Results
Figure 4 shows the mean water level and tidal range along the gradient
from river-dominated to tide-dominated flow regime across the GBM delta
for the three main hydrological seasons. Mean water level gradually
decreases from the reaches with river-dominated flow to tide-dominated
flow (Figure 4). The mean water level is lowest during dry season and
highest during monsoon for reaches of all three flow regimes (Figure 4).
The water level gradient from river-dominated flow regime to
tide-dominated flow regime is steepest during monsoon and mildest during
pre-monsoon season (Figure 4). The reaches with river-dominated flow
regime have the largest seasonal variation of mean water level being
about 5 m between dry season and monsoon. In contrast, seasonal
variation in mean water level is smallest in the reaches under
tide-dominated flow, being about 1 m between dry season and monsoon.
Mean tidal range also varies seasonally, being largest during
pre-monsoon and smallest during monsoon seasons for all reaches across
the GBM delta (Figure 4). Tidal range varies seasonally for all flow
regimes with lowest during monsoon and highest during pre-monsoon
season. The highest variation of about 90 cm occurs in reaches under the
mixed flow regime, and lowest is about 20 cm in the reaches under the
tide-dominated flow regime
Considering the seasonality, the average SSC is highest during monsoon
in the reaches of river-dominated and mixed flow regime, whereas the
average SSC is highest during pre-monsoon in the tide -dominated regime
sections (Figure 5). Along the gradient from river-dominated to
tide-dominated regime, tidal range and average SSC change gradually
along the transect (Figure 4 and Figure 5). The reaches of mixed flow
experience the highest average SSC during the monsoon season (Figure 5).
The results of the different model scenarios are compared for the total sediment accumulation over the seasons and for the trapping efficiency. The results demonstrate that estimated total sediment deposition over the season inside the beel of the re-opened polder varies per season for all flow regimes (Figure 6). At all locations, total sediment accumulation over the seasons is highest during the monsoon season (Figure 6). For the tide-dominated regime, seasonal variation in sediment accumulation is smaller than for the river-dominated and mixed flow regimes. The sediment deposition during monsoon is almost 50% higher than during the dry season for the tide-dominated flow regime, whereas it is ten times higher for the river-dominated regime, and almost 40 times higher for the mixed flow regime (Figure 6).
Along the gradient from river-dominated to tide-dominated flow regime, the calculated total sediment deposition for all three seasons increases substantially. During monsoon, total sediment deposition with unregulated flow is about 4,600 tons at the location of river-dominated flow, about 100,200 tons at the location of mixed flow and about 129,800 tons at the location of tide-dominated flow regime. The total sediment deposition at the location of tide-dominated regime is highest among the flow regimes for all seasons, and almost 28 times higher than at the location of the river-dominated flow regime during monsoon season (Figure 6).
Regulation of flow into the re-opened polder affects sediment deposition within the polder. Model simulations show for the location of river-dominated flow regime that using successive gate operation results in about 18% more sediment deposition during the monsoon period. Conversely, for the location of the tide-dominated and mixed flow regimes unregulated flow into the re-opened polder results in higher sediment deposition (Figure 6). During monsoon, unregulated flow into the re-opened polder results in about 19% more deposition than regulated flow with successive gates at the location of the mixed flow regime, and about 45% more for tide-dominated flow regime.