Application of multi-criteria decision-making approach for ascertaining the avulsion potentiality of the Torsa River course

This work has been a scientific venture to analyse the avulsion potentiality in the course of the Torsa River flowing across the Himalayan Foreland Basin (HFB) in West Bengal, India. A total of nine parameters, based on theoretical propositions regarding their corresponding role in channel avulsion, were selected to calculate the Avulsion Potentiality Index (API). Since the weightage of the topographic and planimetric parameters vary correspondingly in the avulsion process, a supervised technique of assigning weightage to multiple criterias, the analytic hierarchy process (AHP) has been used in this study. The weightage-based compound scoring was conducted using the weighted linear combination model (WLC). API was calculated for the Torsa River considering the equal interval-based compartmentalization of the course as individual spatial units of analysis. It has been an approach where the palaeofluvial events, i.e. old avulsion history and associated fluvial expressions, have been integrated with the present fluvial forms, i.e. channel planform and floodplain topography, to predict the future possibilities. The prediction done using the model was found significant in the real world. 71% of the highly avulsion prone channel segments have experienced channel avulsion during the post-analysis period. The channel segments with high in-channel sedimentation rate and downstream segments of the confined stretches, both anthropogenic and structural, were found more susceptible to avulsion. On-field verification of the avulsion events has dished up the characteristic influences of both the floodplain topography and channel planform parameters on avulsion events in the Torsa River course.