Hydrology and Ground Water

8/8/2019 Hydrology and Ground Water

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Dr. Murari R. R. VARMA

Department of Civil Engineering

Thapar University, Patiala

CE 004Hydrology and Ground Water


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Drainage basin (watershed, catchment)

- Drains surface water to a common outlet

Drainage divide - how is it defined?


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Hydrographs

A stream flow or discharge hydrograph is a graph or table

showing the flow rate as a function of time at a given

location on the stream.

OR

A hydrograph is a graph showing discharge (i.e., stream

flow at the concentration point) versus time

A hydrograph is the response of a given catchment to a

rainfall


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Types

Long term Hydrographs(Annual, Monthly, Seasonal) Calculating Surface water Potential of a stream

Reservoir Studies

Drought studies

Storm or flood hydrograph Results from an isolated storm Single-peaked skewed distribution of discharge

Comprised of all runoff components namely surface, inter flow andbase flow

Hydrographs complex multiple peaks, kinks

Simple hydrographs resulting from isolated storms used foranalysis.


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Chow, 1988


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Chow, 1988


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Chow, 1988


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Factors affecting flood hydrograph

Physiographic factors

Basin Characteristics

Size

Times base larger for larger catchments

Shape

Fan shaped (high peak and narrow hydrographs) Elongated (broad and shallow peaks)

Slope

Time base smaller - steeper recession curves higher slopes

Nature of the valley

Elevation

Drainage density Ratio of total channel length to total drainage area higher DD, higher peaks.


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Physiographic factors

Infiltration characteristics Land use and land cover

Soil type and Geological conditions

Lakes, swamps and Other storages

Channel Characteristics CS, Roughness and Storage capacity

Climatic Factors

Storm Characteristics

Precipitation, intensity, duration, magnitude and movement of storm

Initial loss

Evapotranspiration


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Chow, 1988


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Hydrograph Components Rising Limb or curve

Controlled by basin and climatic factors

Crest Segment

Recession limb or curve

Controlled by catchment characteristics

Due to depletion of Storage (surface and channel, inteflow, groundwater)

Qt = Q0 Krt Barnes (1940) or Qt = Q0 e

-at where a = ln kr

Kr =recession constant of a value less than unity

Kr= krs . Kri. Krb

Krs varies 0.05 to 0.20Kri varies from 0.5 to 0.85

Krb varies 0.85 to 0.99


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Raghunath, 2006


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Base flow Separation

Raghunath, 2006


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DRH Direct Runoff Hydrograph

The SRH Obtained after base flow Separation is DRH

ERH Effective rainfall Hyetograph or hyetograph of rainfall

excess

Area of ERH x Area of catchment = area under a DRH=volume

of direct runoff


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Unit Hydrograph

Predict the flood hydrograph resulting from aknown storm in a catchment

Sherman 1932

The Hydrograph of direct runoff resulting from oneunit depth(usually 1cm) of rainfall excess occurringuniformly over the basin and at a uniform rate for aspecified duration( D hours).

-Unit refers to depth of rainfall excess - 1cm


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Features

- UH a lumped response of the catchment to a unit

rainfall excess ofD hr duration

- It relates only direct runoff to rainfall excess.

- Volume of water contained in the Unit

Hydrograph must be equal to rainfall excess- Area ofUH is equal to volume given by 1 cm

depth of rainfall excess over the catchment.

- Rainfall is considered to have an average intensity

of 1/D cm/h for the duration D hr of the storm.- Distribution is considered to be uniform all over

the catchment


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Basic Assumptions

Direct runoff response to a given effective rainfall in acatchment is time-invariant. DRH for a given ER in acatchment is same irrespective of

when it occurs

Linear Response Direct runoff response to rainfall excess is linear.

X1(t)->

y1(t) and X2(t)->

y2(t) X1(t) + X2(t) -> y1(t) + y2(t)

X2(t) = r X1(t) then y2(t) = r y1(t)

ER in a duration D is r times the unit depth the resultingDRH will have ordinates bearing ratio r to those of

correspondingD-

h unit hydrograph. Since the area resulting DRH should increase by ratio r the

base ofDRH will be the same as that of unit hydrograph.

Enables the method called super position to derive DRH


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if two ER ofDh duration occur consecutively,

their combined effect is obtained by

superposing the respective DRHs


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Application ofUnit Hydrograph

A DRH of a catchment can be calculated if an

appropriate UH is available

IfDh UH and Storm hyetograph are available

ERH is derived

ERH divided into M blocks ofD duration

Rainfall excess in each Dh is operated upon the UH

to obtain Various DRH curves

Ordinates of this DRH are lagged suitably to obtain

proper time sequence and then are collected and

added to obtain the nett DRH.


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Determination of DRH from known UH

and ERH


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Derivation ofUnit Hydrographs A no. of isolated Storm Hydrographs caused by short

spells of rainfall excess each of approximately sameduration. (0.90D to 1.1 Dh) are selected from acontinuously gauged runoff of the stream.

Baseflow is separated for each.

A no ofUnit Hydrographs are plotted on a common

pair of axes. Various UH Will not be identical

Adopt a mean of such curves (average to peak flowsand time peak are calculated)

Curve of best fit judged by eye is drawn


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UH

Criteria for Selecting Storm Events to

Derive UHs

Storms are isolated and occur individually;

Storm coverage should be uniform over the entire

watershed - watershed area should not be too large, say

200 ha < 5000 km

2

; Storms should be flood-producing storms ER is high,

10mm < ER < 50mm is suggested;

Duration of rainfall should be approx. 1/5 to 1/3 of basin

lag;

The number of storm events should be at least 5.


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UH

Derivation of UH (For simple ERH)

1. Analyze hydrograph and perform baseflow separation.

2. Measure the total volume of DRH in equivalent

uniform depth (EUD)

3. Find the effective rainfall such that VDRH = VERH.

4. Assume that ERHs are uniform, the UH can be derived

by dividing the ordinates of DRH by VDRH

5. The duration of the UH is the duration of ERH.

6. In rainfall-runoff analysis, the times of occurrence for

DRH and ERH are commonly made identical.


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Unit Hydrograph from a complex storm

- When suitable isolated storms are not available

- Decompose ameasured composite flood hydrograph intoits component DRHs and base flow

- A common UH of appropriate duration is assumed toexist

- Inverse of derivation of flood hydrograph from UH

Ordinates of Composite DRH

Q1 = R1u1Q2=R1u2+R2u1

Q3=R1u3+R2u2+R3u1

.

Q5 =R1u5+R2u4+R3u3

Solving by optimisation schemes by matrix methods


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UH's are derived from simple isolated storms and if

their durations do differ much ~ +- 20%D groupedunder one average duration D h.

Due to lack of data- for different durations

Other duration are derived from available durations

Two methods

1. Method of Super position

2. S curve

Unit Hydrographs of different Durations


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S-Curve Analysis (1)

S-curve is a DRH (also called S-hydrograph), resulting from acontinuous effective rainfall at a constant rate for an infinite

period.

Produced by summation of an infinite series of Dh Uhs spaced

Dh apart

After D hours, the continuous rainfall producing 1 cm (or 1

inch) of runoff every D hr would reach an equilibrium

discharge, Qs.

The equilibrium discharge, Qs, can be computed as

Qs = [A/D x 10-4

] m3

/hr


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S-Curve Analysis (2)

To derive a UH of duration D-hr from the S-curve, , obtained from D-hr UH, the S-curve is shifted to the right by T-hr. Then, the difference

between the two S-curves represents the direct runoff hydrograph

resulting from a rainfall excess of T/D cm or inches. The -hr UH then

can be easily obtained by dividing the ordinates of SA-SB by T/D.

Note: The S-curve tends to fluctuate about Qs. This means that the

initial UH does not represent actually the runoff at a uniform rate over

time. Such fluctuations usually occur because of lack of precision in

selecting UH duration. That is, the duration of the UH may differ

slightly from the duration used in calculation. Nevertheless, an average

S-curve can usually be drawn through the points without too much

difficulty.


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Use and Limitations of Unit HydrographUse

Development of Flood hydrographs for extreme rainfall

use in design of Hydraulic Structures

Extension of Flood flow records based on rainfall records

Development of Flood forecastingand warning systems

Limitations

Assumptions - uniform distribution, Intensity constant for

duration of rainfall excess

Basins above 5000 Km2 and below 200ha are not

preferred.

Precipitation must be from rainfall:snowmelt cannot be

satisfactorily represented

Large storages like tanks, Ponds , large flood bank storage

affect linear reationships

Precipitation has to uniform.


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Duration of Unit Hydrograph

Should not exceed the time of rise

Should not exceed the basin lag

Time of concentration

D~ of basin lag is ideal choice

>1200 km2 above 12 hr duration preferred.

Distribution graphs

Variation of UH introduced by Bernard (1935)

A D-h UH with ordinates representing the percentage

of surface runoff occurring in successive periods of equal

time intervals ofD-h

Useful in comparing the runoff characteristics of different

catchments.


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UH


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Synthetic Unit Hydrographs

No data ungauged catchments Remote locations

Empirical equations developed from available data is

developed for a region

Relation between basin and salient hydrograph

characteristics. UHs derived thus are called Synthetic UHs.

Methods for synthesising hydrographs for ungauged areas

have been developed by Bernard, Clark, McCarthy and

Snyder. Snyders method Appalachian mountains USA


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Snyders method

3 main parameters - base width (T), peak discharge (Qp)and lag time (basin lag, tp)

Basin lag or lag time (In snyders method midpoint of unit

rainfall excess to peak of the UH)

Represents the mean time of travel of water particlesfrom all parts of catchment to the outlet during a given

storm.


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UH


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UH

Finalizing of Synthetic UH

After obtaining the parameters of from Synders method a tentative UH is drawn

An S curve is developed and Plotted As UH ordinates will bw tentative

will have kinks

Smoothened and logical pattern of S curve is sketched From this S curve a tr hr UH can be derived back

Area under th UH is checked to see if it is 1cm

Time obtained from synthetic UH will be least accurate.


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UH


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UH


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UH

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