Prioritization and Regionalisation of Sub-watershed for Flash Flood Susceptibility in the Upper Teesta River Basin, North Sikkim, India

Flash floods pose significant risks to communities and infrastructure in the Upper Teesta River Basin, North Sikkim. This study examines the 3A1C6 watershed, which includes 11 distinct sub-watersheds: 3A1C6n, 3A1C6m, 3A1C6k, 3A1C6j, 3A1C6h, 3A1C6g, 3A1C6f, 3A1C6d, 3A1C6c, 3A1C6b, and 3A1C6a, as delineated by SLUSI.The study aims to prioritize and regionalize these sub-watersheds concerning flash flood susceptibility. A comprehensive analysis of morphometric and topo-hydrological parameters was conducted, including Stream Frequency (Fs), Compactness Constant (Cc), Constant of Channel Maintenance (C), Bifurcation Ratio (Rb), Drainage Density (D), Elongation Ratio (Re), Circularity Ratio (Rc), Form Factor (Rf), Drainage Texture Ratio (Rt), Topographic Wetness Index (TWI), Stream Power Index (SPI), and Stream Transport Index (STI). These parameters were weighted using the Analytic Hierarchy Process (AHP). Multi-criteria decision-making (MCDM) techniques, including Grey Relational Analysis (GRA), Combinative Distance-based Assessment (CODAS), Evaluation based on Distance from Average Solution (EDAS), and Combined Compromise Solution (CoCoSo), were employed to rank the sub-watersheds for flash flood susceptibility. Spearman’s rank correlation was applied to integrate the rankings from these models. The study reveals that sub-watershed 3A1C6m is the most susceptible to flash floods, followed by 3A1C6h and 3A1C6f. Furthermore, cluster analysis based on principal component analysis identified two homogenous sub-watersheds, which supports effective watershed management. These findings highlight the importance of integrating various methodologies for a comprehensive assessment and management of flood risks.