Development of Channel Characteristics during Post-Avulsion Period-A Case Study on a Part of Torsa River, West Bengal

Abstract

Avulsion in any river course is the change of the system in a complex manner where river has to readjust it through different behavioral responses. Channel shifting in recent time on piedmont part of Sub-Himalayan West Bengal is high flood discharge induced while river captures an old course and neo-tectonic reasons are underneath. In the last decade one of the biggest changes in river courses in West Bengal occurred when Torsa River left its actual course and captured one of its old course called Bura Torsa or Sil Torsa (eastern branch) below Hasimara Railway Bridge in Jalpaiguri district (now Alipurduar district). Abandonment of former course, readjustment into the new course and development of new floodplain are the consequent actions. Development of present channel and plan form characteristics are controlled by few natural factors like vegetation, physiography, discharge, sediment load etc. Geomorphic characteristics of Torsa River are accompanied with development of gravel and sand bars, formation of swamps within the braid plain while high discharge induced by intensive rainfall associated with high suspended load are the major hydrological characteristics. In recent time braiding intensity as well as average width of the floodplains and width of the active corridor of Sil Torsa has increased. Distinctions among phases of channel development of the newer part within the stretch downstream of bifurcation are quite clear.