SEDIMENT DYNAMICS IN THE MANGROVES OF SOUTHERN COASTAL BANGLADESH

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 as a result of sea level rise and sediment starvation. The largest mangrove stand in the world, the Sundarbans National Forest in Bangladesh and India, appears relatively healthy in the modern day, however could be threatened by future changes. In this study, we examine instrument data collected during monsoon-season conditions October 2019 to understand modern sediment transport and deposition processes in this vital resource.

Specifically, we are interested in understanding the relationship between water velocity and suspended sediment concentration (SSC) at various locations along a transect crossing a tidal channel bank. Our data include water velocities from an acoustic Doppler current profiler (ADCP) in the lower intertidal and a tilt current meter (TCM) in the mid-intertidal, SSC from an array of optical backscatter sensors (OBS), and inundation depths from an array of pressure sensors co-located with the OBS across the entire intertidal reach. From our preliminary examination of the data, several interesting patterns emerge. In the near-subtidal, SSC exhibits similar variability in response to changes in both water velocity and depth. In the intertidal, however, SSC is primarily a function of water depth, with maxima of similar value observed in association with water levels <20 cm. There is also surprisingly 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). Less surprising is the baffling observed by the mangrove pneumatophores in the mid-intertidal, which reduce peak velocities (as measured in the lower intertidal) by at least 50%.

In addition to subsequent analysis using the data described above, we will be collecting a companion dataset during dry-season conditions in early March 2020, and plan to have preliminary results available for presentation.