CSDMS 2023: Patterns and Processes Across Scales

Equilibrium elevation of the lower Ganges-Brahmaputra-Meghna Delta

Chris Tasich

Vanderbilt University, United States

The natural elevation of the vast, flat landscape of the lower Ganges-Brahmaputra-Meghna (GBM) remains remarkably stable despite persistent relative sea level rise (rSLR). This stability stems from the tight coupling of the land and tides through a robust negative feedback induced by periodic flooding with sediment-rich water. As water levels increase, the inundation depth and duration also increase resulting in more sediment deposition. This has a stabilizing effect and largely negates the initial increase in water level such that the elevation surface appears unchanged. We refer to this stable elevation as the equilibrium elevation.

Here, we investigate the strength of the inundation feedback and the resulting equilibrium elevation. We identify three main controls on this feedback - (1) annual rate of rSLR, (2) mean tidal range (TR), and (3) mean suspended sediment concentration (SSC). We explore the realistic parameter space of each using a simple, zero-dimensional mass balance model. Specifically, we ask (1) what equilibrium elevations are feasible, (2) how these equilibrium elevations compare to tides (e.g., relative to mean sea level (MSL) or mean high water (MHW)), and (3) how equilibrium elevation impacts the duration (hydroperiod) and intensity (depth) of a typical inundation cycle. Results show an incredibly robust feedback for most conditions with the notable exception of low SSCs (< 0.1 g/L). This low, yet realistic value of SSC represents a tipping point at which the equilibrium elevation drops precipitously. At higher rates of rSLR (> 8mm/yr) and lower TR (< 2 m) the equilibrium elevation results in complete drowning of the platform.

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Of interest for:
  • Marine Working Group
  • Coastal Working Group