Spatial and seasonal variability of sediment accumulation potential
through controlled flooding of polders in the Ganges-Brahmaputra-Meghna
delta of southwest Bangladesh
Abstract
The densely populated coastal areas of the Ganges-Brahmaputra-Meghna
(GBM) delta within Bangladesh are in danger of losing up to one fourth
of their habitable land by 2100 due to relative sea level rise (RSLR).
Tidal River Management (TRM) presents an opportunity to combat RSLR by
raising the land level through controlled sedimentation in re-opened
polder sections. To date, TRM has been applied to tide-dominated coastal
regions, but the potential applicability of TRM for river-dominated flow
and mixed flow regimes is yet to be assessed. We apply a calibrated 2D
numerical hydromorphodynamic model to quantify sediment deposition in a
re-opened polder section (‘beel’) under conditions of river-dominated,
tide-dominated and mixed flow regimes for different seasons and flow
regulations. Simulation results show seasonality in sediment deposition
with monsoon season having the highest. The potential for TRM is largest
along the reaches of the tide-dominated region where sediment deposition
is highest in all three seasons (Pre-monsoon, Monsoon and Dry season),
and almost 28 times higher than river-dominated region during monsoon.
Regulating flow into a polder increases trapping efficiency, but
slightly lower total deposition than without regulation. Our results
show that re-establishing polder flooding without regulating the flow
into the polder is a promising strategy for the mixed and tide-dominated
flow regions in the delta as the sediment deposition can elevate the
land more than the yearly rate of RSLR. Application of controlled
flooding like TRM therefore provides opportunity to match the rate of
RSLR throughout the GBM delta. TRM can potentially be applied to the
sinking deltas around the world to counter RSLR.