Introduction
The low elevation coastal zone of the Ganges-Brahmaputra-Meghna (GBM)
delta of southwestern (SW) Bangladesh is flat with an estimated 21% of
the area having an elevation less than 1 meters above mean sea level
(AMSL) and 78% of the area below 5 meters AMSL (CCC, 2016). By rising
earthen embankments to protect the tidal lowlands from flooding during
high tide, about 139 polders were constructed in the late 1960s and
1970s (van Staveren, Warner, & Khan, 2017). The polders
increased food productivity initially, but hampered the sediment flow in
the floodplains and natural aggradation of the delta plain. This
sediment flow in the GBM delta is large: in total about 1 billion tons
of sediments per year is transported to the sea (Islam, Begum,
Yamaguchi, & Ogawa, 1999). With decreasing supply of fresh water flow
from upstream during dry season, the rivers of SW Bangladesh have lost
capacity to carry the sediment load, while sediment deposition on the
delta plain dramatically reduced. This resulted in massive
within-channel sediment deposition, causing silting-up of the river
channels, decreasing their water conveyance capacity during monsoon and
leading to higher flood water levels. Also, drainage of the low-lying
polders during the rainy season has become increasingly problematic. Sea
level rise (SLR) is one of the consequences of climate change that will
exacerbate the situation by increasing the risk of flooding and water
logging. This will directly affect millions of people living in polders
(van Staveren et al. , 2017).
The Ganges-Brahmaputra-Meghna (GBM) delta is now in danger of loosing up
to one fourth of its habitable land by 2100 due to sea level rise
(Ericson, Vörösmarty, Dingman, Ward, & Meybeck, 2006). The coastal zone
of Bangladesh has over 47,150 km2 area and population
of 38.52 million in the year 2011 (CCC, 2016). From tide gauges and
altimetry observations, Intergovernmental Panel on Climate Change (IPCC)
indicated that the global mean sea level (GMSL) has risen by 3.6 mm/year
over the period of 2005 to 2015 (Oppenheimer et 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 (Oppenheimeret al. , 2019). “likely” denotes the likelihood of an outcome or
result is 66-100% (Oppenheimer et al. , 2019). Sea level rise is
not globally uniform and varies regionally as well (Oppenheimer et
al. , 2019) which adds to the uncertainty of the projection at regional
scale. Goodwin, Haigh, Brown, Nicholls, and Matter (2018) analyzed the
effect of Adjusting Mitigation Pathway (AMP) on global warming and SLR
in the future. AMPs are considered scenarios to restrict future warming
to policy-driven targets, in which future emissions reductions are not
fully determined now but respond to future surface warming each decade
in a self-adjusting manner (Goodwin et al. , 2018). Goodwinet al. (2018) projected the sea level rise to be 0.21 m by 2050
for AMP4.5 scenario. AMP4.5 scenario considers global carbon emission to
have 4.5°C warming above preindustrial by 2300. Sediment starvation and
subsidence will increase the vulnerability of low-lying deltas to SLR
(Brown et al. , 2018). Brown and Nicholls (2015) summarized 205
subsidence data on GBM delta from previous studies. Reported subsidence
rates range from -1.1 mm/year to 43.8 mm/year, with considerable spatial
and temporal variation. Brown and Nicholls (2015) calculated the median
of the rate of subsidence as 2.9 mm/year with standard variation of 3.4
mm/year. The study is indicative of ongoing subsidence in the GBM delta.
However, detailed study needs to be carried out as rate of yearly
subsidence presented till now has large uncertainty with wide range.
Due to progressive siltation, the river channel beds of SW Bangladesh
have become eventually higher than the land inside the polder resulting
in prolonged water logging and drainage congestion (Awal, 2014). In
1997, this led to re-opening of the dike of one of the polders in SW
Bangladesh to re-allow tidal water inside a beel (Adnan, 2006). Beels
are the lowest parts of the delta plain polders, where surface runoff
accumulates through the internal drainage channels (Chakraborty, 2009).
Re-allowing tidal dynamics inside the beel resulted in removal of
sediment from the adjacent river, and the land surface inside the beel
raised substantially with the sediment deposited through the re-opening
of the dike (Adnan, 2006; van Staveren et al., 2017). As this
intervention was primarily applied to retain the drainage capacity and
navigability of tidal river, it was termed as tidal river management
(TRM). Since then, TRM has been applied to 12 beels in SW Bangladesh
(Gain, Benson, Rahman, Datta, & Rouillard, 2017) where tide plays a
dominant role. Current practice of continuously allowing water inside a
polder for several years restricts the use of the land for economic
activities during TRM operation. Such prolonged flooding without proper
compensation (Gain et al. , 2017), and observed uneven sediment
deposition within the beels has questioned the acceptability of TRM by
stakeholders.
The potential of seasonal flooding the land with controlled
sedimentation through re-opening a polder such as TRM to raise the land
will make the land available for part of the year for agriculture and
for alternative livelihood such as fish farming, floating agriculture
when flooded. Seasonal operation of TRM will not only raise the land but
also bring nutrient rich sediments, act as natural pest control (van
Staveren, van Tatenhove, & Warner, 2018) and avail the land for
economic activities during the remaining seasons. The acceptability of
TRM by stakeholders may thus be raised by providing opportunities for
economic activities for earning livelihood round the year even when TRM
is operated. Brown et al. (2018) stated that the mitigation
measures to climate change and controlled sedimentation to maintain
relative delta height is vital for survival over multi-centennial
timescales. For sinking deltas around the world with sufficient sediment
delivered by the rivers, this provides an opportunity to elevate land
above predicted sea level rise by applying controlled flooding like
TRM.The effect of re-opening of polders such as TRM has been studied by
both historical data analysis (de Die, 2013; Gain et al. , 2017;
van Staveren et al., 2017) and mathematical model simulations
(Amir, Khan, Khan, Rasul, & Akram, 2013; Shampa & Pramanik, 2012;
Talchabhadel, Nakagawa, & Kawaike, 2018). Previous studies explored the
effects of increasing number of inlets on sediment deposition inside the
beel, compartmentalization, use of canals inside the polders within the
Tekka-Hari-Kobadak-Shibsa River reach (SW Bangladesh), which is
tide-dominated. Although the primary objective of TRM was to increase
the tidal prism, it resulted at the same time in land elevation increase
for all studied cases.
Previous TRM applications of controlled flooding by re-allowing
sediment-rich water from a tide- dominated river into a polder indicated
that the resulting sediment deposition can raise the polder land surface
substantially but unevenly (Gain et al. , 2017). Yet, controlled
temporary flooding has not been investigated for polders along river
sections with a river-dominated and mixed flow regime. To evaluate the
potential of TRM-like strategies of controlled flooding of polders to
raise the land throughout the delta, their efficiency needs to be
explored first for a wider range of flow regimes across the GBM delta.
The effect of seasonality of river flow, tidal ranges and associated
suspended sediment concentrations (SSC) in the feeding river branches
should be investigated as well for these different flow regime, since
SSC and tidal range are major controls of sediment deposition (Islam,
Middelkoop, Schot, Dekker, & Griffioen, 2019), and show a strong
seasonal variability (IWM, 2010; 2017).
We investigate the applicability of TRM-like controlled flooding of
polders along a gradient from river-dominated to tide-dominated flow
regime across the GBM delta by investigating the resulting sediment
deposition for different flow regimes of the feeding rivers, for
different seasons, and regulation of polder inundation. For this
purpose, we developed scenarios that comprise these different boundary
conditions. Using a calibrated model 2D hydrodynamic flow model for Beel
Pakhimara, which is an ongoing TRM located in SW-Bangladesh (Islamet al. 2019), we simulated sediment deposition for these
scenarios to determine how these boundary conditions affect total
deposition and trapping efficiency.
This is an exploratory study at the delta scale to investigate the
applicability of controlled flooding like TRM across different regions
characterized by river-dominated and mixed flow regimes in the southwest
of Bangladesh. The input variables utilized for selected locations for
each flow regime are considered to be representative of the respective
regions.