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Direct Runoff and Time of Concentration

Direct Runoff

The part of runoff which enters the stream quickly after the rainfall or snow melting.

To design soil conservation structure with proper capacity it is necessary to estimate peak runoff rate.

Peak runoff Rate:

It is the maximum rate of flowing runoff per unit time.

Rational Method:

This is the most common method to predict the peak runoff rate. The peak runoff may be defined as the capacity to be given a structure that must carry the runoff

1. in FPS:

Q = CIA
Where,
Q = Design runoff the ft3/sec.
I = Rainfall Intensity Inches/hr
A = Watershed area in acres.

2. in MKS:

Q =   CIA/36 =  [0.0276x CIA ]

Where,

Q = Peak runoff rate m3/sec
C = Runoff coefficient
I = rainfall intensity [cm/hr ] for duration equal to time of Concentration and    for a given recurrence interval
A = Watershed area,  [ Hectare ].

Time of Concentration:

It is the time required for the runoff water to flow from the most remote point of the area to the outlet.

Rainfall Intensity:

Rainfall intensity is defined as the rate of fall of precipitation, expressed in depth per time [mm/hr]

I = P/T

Where, I = Rainfall Intensity, mm/hr
P = Amount of rainfall, mm
T = duration of rainfall, hr