Enhancing effectiveness of tidal river management in southwest Bangladesh polders by improving sedimentation and shortening inundation time.

• Number of inlets and flow regulations dictate spatial variation of sedimentation. • Regulated flow increases total sediment deposition and trapping efficiency. • Two inlets at opposite sides increase spatial uniformity and total sedimentation. Bangladesh, one of the countries most vulnerable to climate change, is threatened by sea level rise (SLR) and land subsidence. The tidal river management (TRM) practised in coastal regions of Bangladesh has the potential to raise the land by sedimentation, to counteract SLR and subsidence. TRM is an indigenous method in which dikes are breached to readmit sediment-rich water into a polder, which results in sediment being deposited in depressions in the polder called 'beels', while simultaneously preventing silting-up of the tidal rivers. However, after several years of continuous sedimentation from TRM, deposition has been uneven and less than expected. To study the effectiveness of TRM, this research analyses different scenarios to identify which operation schemes most effectively trap sediment to raise the land surface. The scenarios developed considered the number of inlets, flow regulation through the beel using open inlets or gated inlets with different operation schemes, and seasonality. The study area was Pakhimara Beel in southwest Bangladesh, where TRM is ongoing. To simulate the scenarios, a two-dimensional (2D) morphodynamic model with variable cell size was set up, calibrated and used. The simulation results were analysed for total sediment deposition, uniformity of spatial distribution of sediment deposition and trapping efficiency. Sediment deposition shows clear seasonal variability, with greatest deposition in the pre-monsoon period, less during monsoon and least in the dry season. Greatest deposition combined with high spatial uniformity was found for TRM that uses two inlets located at opposite sides connected to different watercourses. Regulated flow using successively opened gates resulted in highest sediment deposition in all seasons, about double that of the existing situation without gate. However, given the complexity and cost of gate operation, TRM with two inlets located at opposite sides of the beel without flow regulation may be considered more feasible, and still effective despite 20–30% less sediment deposition. To also increase acceptability by local affected stakeholders we propose to restrict this improved TRM to the monsoon period, to allow crops to be grown on the land in the dry and pre-monsoon periods, and ensure salinity is minimized. Such well-planned TRM has the potential to also counteract sea level rise in sinking deltas elsewhere in the world by enhancing sedimentation. [ABSTRACT FROM AUTHOR]