Runoff computation by infiltration indices preferable to engineering students of civil engineering.

1. 23/02/2017
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WATER RESOURCES
MANAGEMENT
Topic : Computation of runoff by infiltration method
By : sagar k. vekariya
Infiltration :
Infiltration is the process by
which water on the ground
surface enters the soil.
Infiltration is governed by
two forces: gravity and
capillary action. While
smaller pores offer greater
resistance to gravity, very
small pores pull water
through capillary action in
addition to and even against
the force of gravity.
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Maximum rate at
which water enters the
soil in a given
condition.
Rate at which water
actually enters the
soil during a storm
and is equal to the
infiltration capacity
or the rainfall rate
whichever is less.
Infiltration
Capacity :-
Infiltration
Rate :-
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Types of computing runoff by
infiltration method
By infiltration capacity curve
By infiltration indices
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1) By infiltration capacity curve
In this method
infiltration capacity
curve is used.
The infiltration capacity
curve for a given soil
and moisture conditions
is subtracted from the
curve of rainfall pattern
to derive the excess
rainfall which represent
the runoff.
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2) By infiltration indices
Infiltration indices method is used to
calculate runoff then, when the
catchment area is greater.
Infiltration Index :- Average rate of loss
such that volume of rainfall in excess of
that rate will be equal to direct runoff.
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Types of infiltration index
W-Index
ɸ-Index
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1) W- Index
Windex is the average infiltration rate of total
precipitation (cm) – total runoff (cm)/Duration of
rainfall in hrs.
Where P= total precipitation in cm
R= total runoff in cm
t= Duration of rainfall in hrs.
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2) ɸ-Index
ɸ-Index :- Rate of rainfall above
which rainfall volume equal to
runoff volume.
Where,
k=runoff co-efficient
W= W-Index
i= intensity of rainfall (cm/hr)
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Example :-
1) The following are the rates of rainfall for increases 30
minutes period of a 3 hrs storm.
If value of ɸ-Index is 3.4 cm/hr find
I. net runoff in cm
II. Total rainfall
III. W- Index
Time
(min.)
0 - 30 30 – 60 60 – 90 90 – 120 120 –
150
150 –
180
Rainfall
(cm/hr)
2.6 2.7 7.9 10.5 5.6 2.1
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Solution :-
I. Net runoff in cm :
R = (7.9 – 3.4)(30/60)+(10.5 – 3.4)(30/60)+(5.6 –
3.4)(30/60)
R = 6.9 cm
II. Total rainfall (precipitation) :
P = 30/60 {2.6+2.7+7.9+10.5+5.6+2.1}
P = 15.7 cm
III. W – Index :
W – Index = P – R/t
= 15.7 – 6.9/3
W – Index = 2.93 cm/hr
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