THE EFFECTS OF EROSION ON MIGRATION AND SUSTAINABILITY OF THREE BARFORMS IN THE LOWER MEGHNA RIVER, BANGLADESH

The Ganges-Brahmaputra-Meghna (GBM) delta has been designated as one of the world’s most vulnerable sinking deltas. It is also one of the most dynamic deltas, with the highest sediment discharge and third highest water discharge in the world, with semi-diurnal mesoscale (2-4 m) tides. The high fluvial discharge and tidal velocities (1-2 m/s) create a mixed fluvial-tidal region that spans the entire Lower Meghna River (LMR). Within this region, tidally elongated barforms, locally called ‘chars’, rapidly evolve due to the high sediment and water discharges. As Bangladesh is in the top ten total population and population density worldwide, any available land is quickly utilized for agriculture. This includes the chars, which are inhabited by over five million people.

Chars are relatively low in elevation, being vulnerable to erosion and flooding from monsoon discharge, storm surges, and cyclones. Here we assess the sustainability of local char communities by analyzing the historical evolution and stratigraphic/elevation profiles of three river barforms longitudinally spaced within the LMR. We simultaneously assess patterns of migration to and away from these locations, and the livelihood activities employed by living on these chars. Almost all the stratigraphic profiles show fine and/or medium sands at depth (0.5-2 m), implying that the chars are susceptible to erosion by tidal currents and high river discharge. While erosion takes place, mostly in the form of lateral migration, accretion is simultaneously occurring. For instance, the most upstream site (with a 15 km buffer) has experienced net erosion of -4.88 km² since 1972, but the char itself formed in 1989 and has since accreted 3.8 km². Preliminary household survey data suggest that erosion is significantly related to outmigration, particularly for farmers. Additionally, we find char elevation relative to EGM96 decreases as distance to the mouth of the LMR decreases: from 4.05 m above mean sea level (130 km from mouth), to 2.56 m (100 km from mouth), and 1.85 m (70 km from mouth). This research highlights that char communities closer to the Bay of Bengal are not only vulnerable to bank erosion from their underlying stratigraphy but are more susceptible to flooding caused by storm surges, cyclones, monsoons and relative sea-level rise from their lower elevation.