Modeling metamorphosis of the Old Brahmaputra River and associated impacts on landscapes in the Central Bengal Basin, Bangladesh

This study explores causal mechanisms of river metamorphosis and its impacts on regional landscapes. The study also investigates the implications of metamorphosis on associated ecological resources. Advanced GIS and remote sensing technologies were used to delineate morphological parameters describing metamorphosis of the Old Brahmaputra River from historical maps (i.e., Rannell's Map in 1776, Tassin's Map in 1840, Topographic Survey Map in 1943 ) and remotely sensed optical satellite imagery Sentinel-2 in 2022 . Flood frequencies were investigated for different periods by applying Gumbel’s Analytical Method (GAM), Log-Pearson Type III, and Log-Normal Method to estimate probability of flood vulnerability and impacts of flooding on morphodynamics in the central Bengal Basin. During the periods between 1776 and 2022, the area of sedimentation (77,999.43 ha) was greater than the eroded area (2983.29 ha).This difference was attributed to siltation of the channel bed morphology and corresponding accelerated flood vulnerability that accompanied river metamorphosis. Hydrological variables particularly annual average discharge significantly declined from 22 to ~ 18 m 3 /s per year during the period from 1965 to 2020. The study results demonstrated that the log-normal methods significantly overestimated peak flood discharge compared to Log-Pearson methods and Gumbel’s probability model. The extrapolation of the discharge for the 100-year flood by applying the three methods produced values of 712.66 m 3 /s, 1750.26 m 3 /s, and 2462.92 m 3 /s. Differences of these magnitudes may be critical for planning purposes because these differences in results will generate large-scale projected impacts on morphodynamics of the central Bengal Basin.