TR(BR)-1/97-98 : Regional flood frequency analysis using L moments

Abstract

Estimation of flood magnitudes and their frequencies for planning and design of water resources projects have been engaging attention of the engineers the world over since time immemorial. Whenever, rainfall or river flow records are not available at or near the site of interest, it is difficult for hydrologists or engineers to derive reliable flood estimates directly. In such a situation, the regional flood frequency relationships or the flood formulae developed for the region are the alternative methods which provide estimates of design floods.

Regional flood frequency curves are developed by fitting L-moment based GEV distribution to annual maximum peak flood data of small to medium size catchments of the seven hydro meteorological subzones of zone 3 and combined zone 3 of India. These seven subzones cover an area of about 10,41,661 km'. Effect of regional heterogeneity is studied by comparing the growth factors of various subzones and combined zone 3. The flood frequency curves based on probability weighted moment (PWM) approach have been compared with the flood frequency curves based on L Moment approach. Relationships developed between mean annual peak flood and catchment area are coupled with the respective regional flood frequency curves for derivation of the regional flood formulae. The regional flood frequency curves developed for each subzone together with at site mean annual peak floods may be used for gauged catchments; while for ungauged catchments, regional flood formulae developed for the respective subzones may be adopted for obtaining rational flood estimates.