Flood Estimation Directorate, Central Water and Power Commission (W.W.), R. K. Puram, New Delhi-22.
In this paper, statistical frequency distributions, such as Gumbel’s extreme values distribution, log-normal distribution and exponential distribution— applicable in flood frequency analysis—have been described. Various statistical methods of fitting these distributions have als been explained. As an example, the discharge data for a discharge site on the River Kosi for a continuous period of 19 years (1948-1966) have been subjected to frequency analysis. Limitations in applying statistical methods to flood series have also been indicated. The object of the paper is to provide the methodology and guidelines to the engineers engaged in designing the waterway of structures like railway bridges, culverts, etc., whose “economic life” is about 50 years and for which the maximum flood with a return period of 50 years is to be estimated. It is difficult to decide what length of record is required to estimate precisely 50 years’ flood as various factors influence flood events. However, it is felt continuous records of 20 to 25 years are necessary for a fair estimation of the maximum 50-year flood.
