FLOOD MODELING OF BABAI RIVER BASIN, NEPAL

Abstract

Babai river basin of Nepal is subjected to frequent flooding even washing away of the water level gauging stations. Estimation of surface runoff in the basin due to precipitation and quantifying discharge at desired locations is important aspect of flood forecasting and management plan. In this study HEC-HMS hydrological model is used to simulate rainfall-runoff process in the Babai river basin of Nepal. The model is applied with exponential recession method for base flow estimation and initial and constant loss method for infiltration loss modeling. Two different transform methods viz. Clark and Synder are used in this study. The model is calibrated with two flood events and validated with one flood event by comparing observed and simulated discharge at the Chepang gauging site. Various model performance criteria viz. peak value, total volume generated and Nash-Sutcliffe efficiency along with flood hydrographs are used for evaluating the models. Comparison of result of Clark with Synder shows that Synder method is more satisfactory in terms of higher Nash-Sutcliffe efficiency. The result shows the suitability of HEC-HMS in simulating rainfall-runoff process of Babai river basin. Further, the calibrated HEC-HMS model can be used to simulate flood events in the Babai river basin for flood forecasting. Further, flood frequency analyses is performed to predict design floods for various return periods using historical annual peak values from 1990 to 2014 at Chepang station and theoretical probability distribution viz. Gumbel and Log Pearson III. The 25, 50 and 100-year return period floods generated using Gumbel Distribution were 3124, 3642 and 4156 m3/s, respectively and are seen better predicted than Log-Pearson III method. One of the key technique in preventing and minimizing losses is to provide solid information to the people in general about the flood-risk by preparing flood inundation maps. Flood Inundation maps for various return periods are prepared using hydraulic modeling of river reach of about 88 km using HEC-RAS model. The inundated area is analysed using HEC-RAS for 25, 50 and 100-year return periods flood are found to be of the order of 73.34, 80.17 and 86.35 km2 respectively. The inundation area is characterized into various zones considering on depth of water. It is expected that the developed flood inundation map will be useful in planning & execution of structural and non-structural flood damage reduction measures