Physical controls and a priori estimation of raising land surface elevation across the southwestern Bangladesh delta using tidal river management

Abstract

Abstract. The Ganges–Brahmaputra–Meghna delta in Bangladesh is one of the largest and most densely populated deltas in the world and is threatened by relative sea level rise (RSLR). Renewed sediment deposition through tidal river management (TRM), a controlled flooding with dike breach, inside the lowest parts of the delta polders (so-called beels) can potentially counterbalance the RSLR. The potential of TRM application in different beels across southwestern Bangladesh has been estimated previously but requires further exploration. Neither the seasonal and spatial variations in physical drivers nor the non-linear character of physical drivers and several sensitive parameters for sediment deposition have been taken into account so far. We used a 2D morphodynamic model to explore the physical controls of the following five parameters on the total sediment deposition inside the beels during TRM: river tidal range (TR), river suspended sediment concentration (SSC), inundation depth (ID), width of the inlet (IW), and surface area of the beel (BA). Our model results indicate that these five parameters and their interactions are significant for sediment deposition per day (SPD), where SSC and BA have a high impact, TR and ID have a moderate impact, and IW has a low impact on sediment deposition. Non-linear regression models (NLMs) were developed using the results of 2D models to quantify how sediment deposition inside the beels depends on these parameters. The NLMs have an average coefficient of determination of 0.74 to 0.77. Application of the NLMs to 234 beels in southwestern Bangladesh indicates that TRM operation in beels located closer to the sea will retain more sediment as a result of decreasing SSC further inland. Lower average land surface elevation is one of the reasons that the beels in the western part retain more sediment. Smaller beels have a higher potential to raise the land surface elevation due to the non-linear increase in sediment deposition per day (SPD) with beel area. Compartmentalization of larger beels may increase their potential to raise the land surface elevation. Thus, the length of time of the TRM application in a cyclic order will need to vary across the delta from 1 to multiple years to counterbalance RSLR, depending on the current beel land surface elevation and local TRM sediment accumulation rates. We found that operating TRM only during the monsoon season is sufficient to raise the land surface in 96 % and 80 % of all beels by more than 3 and 5 times the yearly RSLR, respectively. Applying TRM only seasonally offers huge advantages as to keeping the land available for agriculture during the rest of the year. The methodology presented here, applying regression models based on 2D morphodynamic modeling, may be used for the low-lying sinking deltas around the world to provide an a priori estimation of sediment deposition from controlled flooding to counterbalance RSLR.