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.