Flood risk assessment of the Kosi River Basin in North Bihar using Synthetic Aperture Radar (SAR) data and AHP approach

Flood is a recurrent destructive natural calamity in the Kosi River Basin (KRB) in north Bihar in India. Geospatial modelling of these recurrent floods becomes imperative for effective disaster management. The KRB is renowned for its high vulnerability to flooding due to its sudden bending and heavy rainfall in the upper catchment of the basin located in Nepal. This study presents a comprehensive assessment of flood risk over the KRB, by utilizing the Analytical Hierarchy Process approach and Synthetic Aperture Radar data. The risk map was generated by considering multivariate set of factors including physical (elevation, slope), geological and hydrological variables (flood frequency, rainfall intensity, drainage network). Flood inundation and rainfall intensity are calculated over six years (2015–2020) to understand the dynamic nature of floods. The results of this analysis provide detailed flood inundation and risk maps, highlighting areas at varying levels of vulnerability and risk. Higher flood inundation was seen in downstream areas, which accounted for 6526.3 ​km 2 (33%) of geographical areas. Flood inundation was highest in 2020 and 2019 accounting for 27.93% and 20.72% of areas, respectively, whereas the lowest flood inundation was seen in 2015 (4.14%). Areas under higher flood risk were 1383.7 ​km 2 (7%), whereas 3820.9 ​km 2 (19.4%) were at lower flood risk. Extremely flat downstream areas near riverbanks were at higher risk (7% of KRB) that has correspondence with higher flood frequency. The spatially explicit flood risk zone information can be invaluable for disaster preparedness and policymakers. Furthermore, flood risk assessment can reinforce resilience to improve land use planning, insurance planning, flood-prone area management, and raising public awareness of potential flood risks.