In this study, an attempt has been made to study the characteristics of short interval rainfall data of the Punpun river basin lying in Bihar-Jharkhand, which has been identified as the representative basin for the NIH- Centre for Flood Management Studies, Patna. The estimation of flood peaks for small catchments is required for water resources planning, flood forecasting, design of various drainage systems, flood control and design of hydraulic structures.
For this purpose, one of the simple and widely used techniques is based on design excess-rainfall and unit hydrograph approach. In this technique, the design excess-rainfall is convoluted with the unit hydrograph appropriate to the catchment for estimation of flood hydrograph resulting from the design excess rainfall. For small catchments whose time of concentration is a few hours, long term short interval rainfall data are required for estimation of design storms.
The areal extent of the Punpun basin is 8630 sqkm. There are eleven raingauge stations located in the Punpun river basin, out of which only Gaya station has self recording raingauge. The hourly rainfall data of Gaya raingauge station and the daily rainfall data of rest of the ten ordinary raingauge stations have been converted into the hourly data. The Thiessen Polygon method has been used for computing the average areal rainfall for the study area.
Depth area duration (DAD) and depth duration frequency (DDF) curves have been prepared for the study area. The DAD curves have been developed for various durations viz 1 hr, 2 hr, 3 hr, 6 hr, 9 hr, 12 hr, 18 hr and 24 hr. For the aforesaid durations DDF curves have also been prepared for different return periods i.e. 5, 10, 25, 50 and 100 years using the L-moment based general extreme value (GEV) distribution.
Using the excess rainfall of design storm duration, floods of various return periods have been estimated for the sub-basin defined by bridge number 462 of the Punpun basin. For this purpose, DDF curves developed for the Punpun basin upto Hamidnagar gauging site, based on the data of five raingauge stations and the unit hydrograph of the sub-basin defined by the bridge number 462 have been utilized.
The average annual maximum rainfall excess data of the design storm duration for the various years has been convoluted with the unit hydrograph and the annual maximum flood peaks has been calculated for the respective years. The annual maximum peak flood series derived based on this procedure has been used for estimating the floods of various return periods. The floods of various return periods computed by frequency analysis of rainfall and frequency analysis of annual maximum peak floods have been compared.
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