Sediment Dynamics in the Mangroves of Southern Coastal Bangladesh

While much of southwestern Bangladesh is losing land area due to locally accelerated sea level rise, the southern coastline appears to be in a state of pseudo-equilibrium, with erosion and accretion occurring at similar rates. In this study, we use data collected in situ by acoustic and optical instrumentation to examine the physical processes controlling sedimentation in a mangrove forest along the southern coast. These data are then compared with sedimentation rates measured using sediment elevation tables and marker horizons, as well as observations made 100 km further inland near the northern extent of the same mangrove forest. At this inland site, sediment supply, inundation depth, and salinity have been identified as the most important factors controlling sediment deposition to the mangrove platform, which ranges from ~1 cm during the dry season (November – June), to > 2 cm during the monsoon (July-October). Both salinity and sediment availability stand to change dramatically for the worse in response to upstream damming operations, which could threaten the stability of this important ecological and cultural resource. By examining a second location closer to the primary source of both sediment and freshwater (the mouth of the combined Ganges-Brahmaputra-Meghna [GBM] rivers), we can address the regional extent of this potential threat.

Of course, proximity to the sediment source is not the only difference between the two locations. Previous studies have identified that the inland location features a larger tidal range (~5 m vs. ~3 m). Additionally, the coastal site is more directly impacted by cyclones than the inland site, which likely plays an important role in controlling long-term sediment accretion. Results from this study demonstrate the importance of providing regional context to sedimentation studies, as coastal communities adapt to dynamic forcing conditions.