7.9-Flood frequency estimates for Sub-Himalayan region by regional flood frequency approach.

Abstract

In this paper, flood frequency analysis has been carried out for Sub-Himalayan region (Zone-7) using peak flood series data of ten small and medium catchments varying in size from 6 sq.kms. to 2072 sq.kms. The study presented, involves application of Extreme Value Type-I, General Extreme Value and Wakeby distributions using (i) at site data (ii) at site and regional data combined and (iii) regional data alone. Statistical test based on U.S.G.S. method has been performed in order to test the homogeneity of the region. The data of eight sites are utilised for estimating the parameters, keeping the data of remaining two sites for the purpose of testing the methodology based on descriptive ability criteria. The predictive ability of the different methods considered in the study is also tested through Monte Carlo experiments; wherein synthetic flood series love been generated using the regional EV1, GEV and Wakeby parameters derived from the historical data. Generated data sets of specific record lengths (same as the record length of historical data for respective gauging sites) have been considered for the eight sites. For the two independent sites variable record lengths viz.1, 5, 10, 13 ,20, 30 and 40 one at a time, have been considered. The above methodology has been applied to the generated data of different sample sizes for each population for the two independent gauging sites. Performance of different methods has been evaluated based on predictive ability criteria viz, bias, and root mean square error. It is seen that the methods based on EV1(PWM), GEV(PWM) and Wakeby (PWM) approaches using at site and regional data in combined form provide estimates of flood peaks with computationally less bias and comparable root mean square error for the two test catchments.