FLOOD MODELING AND FLOOD RISK ASSESSMENT USING GIS AND HEC-RAS

Abstract

On the destructive scale of natural catastrophes, floods are regarded as relatively high. They may seriously harm people and property. In India, the Godavari River, which crosses multiple states, is notorious for experiencing severe floods due to excessive rainfall, which hampers property and especially lives, particularly in low-lying areas. Out of 356 villages in study area, 161 villages are still facing flood-related problem. In order to solve this problem, research was done to determine the flood risk in the lower Godavari River's catchment area. The study used a specific technique, including two-dimensional hydraulic modelling with HEC-RAS, analysing flood frequency and mapping the floodplain with ArcGIS. The methodology was used to extract river cross-section information using ArcGIS from the digital elevation model (DEM) pertaining to the study area, located between Bhadrachalam and Koida reach of lower Godavari River, including Sabari River’s 35-kilometer stretch starting from Konta. The Manning’s n value, cross-section details and observed 27-year flow data of Bhadrachalam, Konta and Sangam are the input for the HEC-RAS. An unsteady flow simulation was performed to obtain results for depth or elevation in water surface. Then the outputs were exported to ArcGIS for modelling of the floodplain and the results were validated by contrasting both the observed as well as the simulated elevations of water surface at Koida station. Additionally, on the basis of maximum instantaneous flow recorded at sites Bhadrachalam, Konta, and Sangam, flood frequency analysis for 25, 50, 100, and 200 years return periods was carried out using the Log Pearson Type III, Gumbel and Log Normal methods. The expected floods were predicted using the observed inflow data from Bhadrachalam, Konta, and Sangam. In this study, it was observed that Log Pearson Type III was the best fit for Bhadrachalam and Sangam Station whereas Gumbel Max emerged as the most appropriate fit for Konta Station. The flood frequency analysis (FFA) for Bhadrachalam and Sangam station by Log Pearson Type III showed the discharge of 57385 m3/s, 61456 m3/s, 64901 m3/s, 67847 m3/s and 416 m3/s, 452 m3/s, 482 m3/s, 508 m3/s for 25, 50, 100, 200 year return periods respectively. Similarly, the FFA for Konta station by Gumbel Max showed the discharge of 13041 m3/s, 14906 m3/s, 16757 m3/s and 18601 m3/s for return periods of 25, 50, 100 and 200 year respectively.