Assessment of flood hazards using morphometric compound factor and hypsometric integral in lower Gandak basin, India

Catastrophic climate events such as floods significantly impact infrastructure, agriculture, and the economy. The lower Gandak River basin in India is particularly flood-prone, with Bihar experiencing annual losses of life and property due to massive flooding. Identifying flood-prone zones in this region is essential. Recently, geospatial techniques have become vital tools for determining flood-susceptible zones and managing floods. This study uses high-resolution ALOS DEM data to quantitatively analyse various morphometric and morphotectonic characteristics of 35 sub-watersheds (SW1 to SW35) to understand the drainage basin's hydrological behaviour, particularly during the monsoon season. The compound factor weightage, calculated from morphometric analysis of 17 parameters, identifies flood-susceptible zones. Detailed statistical analysis revealed similarities and dissimilarities between each sub-watershed parameter. Sub-watersheds were classified into highly susceptible (26.24-24.00), moderately susceptible (23.18-21.12), and slightly susceptible (20.94-18.29) categories. Highly susceptible sub-watersheds (2, 4, 6, 7, 8, 9, 11, 12, and 13) exhibit high erosion potential and greater flood devastation potential. Moderately susceptible sub-watersheds (1, 5, 10, 14, 15, 17, 20, 22, 23, 24, 30, and 34) show lesser flood effects. Slightly susceptible sub-watersheds (3, 16, 18, 19, 21, 25, 26, 27, 28, 29, 31, 32, 33, and 35) are less affected. Hierarchical cluster and compound factor analyses confirm that highly susceptible sub-watersheds need higher mitigation measures to minimise impacts on life and livestock. Implementing mitigation techniques across all susceptibility classes is crucial to prevent further loss of human life and livestock. The study's outcomes have direct implications for effective watershed management.