6 -computation of runoff

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Methods of computation ofrunoff

Examples


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COMPUTATION OF RUNOFF

METHODS

Use of runoff

coefficient

Use of infiltration

curves

Use of infiltration

indices


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1. USE OF RUNOFF COEFFICIENT

The volume of runoff can be directly computed approximately by using

an equation of the form

Q = k . P

Where

Q = runoff

P = precipitationk = constant depending upon imperviousness of the drainage area

Truly speaking this equation is not rational because:

runoff not only depends upon the precipitation but also depends upon

the recharge of the basin.but

the equation gives more and more reliable results as theimperviousness of the area increases and the value of k tends toapproach unity.


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APPLICATION OF METHOD

Where USED?

Design of storm water drains.

Small water control projects especially for urban

areas where the percentage of impervious area isquite high.

Where should be AVOIDED?

This method of computing runoff should be avoided

for rural areas and for the analysis of major storms.


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2. USE OF INFILTRATION CAPACITY CURVE

The infiltration capacity (IC) curve is a plot of the

infiltration capacity against time.

If the IC curve is super imposed on the rainfall

hyetograph, the resultant amount will represent nothing

but the runoff.

Natural rains of varying intensities, sometimes below and

some time above the prevailing infiltration capacity,

results in a distortion of a capacity time curve.

It is generally assumed that the infiltration capacity atany time is determined by the mass infiltration, which

has occurred up to that time.


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INFILTRATION CAPACITY CURVE


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3. USE OF INFILTRATION INDICES

The infiltration capacity curve already determined on test plot

cannot be applied to large basins or heterogeneous areas.

At any instant, in a large area, the IC as well as the rainfall ratewill vary from point to point.

Moreover subsurface flow or interflow will also be substantial.

Since this water flow is part of infiltration, it will not normally beincluded in the runoff computed by IC curve determined on atest plot.

Hence runoff volumes are generally computed by usinginfiltration indices.

W-index and -index are the two indices which are commonly

used.


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W-index is the average infiltration rate or the infiltrationcapacity averaged over the whole storm period, and is givenby

Windex = F / tr= (P Q) / tr

whereF = total infiltration including initial basin rechargeP = total precipitation

Q = total runofftr= duration of rainfall in hour

-index is defined as, the average rate of loss such that thevolume of rainfall in excess of that rate will be equal to the

volume of direct runoff.

It can be defined the other way round as, the rate of rainfallabove which the rainfall volume equals to the runoffvolume.

-index can be represented graphically as shown.


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-index and W-index will be equal for a uniform rain, but they may not beequal for a non-uniform rainfall. However for rains which are reasonablyuniform or for heavy rains these two indices are found to be nearly equal.


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In a usual case of moderate rain of non-uniformintensities the -index will be somewhat higher thanW-index.

These indices will change with a change ininterception, depression storage and initial soilmoisture.

They will also change with the amount of precipitation.

These indices are not the actual infiltration rate but themeasure of potential basin recharge.

The runoff coefficient k can be determined if the Windexis known by using the equation

k = (P Windex) / P

where, P = rainfall rate


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EXAMPLE # 1

Following are the rates of rainfall for

successive 20 minutes period of a 140

minutes storm:

2.5, 2.5, 10.0, 7.5, 1.25, 1.25, 5.0 cm/hr.

Taking the value ofindex as 3.2 cm/hr, find

out the runoff in cm, the total rainfall and the

value of Windex

.


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Solution:

From the given rainfall rates rainfall hyetograh is plotted, to see

rain intensity pattern, as shown in below Figure.

indexline at a height of 3.2 cm/hr is superimposed.

The hatched area is calculated, so as to obtain the value of

runoff:


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rt

QPW

index


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Example # 2

An isolated 3 hour storm occurred over an area of 120 ha as below:

(i) What is the total rainfall on the catchment in this storm?

(ii) Estimate the runoff from the catchment.(iii) If the index were to remain at the same value, what runoff would

be produced by uniform rainfall of 3.3 cm in 3 hours uniformlyspread all over the catchment?

Partialarea of

catchment (ha)

index(cm/hr)

Rainfall (cm)

1st hour 2nd hour 3rd hour

36 0.9 0.6 2.4 1.318 1.1 0.9 2.1 1.5

66 0.5 1.0 2.0 0.9


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(ii)


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6 -computation of runoff

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