Channel planform dynamics, avulsion and bankline migration: a study in the monsoon-dominated Dwarkeswar river, Eastern India

Abstract

The progressive and sequential shifting of the banklines and centerlines (thalweg) due to bank erosion and channel bar formation, as well as channel planform changes and avulsion of the Dwarkeswar river of West Bengal during the last 100 years, have been studied. For this purpose, police station maps, Survey of India topographical maps, and satellite images were used along with extensive field surveys. The channel was divided into reaches and segments for measuring the amount of migration and quantifying planform indices. Four types of changes have been observed: (1) channel planform change due to meander development and migration, (2) avulsion and consequent alteration in the flow direction, (3) thalweg shifting as a consequence of mid-channel bar formation, and (4) widening of the channel due to bank erosion. The study reveals that the alluvial plain upper (APU) is the most dynamic reach due to the meandering nature of the river. The thalweg shifted up to 1.71 km in this reach. An avulsion occurred in the lower course of the river during a large flood in 1978. The Jhumjhumi Nala became the main course of the river, and the earlier channel had become completely abandoned due to this avulsion. The sinuosity index of the entire study reach has been increasing since 1920, whereas the braiding index has decreased at most of the segments. Therefore, the channel planform is transforming into the meandering pattern from the braided pattern.