Assessing bank dynamics of the Lower Meghna River in Bangladesh: an integrated GIS-DSAS approach

The major conduit of water and sediment discharges from the mighty Ganges-Brahmaputra-Meghna river system to the Bay of Bengal, the Lower Meghna River (LMR), experiences rapid morphological changes with significant environmental and socioeconomic consequences to the surrounding areas. This study measures the rate of bankline shifting, river widening, and change in the braiding index (BI) in the LMR over a period of 30 years (1988-2017) in order to understand the channel dynamics. To extract these parameters, we used the Digital Shoreline Analysis System (DSAS) together with other geospatial techniques on multiple satellite images taken over this period and subsequently validated the spatial outcomes through an extensive field investigation. Our findings revealed that rapid changes occurred in both banks, although at different rates, mostly due to erosion; the right bank has retreated at an average rate of 81 m/yr and the left bank at 28 m/yr, resulting in widening as well as river migration to the west. The channel widening, an overall average increase of about 49%, has been accompanied by a substantial increase in the BI, from 2.2 in 1988 to 6.4 in 2017. This study suggests that the higher erosion rate in the right bank and the consequent westward migration of the river are due to the active tectonic setting of the adjoining areas. Enormous volumes of water and sediment discharges and their periodic variations, heterogeneous bank material properties, climate change, and anthropogenic activities are also possible contributors to this bank instability. This research shows the value of implementing an integrated GIS-DSAS tool for spatiotemporal channel change investigation and could be useful in land-use planning and mitigating bank erosion in the LMR locality.