Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 216

C y × I y × A

Q = (6.8)

360

where

peak flow rate resulting from storm ARI of Y Years (m 3 /s) Q =

C y = runoff coefficient for design event having an ARI of Y years

(dimensionless)

A = area of catchment (ha)

I y = rainfall intensity (mm/h) corresponding to a particular storm duration and

ARI. The duration is set equal to a sub-catchment time of concentration.

The following guidelines are provided for the use of the Rational Method. The appli-
cable average recurrence interval, runoff coefficient, area of catchment, and design
average rainfall intensity should be determined as discussed below.

Design Average Recurrence Interval (ARI)

The hydraulic design for the total drainage system (the underground pipeline plus
the surface drainage system) should cater for 100-year ARI flood flows.

Runoff Coefficient (C)

Runoff coefficients provide for the relationship between runoff and rainfall volumes
and make provision for the intermixing of pervious and impervious surfaces. For
urban storm-runoff estimation, C is about 0.8 -0.9, whereas, for agricultural fields,
it is about 0.4-0.7, depending on the size and pattern of the catchment or drainage
basin.

Time of Concentration ( T c )
The time of concentration at a particular location is generally the time required for
runoff to travel by the longest available flow path to that location.
In many cases, however, a “partial area” effect occurs through the lower part of
the catchment, where flows are higher than those calculated for the entire catch-
ment, because the time of concentration is lower and the design rainfall intensity is
higher.

6.5.3.2 SCS Method

The Soil Conservation Service (SCS) [recent “Natural Resources Conservation
Service”] method is an empirical equation. The SCS runoff equation is

I a ) 2 ( P −

Q = (6.9)

( P − I a ) + S