Assessing Flood Risk and Emergency Preparedness in Gandak River Basin, Bihar, India using AHP

Abstract. India experiences floods more frequently than any other natural disaster with Bihar accounting for 17.2% of the flood prone area. Gandak river, one of the major river basins in north Bihar presents a challenge in terms of long and recurring riverine flood. While floods cannot be entirely prevented, their impact can be mitigated through effective flood risk mapping. This study integrates Remote Sensing (RS) and Geographic Information Systems (GIS) to analyze spatial flood drivers, using Sentinel-1 SAR data processed in Google Earth Engine (GEE) and ArcGIS Pro for geospatial modelling. The study aims to contribute in better emergency preparedness by developing flood frequency (2017-2024) and flood risk map. The Analytic Hierarchy Process (AHP) was used to weight important factors (elevation, slope, drainage density, distance from stream, rainfall, population density, land use land cover, soil bulk density, flood frequency). Flood frequency map highlighted how Gandak River basin experienced flood approximately 118 times over the span of 8 years. Flood extent was maximum for 2020 followed by 2017. Map shows high values near “Ganga Gandak Milan” in Munger district and near “Burhi Gandak and Ganga River confluence”. The flood risk map was categorized into very low, low, moderate and high-risk zone. Approximately 2776.7518 km² of the study area lies in moderate to high flood risk which accounts for 6.27% of the total area. A major portion of the study area lies in low flood risk zone (40788.99 km²/84.4%). Purbi Champaran has the highest area under moderate to high flood risk (691 km²).