The design of structures on a river, such as dams, spillways, earthen embankments, flood control reservoirs, etc., requires information about the maximum (or peak) flood discharge in the river. For gauged watersheds, one can use the collected data and analyse them to get peak flood in the river. But, for un-gauged watersheds, one has to use empirical relations.
In 1985, Dickens made the first attempt in India to obtain an empirical formula for determining the maximum flood dis-change Q (m3/sec) in a river.
Dicken’s formula is written as:
Q = CA3/4
in which A is the area of catchment in sq. Km (i.e., Km ) and C is a constant whose value varies widely between 2.8 to 5.6 for catchments in plains and 14 to 28 for catchments in hills, depending upon the catchment characteristics. Dicken’s formula is used for catchments in north India and central India. For catchments in south India, Ryve’s formula is preferred.
Ryve’s formula is expressed as:
Q = CA2/3
The value of C varies widely between 6.8 (for flat or plain catchments) and 42.40 (for western coast region).
A rational method for estimating peak flood discharge of a river includes intensity of rainfall in the relation for peak flood discharge which is expressed as
Q = CIA
in which I is the intensity of rainfall (in m/hour) and C is the runoff coefficient whose value varies between 0.20 for flat catchment with sandy soil to 0.8 for relatively less pervious catchment.