecasts,
Medium-range forecasts
Long-range forecasts.
Short-Range Forecasts
In this the river stages at successive
stations on a river are correlated with hydrological parameters, such as
precipitation area,
Medium-Range Forecasts
In this rainfall-runoff relationships
are used to predict flood levels with a warning of 2-5 days. Coaxial graphical
correlations of runoff with rainfall and other parameters like the time of the
year, storm duration and antecedent wetness have been developed to a high stage
of refinement by the US Weather Bureau.
Long-Range Forecasts
Using radars and meteorological
satellite data, advance information about critical storm-producing weather
systems, their rain potential and time of occurrence of the event are predicted
well in advance.
FLOOD CONTROL IN INDIA
In
India the Himalayan rivers account for nearly 60% of the flood damage in the
country. Floods in these rivers occur during
Flood forecasting is handled by CWC in close collaboration with the IMD, which lends meteorological data support. Nine flood Met offices both the aid of
recording raingauges provide daily synoptic situations, actual rainfall amounts and rainfall forecasts to CWC. The CWC has 141 flood-forecasting stations situated on various basins to provide a forecasting service to a population of nearly 40 million.A national programme for flood control was launched in 1954 and an amount of about 976 crores has been spent since then till the beginning of the Sixth Five-Year Plan. The Planning Commission has provided an outlay of 1045 crores in the sixth Five-Year Plan for flood control. These figures highlight the seriousness of the flood problem and the efforts made towards mitigating flood damages. The experience gained in the flood control measures in the country are embodied in the report of the Rashtriya Barh Ayog (RBA) (National Flood Commission) submitted in March 1980. This report, containing a large number of recommendations on all aspects of flood, control forms the basis for the evolution of the present national policy on floods.
Back to TopGROUNDWATER
In the previous chapters
various aspects of surface hydrology that deal with surface runoff have been
discussed. Study of
FORMS OF SUBSURFACE
WATER
Water in the soil mantle is called subsurface water and is considered
in two zones (Fig. 9.1), Saturated zone and #9; Aeration zone.
Saturated Zone
This zone, also known as groundwater zone is
the space in which all the pores of the soil are filled with water. The
Fig. 9.1 Classification of subsurface water
Zone of Aeration
In this zone the soil pores are only
partially saturated with water. The space between the land surface and the water
table marks the extent of this zone. Further, the
Soil Water Zone
This lies close to the ground surface in the
major root band of the vegetation from which the water is lost to the atmosphere
by evapotranspiration.
Capillary Fringe
In this the water is held by capillary
action. This zone extends from the water table upwards to the limit of the
Intermediate Zone
This lies between the
All earth materials, from soils to rocks have pore spaces. Although these pores are completely saturated with water below the water table, from the groundwater utilization aspect only such material through which water moves easily and hence can be extracted with ease are significant. On this basis the saturated formations are classified into four categories:
Aquifer Aquitard Aquiclude AquifugeAquifer
An aquifer is a saturated formation of earth
material which not only stores water but
Aquitard
It is a formation through which only seepage is
possible and thus the yield is insignificant compared to an aquifer. It is
partly permeable.
Aquiclude
It is a geological formation which is essentially
impermeable to the flow of water. It may be considered as closed to water
movement even though it may contain large amounts of water due to its high
Aquifuge
It is a geological formation which is neither
porous nor permeable. There are no interconnected openings and hence it cannot
transmit water. Massive compact rock without any fractures is an aquifuge.
The definitions of
aquifer, aquitard and aquiclude as above are relative. A formation which may be considered as an aquifer at a place where water is at a premium (e.g. and zones) may be classified as an aquitard or even aquiclude in an area where plenty of water is available.The availability of groundwater from an aquifer at a place depends upon the rates of withdrawal and replenishment (
recharge). Aquifers play the roles of both a transmission conduct and a storage. Aquifers are classified as unconfined aquifers and confined aquifers on the basis of their occurrence and field situation. An unconfined aquifer (also known as water table aquifer) is one in which a free surface, i.e. a water table exists (Fig. 9.2). Only the saturated zone of this aquifer is of importance in groundwater studies. Recharge of this aquifer takes place through infiltration of precipitation from the ground surface. A well driven into an unconfined aquifer will indicate a static water level corresponding to the water table level at that location.A confined aquifer, also known as
artesian aquifer, is an aquifer which is confined between two impervious beds such as aquicludes or aquifuges (Fig. 9.2). Recharge of this aquifer takes place only in the area where it is exposed at the ground surface. The water in the confined aquifer will be under pressure and hence the piezometric level will be much higher than the top level of the aquifer.Fig. 9.2 Confined and unconfined acquirers
At some locations: the piezometric level can attain a level higher than the land surface and a well driven into the aquifer at such a location will flow freely without the aid of any pump. In fact, the term "artesian" is derived from the fact that a large number of such free-flow wells were found in Artois, a former province in north France. Instances of free-flowing wells having as much as a 50-m head at the ground surface are reported.
Water Table
A water table is the free water surface in an
unconfined aquifer. The static level of a well penetrating an unconfined aquifer
indicates the level of the water table at that point. The water table is
constantly in motion adjusting its surface to achieve a balance between the
recharge and outflow from the subsurface storage. Fluctuations in the water
level in a dug well during various seasons of the year, lowering of the
groundwater table in a region due to heavy pumping of the wells and the rise in
the water table of an irrigated area with poor drainage, are some common
examples of the fluctuation of the water table. In a general sense, the water
table follows the topographic features of the surface. If the water table
intersects the land surface the groundwater comes out to the surface in the form
of
Fig. 9.3 Perched water table
AQUIFER PROPERTIES
The
important properties of an
Porosity
The amount of pore space per unit volume of the
aquifer material is called porosity. It is expressed as
(9.1)
where n = porosity, Vv = volume of voids and Vo = volume of the porous medium, In an unconsolidated material the size distribution, packing and shape of particles determine the
porosity. In hard rocks the porosity is dependent on the extent, spacing and the pattern of fracturing or on the nature of solution channels. In qualitative terms porosity greater than 20% is considered as large, between 5 to 20% as medium and less than 5% as small.Specific Yield
While porosity gives a measure of the
water-storage capability of a formation, not all the water held in the pores is
available for extraction by pumping or draining by gravity. The pores hold back
some water by molecular attraction and surface tension. The actual volume of
water that can be extracted by the force of gravity from a unit volume of
aquifer material is known as the
n = Sy + Sr (9.2)
The representative values of porosity and specific yield of some common earth materials are given in Table 9.1.
TABLE 9.1 POROSITY AND SPECIFIC YIELD OF SELECTED FORMATIONS
Formation |
Porosity % |
Specific Yield % |
Lime stone |
1-10 |
0.5-5 |
Shale |
1-10 |
0.5-5 |
Sand Stone |
10-20 |
5-15 |
Gravel |
30-40 |
15-30 |
Sand |
35-40 |
10-30 |
Clay |
45-55 |
1-10 |
It is seen from Table 9.1 that although both clay and sand have high porosity the specific yield of clay is very small compared to that of sand.
GEOLOGICAL
FORMATIONS AS AQUIFERS
The identification of a geologic formation as a
Potential aquifer for
Limestones contain numerous secondary openings in the form of cavities formed by the solution action of flowing subsurface water. Often these form highly productive aquifers. In Jodhpur district of Rajasthan, cavernous limestones of the Vindhyan system are providing very valuable ground-water for use in this arid zone.
The volcanic rock basalt has permeable zones in the form of vesicles, joints and fractures. Basaltic aquifers are reported to occur in confined as well as underunconfined conditions. In the Satpura range some aquifers of this kind give yields of about 20 m³/h.
Igneous and metamorphic rocks with considerable weathered and fractured horizons offer good potentialities as aquifers. Since weathered and fractured horizons are restricted in their thickness these aquifers have limited thickness. Also, the average permeability of these rocks decreases with depth. The yield is fairly low, being of the order of 5-10 m³/h. Aquifers of this kind are found in the hard rock areas of Karnataka, Tamil Nada, Andhra Pradesh and Bihar.
Back to TopWELLS
Wells form the most important mode
of groundwater extraction from an aquifer. While wells are used in a number of
different applications, they find extensive use in water supply and irrigation
engineering practice. Consider the water in an
Fig.9.5 Well operating in an unconfined aquifer, (definition sketch)
Changes similar to the above take place due to a pumping well in a confined aquifer also, but with the difference that, it is the piezometric surface instead of the water table that undergoes drawdown with the development of the cone of depression. In confined aquifers with considerable
piezometric head, the recovery into the well takes place at a very rapid rate. Back to TopGROUNDWATER BUDGET
The quantum of groundwater available in a
basin is dependent on the inflows and discharges at various points. The
interrelationship between inflows, outflows and accumulation is expressed by the
water budget equation
å I D t - å Q D t = D S (9.39)
where,
å I D t = all forms of
å Q D t = net discharge of groundwater from the basin and includes pumping, surface outflows, seepage into lakes and rivers and evapotranspiration
D S = change in the groundwater storage in the basin over a time D t
Considering a sufficiently long time interval, At of the order of a year, the capability of the groundwater storage to
yield the desired demand and its consequences on the basis can be estimated. It is obvious that too large a withdrawal than what can be replenished naturally leads ultimately to the permanent lowering of the ground water table. This in turn leads to problems such as drying up of open wells and surface storages like swamps and ponds, change in the characteristics of vegetation that can be supported by the basin. Similarly, too much of recharge and scanty withdrawal or drainage leads to waterloggii3g and consequent decrease in the productivity of lands.The maximum rate at which the withdrawal of groundwater in a basin can be carried without producing undesirable results is termed safe yield. This is a general term whose implication depends on the desired objective. The "undesirable" results include permanent lowering of the groundwater table or piezometric head, maximum drawdown exceeding a preset limit leading to inefficient operation of wells and salt-water encroachment in a coastal aquifer. Depending upon what undesirable effect is to be avoided, a safe yield for a basin can be identified.
Back to TopACID RAIN
Rain, which in the course of its history has
combined with chemical elements or pollutants in the atmosphere and reaches the
earth's surface as a weak acid solution.
ACIDITY OF WATER
Amount of acids, given as milli
equivalents of a strong base per 1 litre of water, necessary to titrate the
sample to a certain PH value.
ACTUAL EVAPOTRANSPIRATION
The real evapotranspiration
occurring in a specific situation.
AFFLUENT
Watercourse flowing into a larger watercourse
or into a lake.
ALBEDO
Ratio of reflected to incoming radiation, usually
given in percent.
ALKALINITY OF WATER
Amount of cations balanced by weak
acids, expressed as milli equivalents of neutralized hydrogen ions per litre of
water.
ALLUVIAL PLAIN
Plain formed by the deposition of
alluvial material eroded from areas of higher elevation.
ALLUVIAL STREAM
An alluvial stream is one whose bed is
composed of unconsolidated silt, sand and gravel. The bed is constantly in
motion and highly unstable.
ANEMOMETER
Instrument used for the measurement of wind
speed and direction.
ANTECEDENT
PRECIPITATION
The precipitation occurring during some period antecedent
to the defined event or some part of the defined event.
ANTECEDENT
PRECIPITATION INDEX
A weighed summation of daily precipitation amounts
used as an index of soil moisture. The weight given to each day's precipitation
is usually assumed to be an exponential or reciprocal function of time with the
most recent precipitation receiving the greatest weight.
ANTICYCLONE
An area of
relatively high pressure surrounded by closed isobars, the pressure gradient
being directed from the center so that the wind blows spirally outward in a
clockwise direction in the northern hemisphere, counter- clockwise in the
southern hemisphere.
APPLIED HYDROLOGY
That
branch of hydrology which refers to its applications to field connected with
water resources development and management.
AQUICLUDE
It is a formation which
may contain large volumes of water but does not permit its movement at rates
sufficiently high for economical development e.g. clay and shale.
AQUIFER
It is a formation or a
geological structure which has good
AQUIFUGE
It is a formation which
has no interconnected openings and hence can not absorb or transmit water. It is
neither porous nor permeable.
AQUITARD
It is a formation which
has low to medium permeability which is not sufficient to be a source of water
to flow on a regional scale from one aquifer to the other due to leakage.
Formations having predominance of silt and clay along with kankar form aquitard.
Behave as semi-confining layers.
AREA OF INFLUENCE
The areal extent of the
AREA-ELEVATION-CAPACITY CURVE
Curves showing what part
of the area of a
AREAL PRECIPITATION
Precipitation in a specific area
expressed as the average depth of liquid water over this area.
AREA-VELOCITY METHOD
A method of measuring discharge at
a section in a stream based on the continuity principle. Cross-sectional area of
the stream is measured and velocity of flow is calculated using some type of
instrument, say a current meter. Multiplication of area and velocity gives the
discharge at that station.
ARID CLIMATE
A term applied to regions where
precipitation is so deficient in quantity or occurs at such times that
agriculture is impracticable without irrigation.
ARTESIAN AQUIFER
An
artesian aquifer is overlain and underlain by confining layers such that water
in these aquifers occurs under pressure, which is more than the atmospheric
pressure.
ARTESIAN WELL
A well penetrating an artesian aquifer is
called artesian well. If the water level rises above the bottom of the confining
bed but remains below Ground surface, then it is called artesian well. If water
rises above ground surface, then it is called flowing well.
ARTIFICIAL PRECIPITATION
It means causing precipitation
artificially by the introduction of materials like solid carbon dioxide or
silver iodide into a non-precipitating cloud. The experiments have not yet
become of economic importance.
ARTIFICIAL RECHARGE
Artificial recharge may be defined
as augmenting the natural replenishment of ground water storage by some method
of construction, spreading of water or by artificially changing natural
conditions.
ATMOMETER
Porous, porcelain spheres, cylinders or blocks
commonly used by plant physiologists for measuring evaporation because
evaporation from their surfaces is considered to be quite representative of that
from plants.
AUXILIARY GAUGE
At those hydrometric stations where
variable back-water occurs, it is necessary to utilize fall in a
AVAILABLE HEAD
Amount of fall in a stream which is
available for hydroelectric power development.
AVALANCHE
A moving mass of debris,
AVERAGE ANNUAL FLOOD
A flood equal to the average of the
annual floods during the period of record.
AVERAGE ANNUAL
RAINFALL
Rainfall equal to the average of the annual rainfalls during the
period of record.
BACKWASHING
Reversal of the flow of water under
pressure, for example, in a well to free the screen or strainer and the adjacent
aquifer of clogging material.
BACKWATER CURVE
Longitudinal
profile of the water surface upstream in a stream where the water surface is
raised by a natural or artificial obstruction.
BANK STORAGE
Water absorbed and
stored in the banks of a stream, lake or
BAROMETER
An instrument used for measuring pressure of
the atmosphere.
BASIN
Area drained by a river is
called basin of the river.
BASIN LAG
Actually basin lag (also
known as
BASIN RECHARGE
Basin recharge is the difference between
precipitation and
BASIN RESPONSE
Manner in which a basin
reacts to a meteorological event or sequence of events.
BED LOAD
Bed load may be defined as
the load of bed material in the bed layer where suspension is impossible for
fluid dynamic reasons. Sediment grains in the bed layer are not vertically
supported by flow but rest on the bed almost continuously while sliding, rolling
and jumping along.
BIFURCATION RATIO
The ratio of number of stream segments
of a given order to the number of stream segments of the next higher order.
BRACKISH WATER
Water containing salts at a
concentration significantly less than that of sea water. The concentration of
total dissolved salts is usually in the range 100 10000 mg per
liters.
BUBBLE GAUGE
In this gauge,
compressed air or gas is made to bleed out at very small rate through an outlet
placed at the bottom of the river. A pressure gauge measure gas pressure which
is equal to the water column above the outlet. Small change in water surface
elevation is felt as change in pressure.
CANOPY
INTERCEPTION
Rainfall retained on standing vegetation and evaporated
without dripping off or running down the stems or trunks.
CAPILLARY RISE
The maximum
height to which water will rise due to capillary forces above the water
table.
CAPILLARY WATER
Water held in the soil above the water
table by capillary action.
CATCHMENT ORDER
A catchment order is described depending
on the
CHANNEL DETENTION
Volume of water which can be
temporarily stored in channels during flood periods.
CHANNEL
PRECIPITATION
Precipitation which falls directly on the water surfaces of
lakes and streams.
CHANNEL ROUTING
The routing
of a flood wave in a stream when the only storage is the valley storage.
CHANNEL STORAGE
The quantity
of water within the main channel.
CHEMICAL OXYGEN DEMAND
Mass concentration of oxygen
equivalent to the amount of a specified oxidants consumed by dissolved or
suspended matter when a water sample is treated with that oxidant under defined
conditions.
CLASS-A PAN
It is an instrument
used for estimation of evaporation. It is a standard par of 1210 mm diameter and
255 mm depth used by US Weather Bureau.
CLIMATE
The sum total of all atmospheric or
meteorological influences principally temperature, moisture, wind, pressure and
evaporation which combine to characterise a region and give it individuality by
influencing the nature of its land forms, soil, vegetation and land use.
CLIMATE SHELTER
Climate station instruments that must be
protected from condensation, precipitation and radiation are house in climate
shelters or screens. The typical shelter is white, double-topped, with louvered
sides to permit free circulation of air.
CLIMATIC CYCLE
Actual or supposed recurrences of such
weather phenomena as wet and
CLOUD BURST
Rain storm of high intensity and of a
relatively short duration usually over a relatively small area.
CLOUD SEEDING
In cloud seeding, the clouds which contain
appreciable amount of liquid water under colloidally stable conditions, are made
colloidally unstable by the addition of dry ice, silver iodide or other chemical
agents so that a certain part of this otherwise unavailable water will reach the
ground as precipitation.
COLLUVIUM
Soil which is eroded from sloping land may
become lodged at fence rows and vegetated areas or deposited below breaks in
slopes in the form of colluvium.
COLORADO SUNKEN PAN
This
pan, used to estimate evaporation is 920 mm square and 460 mm deep and buried
into the ground within 100 mm of the top with the advantage that radiation and
aerodynamic characteristics are similar to those of a lake.
COMPOUND HYDROGRAPH
The hydrograph of an intermittent
storm when the flow on account of one substorm continues during the next
substorm.
CONCENTRIC RING INFILTROMETER
This is an instrument used
for the measurement of infiltration. It consists of two concentric rings which
are inserted into the ground and water is maintained on the soil surface to a
common fixed level.
CONCEPTUAL HYDROLOGICAL MODEL
Simplified mathematical
representation of some or all of the processes in the hydrological cycle by a
set of hydrological concepts expressed in mathematical notations and linked
together in a time and space sequence corresponding to that occurring in nature.
Hydrological conceptual models are used for simulation of the behaviour of the
basin.
CONDENSATION NUCLEI
Condensation of water vapour into
cloud droplets takes place on certain hygroscopic particles which are commonly
called condensation nuclei.
CONE OF DEPRESSION
A
downward curve showing the variation of draw down with distance from the well
describes a conic shape in three dimensions called cone of depression.
CONFIDENCE LIMITS
Values which form the lower and upper
limits to the confidence interval.
CONFINED AQUIFER
A confined
aquifer, also known as artesian or pressure aquifer, is an aquifer which is
confined between two impervious beds such as aquicludes or aquifuges and in
which groundwater is confined under pressure greater than atmosphere.
CONFLUENCE
Joining, or the place of junction, of two or
more streams.
CONJUNCTIVE USE
Conjunctive use involves the coordinated
and planned operation of both surface water and ground
CONSUMPTIVE USE
The quantity
of water used by vegetative growth of a given area in transpiration or building
of plant tissue and that evaporated from the soil or from intercepted
precipitation on the area in any specified time. It is expressed in water depth
unit or depth-area units per unit area and for specified periods, such as days,
months and seasons.
CONTAMINATION
Introduction
of any undesirable substance, normally not present, in water, e.g.
micro-organisms, chemicals, waste or sewage, which renders the water unfit for
its intended use.
CONTROL SECTION
Reach of a stream channel in which there
exists a unique stage- discharge relationship.
CONVECTIVE
PRECIPITATION
Precipitation resulting from the upward movement of air
that is warmer than its surrounding. It is generally of a showery nature with
rapid changes of intensities.
CORRELATION
COEFFICIENT
Measure of the inter-dependence between two variates.
COVARIANCE
First product moment of two variates about
their mean values.
CREST SEGMENT
It is that part
of the hydrograph which contains the peak flow. Peak flow occurs when the runoff
from various parts of the catchment simultaneously contribute the maximum amount
of flow at the basin outlet.
CREST STAGE INDICATOR
This indicator is used to
delineate the peak stage of a flood at points other than at a hydrometric
station. Such data are valuable in the establishment of flood profiles.
CRITICAL STORM PERIOD
The duration of that storm which
causes the greatest peak at a station in a drainage basin.
CROP COEFFICIENT
It is an empirical coefficient used in
Blaney-Criddle formula for calculating the
CROSS SECTION
Section of a
stream at right angles to the main (average) direction of flow.
CRYOLOGY
The science of ice in all its forms such as
snow, ice and hall.
CUP-TYPE CURRENT METER
Current meter whose rotor is
composed of a wheel fitted with cups and turning on a vertical axis.
CURRENT METER
It is the most
commonly used instrument in
CYCLONIC PRECIPITATION
The precipitation associated with
the passage of depressions of cyclones.
DAM
Barrier constructed across a valley
for impounding water or creating a reservoir.
DARCY'S LAW
It states that the rate of flow per unit
area of an aquifer is proportional to the gradient of the potential head
measured in the direction of flow.
DATA BASE
Comprehensive set of related data files for a
specific application, usually on a direct access storage device.
DATA PROCESSING
Handling of observational data until
they are in a form ready to be used for a specific purpose.
DEAD STORAGE
Storage volume which can not be released
under normal conditions.
DEAD WATER
Water in a state of slow or no circulation,
usually leading to an oxygen deficit.
DEEP PERCOLATION
Water
which percolates below the root zone and towards a deeper water table.
DEGREE DAY
It is a unit expressing the amount of heat in
term of the departure of one degree per day in the daily mean temperature from
an adopted reference temperature. The number of degree days for an individual
day is the actual departure of the mean temperature from the standard. Standard
temperature is usually taken as 0 C to 32 F.
DENSITY CURRENT
It is defined as the gravitational flow
of one fluid under another fluid of approximately equal density. Density
currents thus separate the turbid water from the clear water and make the turbid
water flow along the river bottom in the vicinity of the dam.
DEPENDABLE FLOW
In the
flow-duration curve, the ordinate Qp at any percentage probability
Pp represents the flow magnitude in an average year that can be
expected to be equalled or exceeded Pp % of time is called
Pp % dependable flow. For perennial streams, Q is a finite value
while for
DEPLETION CURVE
The
depletion curve extends from the point of inflection at the end of the crest
segment to the commencement of natural groundwater flow in a hydrograph. It
represents the withdrawal of water from the storage built up in the basin during
earlier phases of the storm.
DEPRESSION STORAGE
Also called pocket storage, the
volume of water usually expressed as depth on the drainage area which is
required to fill natural depressions, large or small, to their overflow
levels.
DEPTH OF RUNOFF
The total runoff from a drainage area or
basin, divided by the area, expressed in either units of depth or units of
volume per unit area of the basin.
DEPTH-AREA RELATIONSHIP
It is a relation which is
expressed between progressively decreasing average depth of rainfall of a given
duration over a progressively increasing area from centre of maximum
precipitation of a storm outward to its edges in an exponential fashion.
DEPTH-AREA-DURATION
CURVE
A curve which graphically indicates the precipitation amounts for
various areas and durations for a particular rainstorm.
DETENTION
RESERVOIR
Flood-control reservoir with uncontrolled outlets.
DETERMINISTIC HYDROLOGY
Method of analysis of
hydrological processes, using a deterministic approach to investigate the
responses of hydrological systems in terms of various parameters.
DEW
Deposit of water drops on objects at or near the
ground, produced by the condensation of water vapour from the surrounding clear
air.
DIFFUSION WELL
Recharge well that is sunk only into the
unsaturated zone distinguished from an injection well.
DISCHARGE COEFFICIENT
Ratio of the observed or actual
discharge to the theoretically computed discharge.
DOWNSTREAM
In the direction of
the current in a river or stream.
DRAINAGE
Removal of surface
water or groundwater from a given area by gravity or by pumping.
DRAINAGE BASIN
The area from
which a lake, stream or waterway and reservoir receives surface flow which
originates as precipitation.
DRAINAGE COEFFICIENT
Drainage coefficient is the water
depth drained from an area in one day. These coefficients enable the designer to
compare various drainage methods.
DRAINAGE DENSITY
It is
defined as the ratio of the total channel length to the total drainage area. A
large drainage density creates situation conducive for quick disposal of runoff
which is reflected in pronounced peaked discharge.
DRAWDOWN
The drawdown at a given point is the distance
by which water level is lowered.
DRAWDOWN CURVE
A drawdown
curve shows the variation of drawdown with distance from the well. In three
dimensions, the drawdown curve describes a conic shape known as the cone of
depression.
DRIZZLE
A fine sprinkle of numerous
water droplets of size less than 0.5 mm and intensity less than 1mm/h. The drops
are so small that they appear to float in the air.
DROUGHT
In general an extended
period of dry weather or a period of deficient rainfall that may extend over an
indefinite number of days. Without any set quantitative standard by which to
determine the degree of deficiency needed to constitute a drought.
Qualitatively, it may be defined by its effects, as a dry period sufficient in
length and severity to cause at least partial crop failure.
DROUGHT INDEX
Computed value which is related to some
of the cumulative effects of a prolonged and abnormal moisture deficiency. An
index of hydrological drought corresponding to levels below the mean in streams,
lakes, reservoirs and the like. However, an index is agricultural drought must relate to the cumulative
effects of either an absolute or an abnormal transpiration deficit.
DRY YEAR
Year of drought in which
precipitation or streamflow is significantly less than normal.
DRY-WEATHER FLOW
The flow
of water in a stream during the non-rainy season. It is primarily made of water
which seeps from the ground. However, water supplied by snowmelt or regulated
water released from a storage also become a part of it.
DURATION CURVE
A graph
representing the time during which the value of a given parameter e.g. water
level, piezometric head, discharge, concentration of dissolved solids, is
equalled or exceeded regardless of continuity in time.
ECHO-DEPTH RECORDER
An
instrument by which the depth of water is determined by measuring the time taken
by a sound signal to travel to the bottom and return.
EFFECTIVE RAINFALL
Part of the rain that appears as
runoff in the stream.
EFFLUENT STREAM
A stream or stretch of stream which
receives water from groundwater in the
ELECTROMAGNETIC
FLOWMETER
It is an instrument for measuring discharge in a stream. It is
based on the principle that an e.m.f. is induced in the conductor (water in the
present case) when it cuts a normal magnetic field.
ELONGATION RATIO
It is defined as the ratio of diameter
of a circle of the same area as the basin to the maximum basin length. This
ratio runs between 0.6 to 1.0 over a wide variety of climatic and geologic
types.
EMERGENCY SPILLWAY
Auxiliary spillway used in the event
of floods exceeding the capacity of the main spillway.
EMPIRICAL FLOOD FORMULA
Formula expressing peak
discharge as a function of catchment area and other factors.
ENERGY BUDGET METHOD
It is an analytical method for the
determination of evaporation. It is a measurement of continuity of flow of
energy. Energy available for evaporation is determined by considering the
incoming energy, outgoing energy and energy stored in the water body over a
known time interval.
ENGINEERING
HYDROLOGY
That branch of applied hydrology which deals with hydrological
information intended for engineering applications, e.g. planning, designing,
operating and maintaining engineering measures and structures.
ENVELOP CURVES
(a) A smooth curve which envelops all
the plotted points representing maximum recorded
flood peaks and volumes for hydrometeorologically comparable areas.
(b) A
smooth curve covering either all peak values or all trough values of certain
quantities
e.g. rainfall, runoff etc. plotted
against other factors such as area and time. In
general,
none of the peak values goes above the
curve in former case, called the "maximum
envelop"
and non of the minimum points fall below in the later case called the
"minimum
envelop".
EPHEMERAL STREAM
A stream
or a portion of a stream which flows only in direct response to
precipitation.
EQUILIBRIUM EQUATION
Equilibrium equation is used to
determine the
EROSION
It is defined as the wearing
away of land by water, wind, ice or gravity.
EUTROPHIC LAKE
Lake characterized by a great amount of
nutrients and biogenic matters and by highly developed phytoplankton is
summer.
EVAPORATION
The process by which
water is changed from the liquid state to the gaseous state below the boiling
point through the transfer of heat energy.
EVAPORATION OPPORTUNITY
Ratio of actual rate of
evaporation from land or water surface in contact with the atmosphere to the
potential rate of evaporation under existing atmospheric
conditions.
EVAPORATION PAN
An
experimental tank used to determine the amount of evaporation from the surface
of water under measured or observed climatic conditions.
EVAPOTRANSPIRATION
An instrument for measuring
evaporation.
EVAPOTRANSPIRATION
It is the process by which water
moves from the soil to the atmosphere. It consists of transpiration, the
movement of water through the plant to the atmosphere & evaporation, the
movement of water vapour from soil and vegetative surfaces. Thus the entire
surface as well as subsurface water which is released from a basin into the
atmosphere by process of evaporation and transpiration is generally known as
evapotranspiration.
EXCEEDENCE INTERVAL
The exceedence interval is defined
as the average number of years between the occurrence of an event and a greater
event.
EXPERIMENTAL BASIN
Basin in which natural conditions
are deliberately modified and in which the effects of these modifications on the
hydrological cycle are studied.
EXTREME RAINFALL
Amount of
precipitation that is the physical upper limit for a given duration over a
particular basin.
EXTREME VALUE DISTRIBUTION
Fisher Tippet Type I External
distribution applied by Gumbel to the annual maximum flood series and by others
to rainfall series.
FAIR-WEATHER RUNOFF
Also called base flow, it is
composed of
FIELD CAPACITY
The amount of water held in the soil
after the excess gravitational water has drained away and after the rate of
downward movement of water has materially decreased. Essentially the same as
'specific
FIELD EFFICIENCY
The percentage of the total volume of
water delivered to the field that is finally consumed by
evapotranspiration.
FIRN LINE
Boundary which, at the Earth's surface,
separates zone of accumulation of a glacier from the zone of ablation.
FLASH FLOOD
A flood of short duration and abrupt rise
with a relatively high peak rate of flow, usually resulting form a high
intensity of rainfall.
FLOAT GAUGE
This is most common type of automatic float
operated stage recorder. Float operating in a stilling well is balanced by means
of a counter weight over the pulley of a recorder. Displacement of float is
traversed on a chart continuously and stage versus time plot is made.
FLOATING
PAN
It is US Geological Survey evaporation pan set afloat in a lake with
a view to simulate the characteristics of a large body of water. Water level in
the pan is kept at the same level as the lake, leaving a ring of 75
mm.
FLOOD
The flow pattern in a stream,
constituting a distinct progressive rise culminating in a peak or summit
together with the recession that follows the crest.
FLOOD ABATEMENT
Any measure taken outside of stream
channel with the effect of educing the crest of flood flows or changing the
debris load for a flood event.
FLOOD ABSORPTION
The
increase in storage of water in a reservoir, lake, valley or channel resulting
in a reduction of streamflow.
FLOOD CONTROL
Flood control means flood damage
prevention or reduction together with the protection of economic development and
protection of life.
FLOOD DAMAGE
The destruction or
impairment, partial or complete, of human and animal lives, property, goods,
services, flora and fauna or of health etc., resulting from the action of floods
water and the silt and debris they carry. It includes direct and indirect
losses.
FLOOD DISCHARGE
It is the
discharge passing at a particular site during a flood event.
FLOOD FORECASTING
Prediction of stage, discharge, time
of occurrence and duration of a flood, especially of peak discharge, at a
specified point on a stream resulting from precipitation or snowmelt so that
people could be warned well in advance and life and movable goods could be saved
to a large extent.
FLOOD FREQUENCY
(a) The
number of times a flood of a given magnitude is likely to be equaled or exceeded
over
a period of years on the average.
(b) The
number of years in which a flood of a given magnitude is likely to be equaled
or
exceeded once on the average over a period of
years.
FLOOD MARKS
The trace of any kind left on the banks or
flood plain by a flood which may be used, after the flood, to determine the
highest level attained by the water surface during the flood.
FLOOD PLAIN
Land adjoining the
channel which is inundated only during floods.
FLOOD PROOFING
Combination of emergency equation and
structural adjustments for modifying a given property and thus reducing flood
losses.
FLOOD ROUTING
The process of determining progressively
the timing and shape of a flood wave at successive points along a river.
FLOOD SERIES
A List of flood events which occurred
during a specified period of time.
FLOOD STAGE
The elevation of
water surface during a flood relative to a datum, local or national.
FLOOD WAY
The channel of a river
or stream and those portions of the flood plains adjoining the channel, which
are required to carry and discharge the flood water.
FLOW DURATION CURVE
Curve showing the percentage of time
during which the flow of a stream is equal to or greater than given amounts,
regardless of chronological order.
FLOW METER
It is an instrument used for measuring the
rate of flow in a conduit or open channel.
FLOW-MASS CURVE
A graph of the cumulative values of
discharge of runoff, generally as ordinate, plotted against time as abscissa.
The curve has many useful applications such as the determination of reservoir
capacity, operations procedure and flood routing.
FOREST HYDROLOGY
It is the science of water related
phenomena that are influenced by forest cover. It is an interdisciplinary
science, the union of forestry and hydrology.
FORMATION LOSS
It is the head drop required to cause
laminar flow through the porous media.
FREEBOARD
Vertical distance between the normal maximum
level of the surface of liquid in a conduit, reservoir, tank, canal, etc. and
the top of the sides of the retaining structure.
FREQUENCY
ANALYSIS
Procedure involved in interpreting a past record of hydrological
events in terms of future probabilities of occurrence, e.g. estimates of
frequencies of floods, droughts, storage, rainfall, water quality, waves.
FREQUENCY
DISTRIBUTION
Specification of the way in which the frequencies of members
of a population are distributed according to the values of the variates which
they exhibit (DST).
FRESH WATER
Water neither salty
nor bitter to the taste and in general, chemically suitable for human
consumption (having a low content in dissolved solids).
FRONTAL PRECIPITATION
Precipitation caused by the
expansion of air on ascent along or near a frontal surface.
FROST
A light feathery deposit of ice
caused by condensation of water vapour directly in the crystalline form, on
terrestrial objects whose temperature is below freezing, the process being the
same by which dew is formed, except that the later occurs only when the
temperature of the bedewed object is above freezing.
GAINING STREAM
Stream fed by groundwater.
GAUGE
An instrument used for measuring
depth of water.
GAUGING SITE
Location on a
stream where measurements of water level and discharge are regularly
made.
GEOHYDROLOGY
Branch of hydrology related to subsurface
and subterranean water.
GLACIER
Body of land ice formed from
recrystallised snow accumulated on the ground; may form where annual accretion
of snow is greater than ablation by runoff and evaporation. There are two broad
classes
(a) Ice streams which form in mountain valleys and move downslope
under gravity.
(b)
GLACIOMETER
An instrument used for measuring glacial
motion.
GRAVITATIONAL WATER
Water in the unsaturated zone which
moves under the influence of gravity.
GLAZE
Also called freezing rain, it is
reported when rain falls into a cold layer of air and freezes when it strikes
objects on the ground.
GROUND WATER
Water in a
saturated zone of geologic stratum.
GROUND WATER BASIN
It may
be defined as a hydrogeologic unit containing one large aquifer or several
connected and interrelated aquifers. It implies an area containing a groundwater
reservoir capable of furnishing a substantial water supply.
GROUNDWATER RECESSION
Decreasing rate of groundwater
discharge to surface water bodies during periods of no recharge, connected to
the depletion of ground water storage, and expressed by groundwater recessive
curve.
GROUNDWATER
RECHARGE
Process by which water is added from outside to the zone of
saturation of an aquifer, either directly into a formation, or indirectly by way
another formation.
GROUND WATER RUNOFF
That
part of the runoff which consists of water that has passed into the earth and
has entered the zone of saturation and has later been discharged into a water
body.
GULLY
A channel or miniature valley formed as a result
of erosion and caused by concentrated but intermittent flow of water usually
during or immediately after heavy rains. The channel is deep enough to interior
with tillage operations. Gully may be dendritic or branching or linear (long,
narrow and of uniform width). Gully may be U, V or W shaped.
GUMBEL'S DISTRIBUTION
It is one of the most widely used
probability distribution functions for extreme values in hydrologic and
meteorologic studies for prediction of flood peaks, maximum rainfall, maximum
wind speed etc.
HAIL
Small, roughly spherical lumps of
approximately concentric shells of clear ice and compact snow usually ranging
from 5 to 10 mm or more in diameter which fall either separately or agglomerated
into larger irregular lumps precipitated during thunder storms.
HARDNESS OF WATER
That property of water, due mainly to
bicarbonates, chlorides anulphates of calcium and magnesium, which prevents the
production of abundant lather with soap.
HEAD LOSS
Decrease of total head, expressed in units of
height, due to energy dissipation.
HEADWATERS
Streams from the sources of a river.
HEAVY WATER
Water enriched in water molecules containing
heavier (stable and radioactive) isotopes of hydrogen (Deuterium, Tritium) and
Oxygen 18.
HISTOGRAM
Univariate frequency diagram with rectangles
proportional in area to the class frequency, erected on a horizontal axis with
width equal to the class interval.
HISTORICAL DATA
Hydrological and Meteorological data of events which occurred in the
past. Data is collected from natural phenomena that can be observed only once
and then will not occur again.
HURRICANE
These are very intense
low pressure systems with winds in excess of 75 m.p.h. It is an intense cyclone
of tropical origin and of relatively small horizontal dimensions (110-300
miles). Storms of this type are called cyclones in India and typhoons in the far
east. Hurricanes form where sea surface temperature is above 27 C in general;
hurricanes are accompanied by torrential rains.
HYDRAULIC
CONDUCTIVITY
It is a constant that serves as a measure of permeability of
the porous medium. A medium has a unit hydraulic conductivity if it will
transmit in unit time a unit volume of ground water at the prevailing kinematic
viscosity through a
HYDRAULIC GRADIENT
(1) In a closed
conduit the slope of the hydraulic grade line.
(2) In open channels: the
slope of the water surface.
(3) In porous media: measure of the decrease in
head per unit distance in the direction of flow.
HYDROGEOLOGY
That branch of geology relating to effect
of water on earth.
HYDROGRAPH
A graph showing the stage, volume of flow,
velocity, sediment concentration or sediment discharge or some other feature of
flowing water with respect to time at a given place. For example, a graph
showing the discharge of a stream as ordinate against time as abscissa is called
a discharge hydrograph.
HYDROLOGIC CYCLE
A phenomena relating to circulation of
water from the sea, through the atmosphere to the land, and thence, often with
many delays, back to the sea or ocean through various stages and processes, for
example, precipitation,
HYDROLOGIC EQUATION
The
water inventory equation (inflow = o0utflow + change in storage) which expresses
the basic principle that during a given time interval the total inflow to an
area must equal the total outflow plus the net change in storage.
HYDROLOGIC
FAILURE
Failures due to improper assessment of hydrological factors such
as overtopping and consequent failure of an earthen dam due to inadequate
spillway capacity, failure of bridges and culverts due to excess flood flow,
inability of a large reservoir to fill up due to overestimation of stream flow
etc.
HYDROLOGIC NETWORK
It is a network of stations for
measuring hydrologic variables such as rainfall and river stage stations.
Adequacy of hydrologic network is dependent upon the character of the drainage
basin and the critical need of the regulating system.
HYDROMETEOROLOGY
Study
of the atmospheric and land phases of the hydrological cycle, with emphasis on
the interrelationships involved.
HYDROLOGY
The applied science
concerned with the water of the earth in all its stages their occurrences,
distribution and circulation through the unending hydrologic cycle of
precipitation, runoff,
HYDROMETRY STATION
These are the stations where
measurement of discharge in a stream is made at particular points. These must be
sited adequately in the catchment area so that the water potential of the area
can be assessed as accurately as possible.
HYETOGRAPH
A bar graph of average
rainfall, rainfall excess rates or volumes over specified areas during
successive units of time during a storm.
HYSTERESIS
The phenomenon that soil moisture tension
at a given moisture content depends on the past history of wetting and drying
cycles (of soil moisture). The variability of the stage-discharge relation at a
gauging station subject to variable slope where, for the same gauge height, the
discharge on the rising stage is greater than on the falling stage.
ISI STANDARD PAN
This pan evaporimeter specified by IS,
also known as modified class A pan, consists of a pan 1220 mm in diameter, 255
mm deep and made of copper sheet 0.9 mm thick. The top is fully covered with a
hexagonal wire mesh to protect water from birds.
ICE
Solid form of water in nature formed
by the freezing of water, the condensation of atmospheric water vapour directly
into crystals, the compaction of snow with or without the motion of a glacier or
the impregnation of porous snow masses with water which subsequently
freezes.
ICE CAP
Perennial cover of ice and
snow over an extensive are a of land or sea.
INDUCED RECHARGE
It is water entering the ground from a
surface water source as a result of withdrawal of ground water adjacent to the
source. Induced recharge can furnish water free of organic matter and pathogenic
bacteria.
INFILTRATION
The entrance of
water into the soil or other porous material through the interstices or pores of
a soil or other porous medium.
INFILTRATION CAPACITY
Maximum rate at which specified
soil in given condition can absorb water.
INFILTRATION CAPACITY CURVE
A curve showing what the
infiltration rate would be at any period during a specified storm if the
INFILTRATION
INDEX
Infiltration index, in general, expresses infiltration at an
average rate throughout the storm.
INFILTROMETER
A device by
which the rate and amount of water infiltrating into the soil is
determined.
INFLECTION POINT
It is a point on the hydrograph which
represents start of withdrawal of water from the storage built up in the basin
during the earlier phases of the hydrograph. It is the starting point of the
recession limb and end point of
INFLUENT STREAM
A stream or stretch of stream which
contributes water to the zone of saturation. The water surface of such stream
stands at a highest level than the water table or piezometric surface of the
ground water body to which it contributes water.
INITIAL ABSTRACTION
Maximum amount of rainfall that can
be absorbed under specific conditions without producing runoff. Also referred to
as initial losses, it is sum of interception and depression storage.
INITIAL DETENTION
That part of precipitation which does
not appear either as infiltration or as surface runoff during period of
precipitation or immediately thereafter, includes interception by vegetal cover,
depression storage and evaporation during precipitation, does not include
surface detention.
INSTANTANEOUS UNIT HYDROGRAPH
When the unit duration of
the rainfall excess is infinitesimally small, the resulting hydrograph is known
as instantaneous unit hydrograph.
INTENSITY-DURATION-FREQUENCY CURVE
The intensity of
storms decreases with the increase in storage duration. Further a storm of any
given duration will have a larger intensity if its
INTERCEPTION
The process by which precipitation is
caught and held by foliage twigs and branches of the trees, shrubs and other
vegetation, and lost by evaporation, never reaching the surface of the
ground.
INTERCEPTOMETER
Throughfall gauges are called
interceptometers, evidently because of their function in estimating
INTERFLOW
A part of the
precipitation that infilters into the ground moves laterally through upper
crusts of the soil and returns to the surface at some location away from the
point of entry into the soil. This component of runoff is known as interflow.
The amount of interflow depends on the geological conditions of the
catchment.
INTERMITTENT
STREAM
Stream which flows during a season. It has limited contribution
from the ground water. During the wet season, water table is above the stream
bed and there is contribution of base flow to the stream flow. During dry
season, water table drops below stream bed and stream dries out.
IRRIGATION REQUIREMENT
Quantity of water, exclusive
of precipitation, that is required for optimal crop production.
ISOCHRONE
It is a line on a map of a catchment joining
points having equal time of travel to the outlet of the
catchment.
ISOHYET
A line drawn on a map
passing through places having equal amount of rainfall recorded during the same
period at these places (these lines are drawn after giving consideration to the
topography of the region).
ISOPLETH
Lines on a map through
places having equal depths of evapotranspiration.
ISOTHERM
Lines joining points of equal
temperatures.
JUVENILE WATER
It is water derived from magma or molten
mass of igneous rocks during their crystallisation or from lava flows in the
form of stream. It is water that has come to the earth surface from great depths
for the first time.
KARSTIC RIVER
River which originates from a karstic
spring or flows in a karstic region that is, in a region having carbonaceous
rocks as CaCo3.
LAG TIME
It is the time interval
from the centre of mass of rainfall to the centre of mass of hydrograph.
LAKE
An extensive sheet of water,
bounded by land, in a hollow of the earth surface. It is an inland body of water
which is formed more due to glacial
LAND PAN
It is a pan placed on the
ground for the purpose of measuring evaporation.
LARGE WATERSHED
A large
watershed is one in which the effect of channel flow or basin storage is
dominating rather than the effect of overland flow. It has very less sensitivity
to high intensity rainfall of short duration.
LATERAL INFLOW
Inflow of
water to a river, lake or reservoir along any reach from the part of the
catchment adjacent to the reach.
LEACHING REQUIREMENT
Water required for the removal of
salts from the upper soil by relatively salt-free water.
LEAKY AQUIFER
Aquifers which are overlain or underlain
by semi-permeable strata are referred to as leaky aquifers. Such aquifers are
confined in the sense that pumping does not dewater the aquifers, but a
significant portion of the yield may be derived by vertical leakage through the
confining formations into the aquifers.
LIMNOLOGY
That branch of hydrology relating to water of
lakes and ponds.
LINEAR CHANNEL
An imaginary channel in which the rating
curve between discharge and area is a straight line such that at any point, the
velocity of flow is constant for all discharges, but may vary from point to
point along the channel.
LINEAR RESERVOIR
An
imaginary reservoir in which the storage S is directly proportional to the
outflow Q.
LIVE STORAGE
Volume or cubic capacity of a lake or
reservoir between the maximum and minimum operating levels.
LOAD-CARRYING CAPACITY
Maximum sediment quantity per
unit time which can be transported by specified flow in a channel.
LOCAL INFLOW
Water entering a stream between two
gauging stations.
LOG-NORMAL DISTRIBUTION
This is a transformed normal
distribution in which the variate is replaced by its logarithmic value. This is
a screw distribution of unlimited range in both directions.
LOG PEARSON TYPE III DISTRIBUTION
This is one of the
commonly used frequency distribution functions for the prediction of extreme
flood values. In this distribution, the variate is first transformed into
logarithmic form and the transformed data is then analysed.
LOSING STREAM
Stream which contributes water to the
groundwater by infiltration.
LOST RIVER
The term is applied to a stream that
disappears completely underground in a limestone terrain.
LYSIMETER
Tanks with pervious
bottom commonly used for determining evapotranspiration of crops and natural
vegetation by growing the plants in them and measuring the loss of water
necessary to maintain the growth satisfactorily.
MASS CURVE
A curve with values of
cumulative rainfall or runoff etc. plotted against time.
MASS TRANSFER METHOD
This is one of the analytical
methods for the determination of lake evaporation. This method is based on
theories of turbulent mass transfer in boundary layer to calculate the mass
water vapour transfer from the surface to the surrounding atmosphere.
MAXIMUM INTENSITY OF FLOOD
Also called momentary flood
peak, it is the maximum instantaneous rate of flow during a period.
MAXIMUM KNOWN FLOOD
The highest flood which has occurred
within the memory of the inhabitants of a region.
MAXIMUM POSSIBLE FLOOD
It is the greatest flood to be
expected assuming complete coincidence of all factors that would produce
heaviest rainfall and maximum runoff.
MAXIMUM POSSIBLE PRECIPITATION
The maximum amount of
precipitation that can theoretically occur for a certain duration in a drainage
area or basin during the present climatic era.
MAXIMUM PROBABLE FLOOD
The extreme flood that is
physically possible in a region as a result of severe most combinations,
including rare combinations of meteorological and hydrological factors.
MEAN ANNUAL EVAPORATION
The mean value in depth units of
evaporation, the period of observation being of adequate duration to secure
approximate constancy.
MEAN ANNUAL FLOOD
It is defined as a flood having a
recurrence interval of 2.33 years. This is based on Gumbel's distribution which
has the property that gives T = 2.33 years for the average of the annual series
when N is very large.
MEAN ANNUAL PRECIPITATION
The mean of annual amount of
precipitation observed over a period which is sufficiently (say 30 years or
more) to produce a fairly constant mean value.
MEAN MONTHLY RUNOFF
The value of the monthly volume
of water discharged by the stream draining the area, the period of observation
being sufficiently long to secure a fair mean.
MEASURING SECTION
Cross-section of an open channel in
which measurements of depth and velocity are made.
MEDIAN
Middle value of the variate which divides
frequencies in a distribution into two equal portions.
METEOROLOGY
Science of the
atmosphere.
MINIMUM ANNUAL FLOW
The smallest of the annual flows
during the period of record.
MODEL
Model with different scale in
different directions.
MOISTURE ADJUSTMENT FACTOR
It is the ratio of the
maximum total moisture in an atmospheric column of unit cross section in the
region to the total moisture in a similar column that occurred during the
storm.
MOISTURE TENSION
Under-pressure to which water must be
subjected in order to be in hydraulic equilibrium, through a porous permeable
wall or membrane, with the water of the soil, usually expressed in cm of water
or mm of mercury.
MONSOON
It is a wind system with an
annual oscillation, blowing from oceans to continents in summer and in the
reverse direction in winter. These oscillations, which are in response to the
annual heating and cooling of the underlying surface, are quite general, though
the Indian monsoon is most widely known, mainly because of excessive dryness
during the winter and the equally excessive rainfall during the summer
season.
MOVING AVERAGE ANALYSIS
It is a method of flood routing
through a channel. The method involves the concept of wedge and prism
storage.
MULTIPLE CORRELATION
Analysis of the interdependence of
more than two variables.
MULTI-PURPOSE PROJECT
Project designed, constructed and
operated to serve more than purpose, e.g. flood control, hydroelectric power,
navigation, irrigation, fisheries, water supply, recreation.
MUSKINGUM METHOD
It is a method of flood routing through
a channel. The method involves the concept of wedge and prism storage.
NATURAL CONTROL
Reach of a
stream channel where natural conditions exist that the water level upstream a
stable index of the discharge.
NET STORM RAIN
Portion of rainfall during a storm which
reaches a stream channel as direct runoff.
NIPHER SHIELD
Wind shield for precipitation gauges,
shaped like an inverted cone, with base of the cone level with the lip of the
gauge.
NON-RECORDING
RAINGAUGE
These gauges do not produce a continuous plot of rainfall
against time but measure only the total depth of precipitation due to a
particular storm.
NON-UNIFORM FLOW
Flow in which the velocity vector is
not constant along every streamline.
NORMAL
DISTRIBUTION
Mathematically defined, symmetrical, bell-shaped, continuous
probability distribution traditionally assumed to represent random
errors.
NORMAL RAINFALL
Evan value
of rainfall taken over a period of such length that the mean over any longer
period does not significantly effect the value obtained. It is used as a
standard of comparison. It is generally taken as the average value of rainfall
at a particular date, month or year over a specified 30 year period.
NORMAL RATIO METHOD
It
is a method of estimating the missing data of precipitation at a station by
weighing the precipitation at various neighbouring stations by the ratios of
normal annual precipitation. This method is used when the normal annual
precipitation of the station having missing record differs from the normal
annual precipitation of the stations having known record by more than
10%.
NORTH-EAST MONSOON
It is north easterly flow of air that
picks up moisture in the Bay of Bengal and this air mass then strikes the east
coast of the southern peninsula (Tamilnadu) and causes rainfall.
N-YEAR FLOOD
A flood which has a probability of being
equaled or exceeded once in N years or has one chance in N of occurring in any
one year.
OPEN CHANNEL FLOW
Flowing water having its surface
exposed to the atmosphere.
OPTIMAL DESIGN
System design based on the selection or
combination of all pertinent variables so as to maximize some objective function
(such as net benefits) with the requirements of the design criteria.
OPTIMAL YIELD
Amount of water which can be withdrawn
annually from an aquifer or from a basin according to some pre-determined
criterion of optimal exploitation.
OROGRAPHIC
PRECIPITATION
Precipitation caused by dynamic cooling of air as an air
current rises over a mountain barrier.
OUTFALL
Lowest point on the boundary
of a drainage system.
OVER BANK FLOW
The portion of stream flows which exceed
the carrying capacity of the normal channel and overflow the adjoining
OVERDRAFT
Amount of water withdrawn from
a water resources system in excess of the safe yield.
OVERLAND FLOW
The flow of water over the land surface
towards stream channels before it becomes channelised is called overland
flow.
PAN COEFFICIENT
It is the
ratio of actual evaporation to the observed evaporation in evaporation pan. Evaporation
pans do not give exact evaporation values because
(i) they differ in the
heat storing capacity and heat transfer from the sides and bottom
(ii) the
height of rim in the pan affects wind action on the surface
(iii) heat
transfer characteristics of the pan material is different from that of the
reservoir.
PARTIAL DURATION SERIES
Also called partial series, it
is a series of data which are so selected that their magnitude is greater than a
certain base value.
PARTIALLY PENETRATING WELL
Well in which the length
of water entry is less than the thickness of the saturated aquifer which it
penetrates.
PEAK DISCHARGE
Maximum
instantaneous discharge of a given hydrograph.
PEARSON DISTRIBUTION
Group of probability
distribution of varying skewness and other properties which were proposed by
karl pearson and which are sometimes fitted to hydrological data.
PELLICULAR WATER
Water in the zone of aeration is held
against gravity most of the time, in capillary interstices and in thin films
over surfaces of the grains due to strong molecular attraction of the solid
particles at the solid/liquid interface.
PERCENT RUNOFF
The amount of runoff expressed as
percentage of total rainfall on a given area.
PERCENT NORMAL METHOD
It is a method of finding average
precipitation in mountaineous areas where arithmetic means and Thiessen weights
can not be applied accurately. In this method, storm precipitation values at
each station can be expressed in percent of its annual normal, and these
percentage values are averaged for the basin. The basin normal annual
precipitation multiplied by this storm percent value provides an average storm
precipitation. Use of this percent normal method reduces the need for a
consistent reporting network.
PERCHED WATER
TABLE
Sometimes a lens or localised patch of impervious strata can occur
inside an unconfined aquifer in such a way that it retains water table above the
general water table. Such a water table retained around the impervious material
is known as perched water table.
PERCOLATION
It is flow through a
porous substance.
PERENNIAL STREAM
A
perennial stream is one which always carries some flow. There is considerable
amount of ground water flow throughout the year. Even during dry seasons the
water table will be above the bed of the stream.
PERMANENT CONTROL
If the
stage discharge relationship for a gauging section is constant and does not
change with time, the control is said to be permanent control. A majority of
streams and rivers, especially non alluvial exhibit permanent control.
PERMANENT WILTING PERCENTAGE
Moisture content of the
soil at which the leaves of plants growing in that soil become permanently
wilted.
PERMEABILITY
Permeability of a
rock or soils defines its ability to transmit a fluid. This is property of the
medium only and is independent of fluid properties.
PERMEABILITY COEFFICIENT
It is defined as the rate of
discharge per unit cross sectional area of a porous medium under unit hydraulic
gradient.
pH
Absolute value of the decimal logarithm of the
hydrogen-ion concentration (activity). Used as an indicator of acidity (pH<7)
or alkalinity (pH > 7).
PERMEAMETER
It is an instrument for measuring
coefficient of permeability of a porous medium.
PHI-INDEX
The index is an average rate of infiltration
derived from a time intensity graph of rainfall in such a manner that the volume
of rainfall in excess of this rate will equal the volume of storm runoff.
PHYTOMETER
Device for measuring transpiration,
consisting of vessel containing soil in which one or more plants are rooted and
sealed so that water can escape only by transpiration from plants.
PIEZOMETRIC HEAD
(1)
Elevation to which water will rise in a piezometer connected to a point in an
aquifer.
(2) Sum of the elevation and pressure head in a liquid, expressed
in units of height.
PLOTTING POSITION
The
purpose of frequency analysis of an annual series is to obtain a relation
between the magnitude of the event and its probability of exceedence. The
exceedence probability of the event obtained by the use of an empirical formula
is called, plotting position. Various empirical formulae used are California,
Hazen, Weibull etc.
POINT RAINFALL
Rainfall at
a particular site recorded by a raingauge.
POROSITY
Volume of voids filled
with stagnant water, which practically does not participate in the general flow,
per unit gross soil volume, voids inclusively.
POTAMOLOGY
That branch of hydrology which pertains to
surface streams, the science of rivers.
POTENTIAL
EVAPOTRANSPIRATION
The amount of water utilised by plant growth including
evaporation from the soil if the soil contains sufficient moisture for plant
growth at all times.
PRECIPITABLE WATER
It is the total amount of water
vapors in the atmosphere, frequently expressed as depth of precipitable water.
This term is a misnomer, since no natural precipitation process removes all the
moisture from the air.
PRECIPITATION
The total supply of water derived from the
atmosphere in the form of rain, snow, dew, mist, frost, hail, sleet etc. It is
usually expressed as depth of liquid water on a horizontal surface in a day,
month or year and designated so daily, monthly or annual precipitation.
PRISM STORAGE
It is that
portion of the total channel storage during a flood which corresponds to a
condition of steady flow that is when inflow and outflow are equal. It is the
volume formed by an imaginary plane parallel to the channel bottom drawn at the
outflow section water surface.
PROBABILITY
Basic statistical
concept either expressing in some way a "degree of belief" or taken as a
limiting relative frequency of occurrence in an infinite series.
PROBABILITY
DISTRIBUTION
Distribution given the probability of a value of a variate
as a function of the variate.
PROBABILITY PAPER
Graph
paper designed in such a way that the cumulative probability of a theoretical
distribution plots as a straight line, e.g. normal probability paper, log-normal
probability paper, extreme value probability paper.
PROBABLE MAXIMUM FLOOD
It is defined as that flood
which is estimated to result, if the most critical combination of severe
meteorological and hydrological conditions considered reasonably possible in the
region, were to occur.
PROBABLE MAXIMUM
PRECIPITATION
It is the theoretically greatest depth of precipitation for
a given duration that is physically possible over a particular drainage basin at a particular
season.
PROMPT INTERFLOW
It is the
interflow with least time lag between infiltration and outflow.
PSYCHROMETER
A hygrometer, or instrument for measuring
the aqueous vapour in the atmosphere, consisting essentially of two similar
thermometers, the bulb of one being kept wet.
PUMPING TEST
Extraction of water from a well at one or
more selected discharge rates, during which piezometric or phreatic levels are
measured regularly at the pumped well and at nearby observation wells, the data
are used for determining the aquifer parameters in the vicinity of the pumped
well.
QUICK RETURN FLOW
Also
called interflow, it is a part of precipitation which after infiltration moves
laterally through upper crusts of the soil and returns to the surface at some
location away from the point of entry into the soil.
RADIOACTIVE DATING
Method of age determination based on
the property of radioactive decay of isotopes.
RADIOACTIVE TRACER
Radioactive material detectable by
its nuclear radiation and suitable for water tracing even at very low
concentrations.
RADIUS OF
INFLUENCE
Radial extent of the cone of depression is called radius of
influence. It is the distance between centre of well and outer periphery of
RAIN GAUGE
Also called pluviometer, ombrometer and
hyetometer, it is an instrument for measuring the quantity of rain that falls at
a given place and time.
RAIN SHADOW
A region on the leeward side of a mountain
or mountain range where the rainfall is much less than one the windward
side.
RAINDROP EVAPORATION
Evaporation from the raindrops in
the process of their fall from the atmosphere to the earth.
RAINFALL
The total liquid products
of precipitation or condensation from the atmosphere as received and measured in
a rain gauge.
RAINFALL DISTRIBUTION COEFFICIENT
The distribution
coefficient for any storm is the ratio of the maximum rainfall at any point to
the mean rainfall in the basin.
RAINFALL EXCESS
Also called
net rainfall or effective rainfall, it is part of the rainfall that appears as
runoff in the stream.
RAINFALL INTENSITY
The
amount of rainfall occurring in a unit interval of time, generally expressed in
mm per hour.
RAINFALL SIMULATOR
It is
one type of infiltrometer in which water is applied in the form of natural rain
and at rates comparable with natural rainfall. Specially designed nozzles
produce raindrops falling from a height of 2 m and are capable of producing
various intensities of rainfall.
RANDOM PROCESS
Stochastic
process in which the numbers of the time series are independent among
themselves.
RATING CURVE
A curve showing
the relation between stage and discharge of stream at a given gauging
station.
REACH
Length of open channel between
two defined cross sections.
RECHARGE
It is a natural or
artificial process by which water is added from outside to the zone of
saturation of an aquifer, either directly into a formation or indirectly by way
of another formation.
RECORDING
RAINGAUGE
Recording raingauges produce a continuous plot of rainfall
against time and provide valuable data of intensity and duration of rainfall for
hydrological analysis of storms. Tipping bucket type raingauge gives intensity
of rainfall whereas weighing bucket type raingauge gives mass curve of
rainfall.
RECOVERY TEST
Pumping test consisting of the measurement
at predetermined time intervals, of the rise of the piezometric level or water
table in a pumped well or in the surrounding observation wells after stoppage of
pumping.
RECURRENCE
INTERVAL
Statistical parameter used in frequency analysis as a measure of
most probable time interval between occurrence of a given event and that of an
equal or greater event.
REGRESSION ANALYSIS
Statistical method developed to
investigate the interdependence or relationship between two or more measurable
variates. The most common form of regression analysis is linear
regression.
REGRESSION CURVE
The falling limb, after the point of
contra flexure, of hydrograph after a flood event. This represents withdrawal of
water from storage in the valley, stream channel and the subsurface
runoff.
RELATIVE HUMIDITY
At a given pressure and temperature,
the percentage ratio of the mole fraction of the water vapour to the mole
fraction that the air would have if it were saturated with respect to water at
the same pressure and temperature.
REPRESENTATIVE BASIN
Basin in which hydrological
stations are installed to make simultaneous hydrometeorological and hydrometric
observations so that the measurements would represent a broad area in lieu of
making measurements on all basins in a given region.
RESERVOIR
Body of water, either
natural or man-made, used for storage, regulation and control of water
resources.
RESERVOIR RELEASE RULES
Rules governing the way in which
volumes of water are released from a reservoir in order to meet demand,
downstream protection, expected future low flows and other
considerations.
RESERVOIR ROUTING
The
routing of a flood wave through a reservoir.
RESIDUAL MASS CURVE
Graph of the cumulative departures
from a given reference, such as the arithmetic average, versus time or
date.
RETARDING RESERVOIR
A reservoir wherein water is stored
for a relatively brief period of time, part of it being retained until the
stream can safely carry the ordinary flow plus the released water.
RETENTION
That part of the
precipitation falling on a drainage area which does not escape as surface stream
flow during a given period. It is the difference between total precipitation and
total runoff during the period and represents evaporation, transpiration,
subsurface leakage, infiltration and when short periods are considered,
temporary surface and underground storage on an area. When periods of several
years are considered, it approximates consumptive use.
RETENTION CURVE
Graph representing the suction pressure
versus the moisture or water content, in an unsaturated soil.
RETURN FLOW
That portion of the
water, diverted from a river or stream, which ultimately finds its way back
through surface runoff (visible flow) and as percolation or seepage through the
bed and banks (invisible flow).
RETURN PERIOD
Also called
recurrence interval, it is statistical parameter used in frequency analysis as
measure of most probable time interval between occurrence of a given event and
that of an equal or greater event.
RISING LIMB
The ascending
portion of a hydrograph.
RIVER STAGE RECORDER
These are instruments used for
measuring discharge in a river at a particular gauging station. They record the
water surface elevation in the river above the datum which is related to the
discharge in the stream.
ROOT ZONE
Layer of soil containing plant roots.
RULE CURVE
A curve devised to indicate operation of a
reservoir so as to obtain the best results based on past experience and to be
applied to future operation with a view to attain best use of the reservoir for
its intended purposes.
RUNOFF
It is defined as that portion of precipitation
which is not absorbed by the deep strata but finds its way into the streams
after meeting the persistent demands of evapotranspiration including
interception and other losses. It includes surface runoff received into the
channels after rainfall, delayed runoff that enters the streams after passing
through portion of the earth and other delayed runoff that has been temporarily
detained as snow cover or stored in natural lakes or swamps.
RUNOFF COEFFICIENT
The ratio of runoff to precipitation.
S-CURVE
A graph showing the summation of the ordinates
of a series of unit hydrographs spaced at unit rainfall duration interval. It
represents the hydrograph of average rate of rainfall excess of the unit
duration continued indefinitely.
SAFE YIELD
It is defined as the amount of water which
can be withdrawn annually from a
SALINITY
Measure of
the concentration of dissolved salts, mainly sodium chloride, in saline water or
sea water.
SALT-WATER INTRUSION
Phenomenon occurring when a body of
salt water invades a body of fresh water. It can occur either in surface or
groundwater bodies.
SATURATION VAPOUR
PRESSURE
Maximum possible partial pressure of water vapour in the air or
atmosphere at a given temperature.
SEDIMENT LOAD
Also called sediment discharge, it is the
quantity of sediment (suspended load and
SEDIMENTATION
It is the process involving settling and
deposition of sediments in water by gravity.
SEEPAGE
Slow percolation generally associated with flow
in an unsaturated medium. Seepage into a body is termed "influent seepage" and
that away from a body as "effluent seepage". The difference between percolation
and seepage is that the latter is through unsaturated material while the former
is through saturated material.
SEMI ARID
A term applied to a zone or
SEMI-CONFINED AQUIFER
Aquifer overlain and/or underlain
by a relatively thin semi-pervious layer, through which flow into or out of the
aquifer can take place.
SEMI HUMID
Land or climate, neither entirely arid nor
strictly humid, with pronounced tendency towards humid character.
SHIFTING CONTROL
If stage discharge relationship for a
gauging section changes with time, it is called shifting control. The
relationship can change due to
(i) changing characteristics caused by weed
growth, dredging or channel encroachment
(ii) aggradation or degradation
phenomenon in an alluvial channel
(iii) unsteady flow effects of a rapidly
changing stage
(iv) variable back water effects affecting the gauging
station.
SHORT-TERM HYDROLOGICAL FORECASTING
Forecast of the
future value of an element of the regime of a water body for a period ending up
to two days from the issue of the forecast.
SLEET
It is frozen raindrop or transparent grains which
form when rain falls through air at subfreezing temperature. In Britain, sleet
denotes precipitation of snow and rain simultaneously.
SLOPE-AREA METHOD
It is a very versatile indirect method
of discharge estimation which requires
(i) selection of a reach in which
cross sectional properties including bed elevations are
known
at its ends
(ii) the value of Manning's N and
(iii) water surface elevations at the two end sections.
SMALL WATERSHED
A small watershed may be defined as one
that is so small that its sensitivity to high intensity rainfall of short
duration and to land use are not suppressed by the
SNOW
Precipitation from the atmosphere
in the form of branched hexagonal crystals or stars, often mixed with simple ice
crystals, which fall more or less continuously from a solid cloud sheet. These
crystals may fall either separately or in coherent clusters forming snow
flakes.
SNOW COARSE
A snow course consists of a series of
sampling points, usually not fewer than 10. The points are located along a
predetermined geometric pattern at a spacing of 50 to 100 ft. The ends and
pivots of the pattern are permanently marked to make certain that the snow is
surveyed at the same locations year after year.
SNOW GAUGE
An instrument used to measure the amount of
snowfall. Weighing type storage gauges operate in mountainous regions for 1 to 2
months without servicing while some non recording storage gauges are designed to
operate for an entire season without attention.
SNOW MELT
It is the transformation of snow into liquid
water.
SNOW PACK
Field of naturally packed snow that ordinarily
melts slowly and yields water during early summer months.
SOFT WATER
Water without significant
hardness.
SOIL CONSERVATION
Soil
conservation implies all the effective measures taken to reduce the amount of
soil erosion. Erosion resulting from intense rainfall can be reduced by (i)
protecting the soil surface from raindrop impact through the use of crop cover
or mulch and (ii) reduction of surface runoff by encouraging infiltration
through proper selection of cover.
SOIL MOISTURE DEFICIT
The amount of water that should be
applied to the soil to cause thorough drainage and is substantially equal to the
soil moisture deficit then existing.
SOIL WATER ZONE
This zone
lies close to the ground surface in the major root band of the vegetation from
which water is lost to the atmosphere by evapotranspiration.
SOUTH-WEST MONSOON
It is
south westerly flow of air which originates in the Indian Ocean, picks up
moisture and advances over India in two branches i.e. the Arabian sea branch and
the Bay of Bengal branch. The former sets in at the extreme southern part of
Kerala and the latter in Assam, almost simultaneously in the first week of June.
This is the principle rain causing monsoon in India.
SPECIFIC CAPACITY
Specific capacity of a well is the
discharge per unit drawdown and is a measure of the performance of the well. For
a given well, it is not constant but decreases with increase in discharge and
time.
SPECIFIC RETENTION
It is
a ratio, expressed as a percentage, of the volume of water retained in the soil
against the force of gravity, after complete saturation, to the volume of the
soil.
SPECIFIC YIELD
It is a ratio,
expressed as percentage, of the volume of water which is drained by the force of
gravity, after complete saturation, to the volume of the soil.
SPILLWAY DESIGN FLOOD
Design flood used for the specific
purpose of designing the spillway of a storage structure. This term is
frequently used to denote the maximum discharge that can be passed in a
hydraulic structure without any damage or serious threat to the stability of the
structure.
SPRING
Spring is a natural discharge
point where ground water issues from soil or rock in concentrated flow.
STABLE CHANNEL
Channel in which accretion balances scour
on the average.
STAGE
The stage of a river is defined
as its water surface elevation measured above a datum. This datum can be the
mean sea level or any arbitrary datum connected independently to the mean sea
level.
STAGE HYDROGRAPH
The stage
data, often presented in the form of a plot of stage against chronological time
is known as stage hydrograph. A plot of stage versus time.
STAGE-DISCHARGE
CURVE
The relationship between the stage in a river and the corresponding
discharge at the particular section is represented in the form of a curve called
Stage-discharge curve.
STANDARD DEVIATION
Widely
used measure of dispersion of a frequency distribution or of a set of
values.
STANDARD ERROR
Positive square root of the variance of
the sampling distribution of a statistic.
STANDARD PROJECT FLOOD
The flood resulting from the most
severe combinations of meteorological and hydrological conditions considered
reasonably characteristic of the region. Extremely rare combination of factors
are excluded.
STANDARD PROJECT STORM
The standard project storm
estimate for a particular drainage area and season for year in which snow melt
is not a major consideration should represent the most severe flood producing
rainfall depth area duration relationship and isohyetal pattern of any storm
that is considered reasonably characteristic of the region in ;which the
drainage basin is located, giving consideration to the runoff characteristics
and existence of water regulation structure in the basin.
STATION YEAR METHOD
This is a method of extending the
length of record for a frequency curve at a station, based on the assumption
that records for the same or different periods of records at a number of
stations may be considered as a composite record for a single station for a
period equal to the total number of years involved. For example, if 50 years of
record is available for 100 stations, it might be assumed that this was
equivalent to 5000, years at a single station.
STATISTICAL HYDROLOGY
Hydrological processes and
phenomena which are described and analyzed by using the methods of probability
theory.
STEM FLOW
It is that part of rainfall that, having been
intercepted by the canopy, reaches the ground by running down the stems.
STOCHASTIC SYSTEM
It is a system in which the various
processes are dependent on chance as well as time. All the hydrologic data is,
more or less, stochastic in nature.
STORAGE COEFFICIENT
It is a formation constant of an
aquifer which represents the volume of water released by a column of
STORAGE TIME CONSTANT
It is a coefficient used in the
Muskingum equation of channel routing. It is approximately equal to the time of
travel of a flood wave through the channel reach. It has the dimensions of
time.
STORM
It is a term commonly used for
violent atmospheric motion, such as gale, thunderstorm, rainstorm, snowstorm or
dust storm.
STORM INTERVAL
The interval of time from the beginning
of a rain, through the rain period and the subsequent dry or rainless period to
the beginning of the next subsequent rain period.
STORM
MAXIMIZATION
Procedure used to derive probable maximum
precipitation.
STORM RUNOFF
It is that part of
runoff which enters the stream immediately after the precipitation. It includes
surface runoff, prompt interflow and
STORM SEEPAGE
A part of
precipitation that in filters, more literally through upper crust of the soil
and returns to the surface at some location away from the point of entry into
the soil. This component of runoff is known variously as storm seepage or
interflow or throughflow or storm flow.
STORM TRACK
The path traversed by the centre of the
storm.
STREAM
A natural channel for
conveying water.
STREAM ORDER
It is a
classification of river basin reflecting the degree of branching or bifurcation
within a basin. A first order stream has no tributary. A second order stream has
two or more tributaries of the next lower order. A catchment is described as
first, second or higher order depending on the stream order at the
outlet.
STREAMFLOW
It is movement of
water under the force of gravity through well defined surface channel. It is the
total runoff confined in stream channel.
SUBSURFACE FLOW
Any flow
below the surface of the ground which may contribute to
SURFACE RETENTION
Also known as surface storage or
initial detention, it refers to that part of precipitation which does not appear
either as infiltration or as surface runoff during the period of infiltration or
as surface runoff during the period of precipitation or immediately thereafter.
Thus surface retention includes interception by vegetal cover,
SURFACE RUNOFF
The water
which reaches the stream by travelling over the soil surface or falls directly
into the stream channels.
SURFACE WATER HYDROLOGY
That branch of hydrology which
deals with hydrological phenomena and processes which occur on the Earth's
surface, emphasizing overland flows.
SUSTAINED RUNOFF
Also known as base flow or fair weather
flow, it is composed of ground water runoff and delayed subsurface
runoff.
SYNTHETIC UNIT HYDROGRAPH
A unit hydrograph developed on
the basis of estimation of coefficients expressing various physical features of
a catchment.
TELEMETERING RAINGAUGE
These raingauges are of the
recording type and contain electronic units to transmit the data on rainfall to
the base station both at regular interval or on interrogation. The tipping
bucket type raingauge, being ideally suited, is usually adopted for this
purpose. Telemetering gauges are of utmost use in gathering rainfall data from
mountainous and generally inaccessible places.
TENSIOMETER
It is a device used for measuring soil water
tension. It operates only up to about 0.85 atm. tension.
THIESSEN POLYGON
The points
of location of rain gauges on a map are joined by straight lines and their
perpendicular bisectors are drawn. The polygon formed around each raingauge
station by these perpendiculars is called, after its originator, a Thiessen
polygon.
THROUGHFALL
Also known as
interflow or storm seepage, it is that part of precipitation which after
infiltration moves laterally through upper crusts of the soil and returns to the
surface at some location away from the point of entry into the soil.
TIME BASE
Time base of a
hydrograph is considered to be the time from which the concentration curve
begins until the direct runoff component essentially reaches zero.
TIME OF
CONCENTRATION
The time taken by the runoff from the farthest point of the
catchment to reach the point under consideration.
TIME-AREA HISTOGRAM
It is a bar plot of inter isochrone
area Vs. time. If a rainfall excess of 1 cm occurs instantaneously and uniformly
over the
TORNADO
A rotary storm, one of the most violent type of
storms known, of small diameter, which travels across the country and leaves
great devastation along a narrow path. Its chief characteristics are
(a)
Under a heavy cumulonimbus cloud there hangs a funnel shaped cloud, which marks
the
vertex and, as the storm moves along, may or may
not touch the earth.
(b) Heavy precipitation and usually
TORRENT
A stream of water flowing with great velocity or
turbulence, as during a freshet or down a steep incline.
TRANSMISSIBILITY
It is defined as the rate of flow
through an aquifer of unit width and total saturation depth under unit hydraulic
gradient. It is equal to product of full saturation depth of aquifer and its
coefficient of
TRANSPOSITION OF STORM
It means application of a storm
from one area to some other area within the same region of meteorological
homogeneity. It requires the determination of whether the particular storm could
have occurred in the area to which it is to be transposed.
TEMPERATURE LAPSE RATE
Decrease of the air temperature
in degrees Celsius per 100 m of latitude increase.
TIME SERIES
Set of observations,
in order, taken at successive points of time, commonly at a fixed
interval.
TOTAL DISSOLVED SOLIDS
Total weight of dissolved mineral
constituents in water per unit volume (or weight) of water in the sample.
TRAP EFFICIENCY
It is the ability of the reservoir to
trap and retain sediment and is expressed as the percent of sediment yield,
which is retained in the basin. It increases with the increase in the capacity
of the reservoir but it decreases with outflow discharge increase.
TREND
Unidirectional, monotonous
(diminishing or increasing) change in the average value of a hydrological
variable.
TRIBUTARY
Watercourse flowing into a larger watercourse
or into a lake.
TROPICAL CYCLONE
Also
called cyclone in India, hurricane in U.S.A. and typhoon in south east Asia, it
is a wind system with an intensely storm depression with msl pressures sometimes
below 915 mbars. Normal and areal extent is about 100 200 Km in diameter.
Isobars are closely spaced and winds are anti-clock wise in northern hemisphere.
Rainfall is normally heavy in the entire area occupied by the cyclone.
ULTRASONIC FLOW
METER
The device, which estimates the discharge in a stream, depends for
its operation upon the Doppler effect of ultrasonic waves passing through water.
A transmitter directs a signal towards a receiver some distance upstream. The
ultrasonic waves moving upstream are compressed, those returning are attenuated.
The magnitude of this effect can be recorded and related to water
velocity.
UNCONFINED AQUIFER
Also
called non-artesian aquifer, it is a water bearing strata having no confined
impermeable overburden. In this aquifer, water table varies in undulating form
and in slope depending on areas of recharge, pumpage from wells and
permeability.
UNDERFLOW
Movement of water through a pervious stratum
under the bed of a river.
UNIT HYDROGRAPH
Hydrograph of storm runoff at a given
point on a given stream which will result from an isolated rainfall excess of
unit duration occurring over the contributing drainage area and resulting in a
unit of runoff.
UNIT RAINFALL DURATION
The duration of runoff producing
rainfall or rainfall excess that results in a unit hydrograph.
UNSATURATED ZONE
That portion of the lithosphere in
which the interstices are filled partly with air and partly with water, held or
suspended by molecular forces.
UNSTABLE CHANNEL
Channel in which stage-discharge
relation changes in the course of time.
UPSTREAM
In the direction opposite
to the main current.
UNSTEADY FLOW
Flow in
which, the velocity changes in magnitude or direction with respect to
time.
URBAN HYDROLOGY
It is defined as the interdisciplinary
science of water and its interrelationships with urban man. It is the study of
hydrological processes both within and outside the urban environment that are
affected by urbanisation.
VADOSE WATER
Water in the zone of aeration is called
vadose water.
VALLEY STORAGE
(a) The volume below the water surface
profile.
(b) The natural storage capacity or volume occupied by a stream in
flood after it has over
flown its banks. It
includes the channel storage and lateral storage.
VIRGIN FLOW
It is the stream
flow unaffected by artificial divergence, storage of other works of man in or on
the stream channels or in the drainage basin or watershed.
W-INDEX
The average rate of infiltration during the time
the
WADING ROD
A graduated rod to which a current meter is
attached for measuring the velocity in shallow water. This rod measures the
depth below water surface at which velocity is being measured.
WARNING STAGE
The river stage at which it is necessary
to begin issuing warnings of river forecasts to enable adequate precautionary
measures to be taken to avoid damage or inconvenience due to flooding.
WATER BODY
Mass of water distinct from other masses of water.
WATER BUDGET METHOD
It is an analytical method for the
determination of evaporation. It is measurement of continuity of flow of water.
This holds true for any time interval and applies to any drainage basin and to
the earth as a whole. It involves writing the hydrological continuity equation
for the reservoir and determining the evaporation from a knowledge or estimation
of the variables.
WATER CONSERVATION
Measures introduced to reduce the
amount of water used for any purpose and/or to protect it from pollution.
WATER EQUIVALENT OF SNOW
It means depth of water which
would result from melting of snow. It depends on the snow density as well as its
depth.
WATER LOGGING
Condition of land when the
WATER RESOURCES
Supply of
water in a given area or basin interpreted in terms of availability of surface
and underground water.
WATER RESOURCES MANAGEMENT
Planned development,
distribution and use of water resources.
WATER TABLE
Water table is the
upper surface of the zone of saturation at atmospheric pressure. It is the level
at which water stands in a well penetrating the unconfined
aquifers.
WATER YEAR
Continuous twelve
month period selected for maintaining or presenting records of flow, and/or use
of water of any river system.
WATERSHED
The terms drainage basin
and watershed are often considered synonymous but strictly speaking, a watershed
is the divide separating one drainage basin from another.
WATERSHED LEAKAGE
The geological formation under many
drainage basins is such that precipitation falling on one basin finds its way
underground through fissures and water bearing strata to an outlet either in a
nearby or a remote drainage basin, or directly to the sea. This is called
watershed leakage.
WATERSHED MANAGEMENT
Management of watershed to conserve
soil and water requires that the land be used within its capabilities and
treated according to its need. The objectives are to protect the land against
all forms of soil deterioration, to rebuild eroded and depleted soils, to build
up soil fertility, to stabilise critical runoff and sediment producing areas, to
improve grass lands, woodlands and wild life lands, to conserve water for
beneficial use, to provide needed drainage and irrigation and to reduce flood
land sediment drainage.
WEDGE STORAGE
It is the wedge
like volume formed between the actual water surface profile and the top surface
of the prism storage. It is the difference between the total channel storage and
the prism storage.
WEIBULL
DISTRIBUTION
Fisher-Tippett Type III External distribution used usually
in drought studies.
WELL
A water well is a hole or shaft,
usually vertical, excavated in the earth for bringing ground water to the earth.
Occasionally, wells serve other purposes such as subsurface exploration and
observation, artificial recharge and disposal of waste waters.
WELL LOSS
It is the head loss caused by flow through the
well screen and flow inside of the well to the pump intake.
WET YEAR
Year in which precipitation or stream flow is
significantly above normal.
WILTING COEFFICIENT
The moisture content of the soil
expressed as a percentage of the dry weight at the time when the leaves of a
plant growing in the soil first undergo a permanent reduction in their moisture
content, as a result of the deficiency in the soil moisture
supply.
WIRE GAUGE
This is used for
determining the stage of a river by lowering a weight to the water surface. It
is mostly attached on the bridges and is read by means of a mechanical counter
attached to the reel on which wire is wound.
YIELD
Total volume or flow from a drainage basin for a
long stipulated period of time, for example annual yield of drainage basin is
the mean annual runoff.
ZERO MOISTURE INDEX
The index of moisture when the
precipitation is just adequate to supply all the water that would be needed for
maximum evaporation and transpiration in the course of a year.
ZONE OF AERATION
That
portion of lithosphere in which the interstices are filled partly with water
which is held or suspended by molecular forces.
ZONE OF SATURATION
That
part of lithosphere in which the pores are completely filled with water and
pressure is at or above atmospheric.