Rejuvenating Poldered Landscapes: A Numerical Model of Elevation Equilibrium in Coastal Bangladesh

Abstract:

The low-lying, coastal region of Bangladesh has relied on poldering (surrounding islands and flood-prone areas with embankments) since the 1960s to mitigate flooding and tidal inundation. The result has been an increase in total arable land and the ability to sustain food production for one of the most densely populated countries in the world. However, poldering has had the unintended consequences of starving embanked landscapes of sediment. To mitigate the effects of subsiding interiors, some poldered communities have used tidal river management (TRM) to allow water and sediment exchange between the polders and the tidal network. Anecdotal reports claim great success in some locations, but not in others. To date, there has been very little quantitative analysis.

Here, we use a simple numerical model of tidal inundation and subsequent sediment accretion to examine the potential impacts of TRM at a poldered island, Polder 32 (P32), and the adjacent mangrove forest in southwest Bangladesh. Our model employs mass balance with variable incoming suspended sediment concentration (SSC). We use tidal gauge and SSC data as inputs and test the model against measured accretion values at P32. Sensitivity analysis of model parameters narrows the range of realistic parameter inputs. Preliminary results suggest that it would take ~10-20 years for P32 to re-equilibrate to the natural surrounding land elevations with a restored direct tidal channel connection. Since a direct tidal channel connection is unfeasible and would displace the local population, future work will attempt to constrain time frames for inundation to more closely model TRM efforts. We also plan to add a bedload component and multidimensionality. The present model provides a simple framework for understanding sediment accretion in southwest Bangladesh and helps generate more complex questions about making the delta more sustainable in the face of sea level rise and population growth.