Conclusion
We explored the potential of re-opening of polders to re-establish
sediment deposition and land raise in the SW Ganges-Brahmaputra-Megna
(GBM) delta. For this purpose we carried out scenario analyses
representing the seasonal variability river discharge, suspended
sediment concentration (SSC) and tidal range in river branches across
the delta, and applying different flow regulation operations for the
polder. Our scenario analyses demonstrate that:
- Flow regimes are primary controls of sediment deposition inside a
polder of TRM operation. Along the gradient from river-dominated
(north) to tide-dominated (south) flow regime, the total sediment
deposition increases considerably. Sediment accumulation within
polders along the reaches with a tide-dominated flow regime can be 28
times higher than in polders along the reaches of river-dominated flow
regime. Therefore, the potential for TRM is much higher along the
reaches of tide-dominated flow regions. With SLR the region with
tide-dominated flow regime will presumably shift inland making TRM
effective for larger areas in the future.
- Seasonality of sediment deposition is evident in all flow regions.
Highest sediment deposition by re-opening the polder occurs during
monsoon season for all reaches of the GBM delta. Highest river
discharge and relatively higher SSC during monsoon season provides
highest sediment load in the rivers for raising the polder land
surface. In the tide-dominated region the seasonal variation of
sediment deposition is remarkably small, due to the strong tidal
effect in all seasons.
- Tidal range, SSC and discharge in the river govern the sediment
dynamics and sediment deposition inside the polder. Increase in river
discharge, SSC and tidal range result in larger sediment deposition
inside the polder. Sediment deposition is highest during monsoon owing
to with highest river discharge and relatively higher SSC in the
feeder rivers, and lowest during dry season when river discharge and
SSC in the feeder river is lowest. The highest tidal range along with
only slightly lower SSC during monsoon translates to highest sediment
deposition in the polders in the reaches with tide-dominated regime.
- Regulation of flow into the re-opened polder considerably affects
total sediment deposition in the polder. Unregulated flow results in
highest sediment deposition during monsoon for the regions with
tide-dominated flow and mixed flow regimes which experience tidal
effect even in monsoon. Thus, it is potentially an effective means to
increase sediment deposition.
- All polders of GBM delta are situated within the mixed flow region and
tide-dominated flow region. The total sediment deposition results in
an increment of land elevation for mixed flow region and
tide-dominated flow region that is more than the highest rate of
projected relative sea level rise. Although the uncertainty of
projected SLR for 2050 is relatively low (Oppenheimer et al. ,
2019), subsidence included in RSLR have large uncertainty and wide
range (Brown & Nicholls, 2015) with large spatial and temporal
variation. It can be safely assumed that RSLR will not induce flooding
for river-dominated flow region because the average land elevation is
about 10 m AMSL with average of highest water level along the river
reaches of about 7 m AMSL.
The effect of projected SLR and subsidence can thus be countered with
the application of indigenous practice like TRM to re-open the dike to
allow sediments inside polders through controlled flooding, even for the
low elevation coastal zone of Bangladesh. This study illustrates that
with enough sediment availability, sinking deltas around the world can
combat the projected RSLR by applying controlled flooding with
re-opening the dike like TRM.