Critical duration of rainfall for flood estimation

Many methods of estimating design floods from rainfall data involve a trial and error procedure to determine the duration of the design rainfall. An analytical expression for estimating the appropriate duration for use with a particular unit hydrograph is derived. This analytical expression permits the effects of other parameters significant to the estimation of the design flood, such as different storm characteristics, to be evaluated with ease. The analytical approach has considerable flexibility in that the mathematical expression linking unit hydrograph and rainfall characteristics permits the independent adjustment of one of the sets of characteristics. This is particularly advantageous where some form of dimensionless unit hydrograph is available for flood estimation. It means that storm characteristics may be varied from watershed to watershed without any changes being required in the expression for the unit hydrograph. A ‘flexible’ dimensionless unit hydrograph is also developed.