Discussion

The model simulations demonstrate how sediment deposition in re-opened polders along the gradient from river-dominated to tidal flow regimes in the GBM delta is controlled by these regimes, SSC and gate regulation. This supports to identify the regions where such water management practice is effective with regards to increasing land elevation by sediment deposition to combat RSLR.
Water level, tidal range and SSC vary seasonally and along the gradient from river-dominated to tide-dominated flow region (Figure 4 and Figure 5). During monsoon heavy rainfall causes excess overland flow, flood and land erosion in upstream areas. This results in high discharge, increased river water level (Figure 2) and higher SSC in the rivers (Figure 5), all contributing to a larger sediment load in the rivers available for sediment deposition by flooding of the polders.
During the dry season, all river reaches within the GBM delta are influenced by the tide, specially the rivers lacking river flow from upstream, such as the Gorai (Figure 4). Consequently, sediment deposition by re-opening the polders during the dry season depends on the sediment delivered from the sea by the tides. The primary rivers of Bangladesh, Ganges, Brahmaputra and Meghna, reach the sea via the Meghna Estuary. Haque, Sumaiya, & Rahman (2016) indicated that part of sediment discharged through the Meghna estuary into the eastern Gulf of Bengal re-enters the estuaries in the west, such as through the estuary of the Pasur River. The amount of sediment that that re-enters the estuaries in the west is much higher than the sediment load directly transferred via the Gorai river from the Ganges towards the western estuaries (Milliman & Haq, 1996; Rogers, Goodbred Jr, & Mondal, 2013). Consequently, the SSC in the feeding rivers is in all seasons much lower in the river-dominated flow region than in the tide-dominated and mixed flow regions (Figure 5). With higher SSC and daily inflow- and outflow of water due to the tides a much larger sediment load is transferred into the beels at the locations of mixed flow and tide-dominated flow regime than for the river-dominated flow regime (Figure 4). Accordingly, sediment deposition inside the beel is larger for the mixed flow and tide-dominated flow regions (Figure 4-6). Still, during monsoon, with higher water discharge from upstream, less sediment is pushed inland from the sea (Figure 2 and Figure 5), resulting into slightly lower SSC than during pre-monsoon and dry seasons in the tide-dominated flow region (Figure 5, location 3). Moreover, the cumulative effect of high sediment loads in the upstream rivers and tide driven sediments brought towards inland results in highest SSC during monsoon in the mixed flow region, even higher than for the tide-dominated flow region (Figure 5).
Total deposition depends on a combination of factors. The total sediment deposition inside the beel during a season depends on the length of that season. The longest season - monsoon – is characterized by the highest river water discharge (Sarker, 2006; Shaha & Cho, 2016) and relatively higher SSC (Figure 5) which result in highest sediment deposition for all flow regions (Figure 6). Due to highest mean tidal range (Figure 4) in the reaches of the tide-dominated flow region during monsoon, sediment deposition is the largest, (Figure 6) even though average SSC is slightly lower (Figure 5) in the feeding river when compared to the mixed flow region. At the location of river-dominated flow sediment enters the beel and leaves slowly with receding peak discharge in the river during monsoon. Here, the beel is not daily re-supplied with fresh sediments, since there is very small tidal variation during monsoon (which is unable to drive a daily, tidal sequence of water inflow and outflow within the polder (Figure 4). This combined with lower SSC results in lowest sediment deposition for the location of river-dominated flow regime compared to other flow regimes during monsoon season (Figure 6). It can be inferred that tide-driven daily supply of fresh sediment towards the beel within the polder is essential for higher sediment deposition. The combined effect of lowest average SSC (Figure 5) and water discharge (Shaha & Cho, 2016) during dry season in the feeder rivers translates to lowest sediment deposition for all three locations representing different flow regimes.
The trapping efficiency is highest during pre-monsoon for the locations of tide-dominated flow and mixed flow regime (Figure 7) as the larger tidal range results in a larger daily delivery of sediment inside the beel (Figure 4). Because of the absence of tidal dynamics and gradual recession of flood water in the river (Figure 2) during monsoon, the volume of water retained inside the beel is mostly undisturbed at the location of the river-dominated regime increasing the residence time and rate of sediment deposition (Figure 6). This results in highest trapping efficiency for the location of river-dominated flow regime (Figure 7), although total deposition is lower due to lower SSC of the incoming water.
Flow regulation results in a higher trapping efficiency, because the residence time of water and sediment inside the beel increases. However, in spite of the higher trapping efficiency, total sediment deposition is less. This is because with unregulated inlets, suspended sediment is daily resupplied inside the beel with tides. Sediment can enter the beel only half of the time with flow regulations because the inlets are closed and opened in 12 hours cyclic order (see section 2.4). The double time of beel opening without regulation is results in higher sediment deposition. Remarkably, the same regulation scheme has the opposite effect for the river-dominated flow regime. The tidal range for the river-dominated flow regime is very small during monsoon; hence there is no tidal dynamics and cyclicity of sediment delivered into the beel. Instead, sediment enters the beel with high water level in the river and leaves it slowly with the recession of flood water. The sediments carried by the rivers of SW Bangladesh is very fine (less than 63 µm) having low settling velocity (about 0.05 cm/s) (Barua, Kuehl, Miller, & Moore, 1994). Due to sthis low settling velocity, a large portion of sediments entering the beel stays in suspension and recedes with tide as even the highest trapping efficiency achieved with flow regulation is about 30%.
More sediment can be delivered with simultaneous gate operation than with the successive gate, because two inlets open at the same time instead of one. This obviously results in larger sediment deposition for tide-dominated flow regime and mixed flow regime. However, the effect is opposite during monsoon at the location of river-dominated flow regime. With both gates open at the same time for simultaneous gate operation, the average velocity of receding flow inside the beel is higher than the successive gate with which one gate is open at an instance. Due to higher receding velocity, more sediment returns to the feeding river which before it can settle in the polder, as the tidal range is lowest during monsoon at the location of river-dominated regime, resulting in lower sediment deposition.
In the “Special Report on the Ocean and Cryosphere in a Changing Climate”, IPCC indicates that the global mean sea level (GMSL) has risen by 3.6 mm/year over the period of 2005 to 2015 (Oppenheimeret al. , 2019). GMSL is predicted to rise between 0.24 m under RCP 2.6 (0.17–0.32 m, likely range) and 0.32 m under RCP 8.5 (0.23–0.40 m, likely range) by 2050 (Oppenheimer et al. , 2019) with medium confidence (Oppenheimer et al. , 2019). Goodwin et al.(2018) projected the sea level rise to be 0.21 m by 2050 for AMP4.5 scenario. The study on subsidence of GBM delta by Brown and Nicholls (2015) compiled of 205 literatures indicates that the rate of subsidence ranges from -1.1 mm/year to 43.8 mm/year which varies spatially and temporally with the median as 2.9 mm/year and standard variation of 3.4 mm/year. When a linear trend for the rate of SLR per year and the median of the subsidence rate per year are considered, the rate of relative SLR (RSLR) ranges from 7.6 mm/year for AMP4.5 and 10 mm/year for RCP8.5. We consider the density of the sediments in the floodplain as 1300 kg/m3 suggested by Allison and Kepple (2001) and Rogers and Overeem (2017) for GBM delta to calculate the change in land elevation through sediment deposition. The sediment deposition during the monsoon season by unregulated flow into the beel within the polder for river-dominated flow, mixed flow and tide-dominated flow regions are estimated as about 0.5 mm, 11 mm and 14.3 mm, respectively.
Rogers and Overeem (2017) indicate that in the GBM delta the agredation rate by sedimentation can be more than the estimated average rate of local sea level rise. This is in agreement with our findings. Amiret al. (2013); de Die, (2013); Gain et al. , (2017); Shampa and Pramanik (2012); Talchabhadel et al. (2018); van Staverenet al., (2017) investigated the polders along the river reaches with tide-dominated flow and inferred that allowing sediments inside the polder with TRM raises the land level with sedimentation. However, they did not compare the rate of land level rise to the RSLR. Our estimation shows that controlled flooding for sediment accumulation has high potential for tide dominated and mixed flow region. As all the polders are within tide-dominated flow region and mixed flow region (Figure 1), controlled flooding by re-opening the polder during monsoon can potentially assist the polders to mainntain the height to overcome projected RSLR for the worst case scenario. However, the rate of SLR as well as the rate of subsidence considered have large uncertainly and wide range. As the average land elevation for river-dominated flow regime is about 10 m AMSL, it can safely be asumed that it will not be flooded due to relative SLR even in monsoon with the average of highest water level along the river reaches of about 7 m. However, with SLR the region of tide-dominated and mixed flow regimes will presumably shift upstream making TRM effective for larger areas in the future.