Riverbank erosion and char stability along the fluvial-to-tidal transition zone in the Lower Meghna River and Tentulia Channel in the Ganges-Brahmaputra-Meghna Delta, Bangladesh

The Ganges-Brahmaputra-Meghna (GBM) delta has been named one of the world's most vulnerable sinking deltas. It is also one of the most dynamic deltas, with the world's highest sediment discharge and third highest water discharge, with semi-diurnal mesoscale (2-4 m) tides. The high fluvial discharge and significant tidal velocities (1-2 m/s) create an expansive fluvial-to-tidal transition zone (FTTZ) along the entire Lower Meghna River (LMR). Within this FTTZ, tidally elongated channel bars, locally called 'chars', rapidly evolve due to the high sediment and water discharges. Chars along the LMR have a total population exceeding 5 million people; thus, evaluating the interactions of fluvial and tidal processes along the FTTZ is crucial to understanding vulnerability among char communities with respect to flooding and sea-level rise. Here, we utilize a multi-faceted approach to assess the geomorphology, sedimentology, and hydrology of three chars longitudinally spaced within the LMR and one of its distributaries - the Tentulia Channel. Decadal land change analyses revealed that the FTTZ along the upper LMR and Tentulia Channel has been gaining land at a rate of 4.7 km2/yr, with erosion being most prevalent upstream and most accretion occurring within 50 km of the coast. Almost all the stratigraphic profiles show fine and/or medium sands at depth (0.5-2 m), implying that the three chars are similarly susceptible to erosion by tidal currents and high river discharge. Thus, we attribute the increased stability of chars in the downstream direction to increased tidal influence rather than sedimentology. Land elevation and water level surveys reveal that elevation relative to sea level decreases as the distance to the mouth of the LMR decreases: from 4.05 m (130 km from the mouth), to 2.56 m (100 km from the mouth), and 1.85 m (70 km from the mouth). By incorporating local water level data with the elevation surveys, we conclude that fluvial, namely monsoonal, processes control the geomorphology of the chars near the confluence of the LMR and the head of Tentulia Channel. The shift between fluvial-dominated and tidal-dominated regions of the FTTZ occurs just downstream of Char 3 (-70 km from the LMR mouth). Therefore, char communities near the LMR confluence are most vulnerable to riverbank erosion and flooding associated with the monsoon season, and chars proximal to the coast are most vulnerable to erosion and flooding related to storm surges and relative sea-level rise.