For the design of structures on a river, such as dams, spillways, earthen embankments, flood control reservoirs, etc., it is important to know how much water can flow through the river during its biggest flood. For watersheds that have been measured, the collected data can be analysed to find the river’s peak flood. But for watersheds that can’t be measured, you have to use empirical relationships.
In 1985, Dickens was the first person in India to try to find an empirical formula for figuring out a river’s maximum flood change Q (m3/s).
Figure 1: Bhakra Dam
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.
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.