Reconstructing the channel shifting pattern of the Torsa River on the Himalayan Foreland Basin over the last 250 years

The varied physiography, incidences of high seasonal discharge, influences of neo-tectonic activity and the young geological foundation with less consolidated cohesive and non-cohesive sediment have left the Himalayan foreland basin a formidable ground, where silt-laden rivers tend to migrate frequently. A set of maps prepared after 1764, space photographs captured in 1970 and current satellite images from 2015 and 2017 were studied to reconstruct the fluvial dynamics of the Torsa River on the foreland basin of Sikkim-Bhutan Himalaya considering a time span of nearly 250 years. Evidence collected from colonial literature, the above-mentioned satellite images and a field survey, were combined to verify results taken from the old maps used as the base of the study. The application of satellite remote sensing and analysis of the topographic signatures of the palaeo-courses in the form of the palaeo-levee, abandoned courses and ox-bow lakes were the major operational attributes in this study. As a consequence of the channel migration of Torsa River since 1764, the historical floodplain of Torsa has been topographically marked by beheaded old distributaries, a misfit channel system and the presence of abandoned segments. Morphometric changes in the old courses, major flood events and neo-tectonic activity guided an overall trend of channel migration eastwards and has led to a couple of channel oscillation events in the Torsa River over the last 250 years. The mechanism of the avulsion events was thoroughly driven by sedimentation-induced channel morphometric changes and occasional high discharge.

Fluvial Processes and Channel Stability of the Torsa River, West Bengal (India)

Fluvial processes such as bank erosion plays an important role to change the channel stability of the Torsa River in the downstream region. The present study was focused on stream stability assessment of the Torsa River. The study area is situated between the downstream of the Jaldapara Reserve Forest and confluence of Kaljani River. Data of different parameters about 64 bank segments of the Torsa River were prepared using the field work techniques for assessing the stream bank conditions using lateral, vertical and overall reach stability models. The individual results of BEHI and NBS ratings show that out of 64 bank segments only 35 and 19 bank segments classified in higher categories. Overall lateral stability analysis shows that most of the sample bank segments are in an unstable condition. All bank segments are vertically unstable and degrading. Overall reach stability analysis shows widespread instability. BEHI and NBS results are almost similar for most of the bank segments and therefore, BEHI and NBS can be suitable bank erosion hazard predictive models in the study for channel stability analysis.