Mangrove Sediment Dynamics in the Sundarbans National Forest as part of the Ganges-Brahmaputra-Meghna Delta, Bangladesh

Abstract:

Mangroves have been identified as important buffers of coastal storm and wave energy, biological resources, and carbon sinks. Despite their obvious importance, mangroves face a variety of threats, including drowning due to the combined effects of sea level rise (eustatic and relative) and sediment starvation (natural and anthropogenic). The largest mangrove stand in the world, the Sundarbans National Forest in Bangladesh and India, appears relatively healthy in the modern day, as the total area remains nearly constant. This mangrove stand is located on the topset of the Ganges-Brahmaputra-Meghna (GBM) Delta, through which the namesake rivers carry >1x10^9 tons of sediment per year. Future changes to water level, sediment supply, and storm frequency and intensity may upset the delicate balance, threatening the delta health and sustainability. In this study, we examine modern sediment transport and deposition processes, with the goal of understanding which components are most vital for protecting this resource.

Specifically, we are studying the processes responsible for delivering sediment from tidal channels to the delta topset during monsoon conditions. We collected in situ measurements of water velocity, depth, and suspended sediment concentration (SSC) along a transect through the intertidal zone, for the duration of a complete tidal cycle. In the near-subtidal, SSC exhibits similar variability in response to changes in both water velocity and depth. In the intertidal, SSC is primarily a function of water depth, with maxima of similar value observed associated with water levels <20 cm. There is little change in average SSC as we move from the middle to upper intertidal, likely a product of the slow settling velocities associated with the fine-grained sediment observed here (~16 um D50). This result is surprising given the presence of pneumatophores in the middle and upper intertidal, which are known to enhance sediment trapping. We do observe baffling, with a reduction in peak velocities by at least 50% (measured in the lower intertidal), and future research will address how this impacts sediment deposition and accumulation.