Avulsion potential modeling using integrated Fuzzy-AHP in lower catchment of Chel river of North Bengal, India

Abstract

The paper aims to determine the potential avulsion zones on the piedmont segment of the Chel river basin. A total of 12 hydro-geomorphic causative variables have been selected considering the river basin's present geomorphological and hydrological characteristics. The analysis focused on spatial modeling considering Multi-Criteria Evaluation (MCE) technique. The paper's main contribution is to identify the active avulsion zones on the piedmont of Darjeeling Himalaya triggered by high monsoon rainfall (5479.3 mm maximum annual average). The model's input comes from high-resolution Digital Elevation Model (DEM) based on interpolated rasters, proximity rasters, and rainfall raster. The raster overlay inputs have been fuzzified to scale down within 0-1 and produce the final factor weights. The Analytical Hierarchic Process (AHP) checks the priorities judged by each variable's pair wise relative importance as a part of MCE. Finally, the model indicates that suitable sites on a continuous scale of suitability range between 0 and 1. The rainfall raster gets the highest weightage in the AHP priority scoring. The model correctly classified 85.7% of sites suitable to avulsion, and it has also been proven from the recent (2017-2020) field photographs. The Fuzzy-MCE technique has revealed the role of high rainfall in triggering avulsion on the piedmont surface. The final composite layer indicates the mountain outlet (junction of Chel, Manzing and Fagu Khola) as the most potential site for avulsion.