Methods
Study area
The study covers the 41,900 km2 southwest part of GBM
delta of Bangladesh, bounded by Ganges River and Padma River in the
north, Meghna River in the east, Bay of Bengal in the south and the
international border between Bangladesh and India in the west (Figure
1). The population of the study area is about 38.52 million according to
the recent census conducted by Bangladesh Bureau of Statistics (BBS)
(BBS, 2012). Polders are located in the southern part of the study area.
The Gorai River in the western part and the Arial Khan River in the
eastern part are the primary sources of fresh water flow. The river flow
in southwest Bangladesh is affected by tides entering from the south..
The areas experiencing reversal of water flow direction (horizontal
tide) due to tide during dry season were designated as tide affected
areas by FAO (1985) and Wilson et al. (2017).
During monsoon the rivers receive large volumes of flood water from the
Ganges and the Padma Rivers, which causes wide-spread flooding in the
delta. The tidal flats located in the southern part of the study area
experience flooding as well, but primarily due to high tide. The extent
of river floodplain and tidal floodplain for the coastal areas of
Bangladesh were depicted by Brammer (1990). Brammer (1990) defined as
tidal floodplain those areas that consist of numerous tidal rivers and
creeks, being subject to flooding during high tide even in dry season,
and as river floodplain those areas that become flooded seasonally due
to high water level in the rivers specially during monsoon. The
tide-affected area defined by FAO (1985) and Wilson et al. (2017)
overlaps the southern part of the river floodplain delineated by Brammer
(1990) (Figure 1).
We divided the study area in three regions with their own flow regimes:
river-dominated flow, tide-dominated flow and mixed flow. The tidal
floodplain is identified as the region of tide-dominated flow regime,
the area with river floodplain but affected by tide is defined as region
of mixed flow regime and remaining as region of river-dominated flow
regime (Figure 1). Within each region we selected a representative
location for our analyses: 1- river-dominated flow regime, 2 - mixed
flow regime, and 3 - tide-dominated flow regime (Figure 1). Average
elevation of the region of river-dominated flow, mixed flow and
tide-dominated flow regimes, calculated using the elevation data
collected from Shuttle Radar Topography Mission (SRTM) are about 10 m,
2.2 m and 1.7 m AMSL, respectively. Most of the polders lie within the
region of tide-dominated flow regime, few in the region of mixed flow
regime and none within the region of river-dominated flow regime (Figure
1).
The average monthly rainfall for the years 1901 to 2015 (Arefin &
Mallik, 2017; Figure 3) and the average monthly discharge of Gorai River
branch (IWM, 2017; Figure 3) indicate that rainfall is highest in June
and discharge is highest in August. Seasons for the study area were
defined by combining these rainfall and discharge data with the
suggestions made by Lázár et al. (2015) into dry season (November
to February), pre-monsoon (March to May) and monsoon (June to October).
The Gorai-Nabaganga-Pasur River reach of SW Bangladesh was selected for
this study (Figure 1). The Gorai River is a distributary of the Ganges
River, has its bifurcation from Ganges River at about 50 km downstream
of the Bangladesh-India border, and reaches the sea about 300 km further
downstream as Pasur River and other branches (Figure 1). The Gorai has
very little fresh water flow in the dry season, but discharges large
amounts of water during the monsoon (Moly, Rahman, & Saadat, 2015)
(Figure 3). The water level over the entire Gorai River is influenced
bsy the tides (Figure 2) during the dry and pre-monsoon seasons. During
monsoon the high river discharge nullifies the effect of tides (Figure
2). The upper Gorai River represents the region of river-dominated flow
regime during monsoon. Therefore, we used the hydro-morphodynamic
conditions at Location 1 (“Gorai Railway Bridge” measurement station
of Bangladesh Water Development Board (BWDB) on the Gorai River reach)
(Figure 1) as input for the modeling under the river-dominated flow
regime.
The Pasur River is the most downstream section of the
Gorai-Nabaganga-Pasur River system and meets the sea in south. The Pasur
lies in the region of tide-dominated flow regime (Figure 1). The
hydro-morphodynamic conditions at Location 3 (“Mongla” measurement
station of BWDB on the Pasur River reach) were used as input for the
modeling under the tide-dominated flow regime (Figure 1). The flow of
the Pasur River is dominated by tides around the year (Figure 2); still,
the mean water level of the Pasur rises during the monsoon, then still
experiencing an average tidal range of about 2 m (Figure 2 and Figure
4). Salinity varies seasonally between about 20 ds/m (deci-siemens per
meter) during the pre-monsoon season and about 5 ds/m in monsoon (Ghosh,
Kumar, & Roy, 2016).
The Nabaganga River is the section of the Gorai-Nabaganga-Pasur River
reach which is about 150 km north from the sea shore, and lies within
the region of mixed flow regime (Figure 1). The hydro-morphodynamic
conditions at Location 2 (“Gazirhat” measurement station of BWDB on
Nabaganga River) were used as input for the modeling under the mixed
flow regime (Figure 1). The mixed flow region is affected by tide during
all seasons, although during monsoon tidal ranges are damped due to the
flood water coming from upstream (Figure 2). Average tidal range at
location 2 varies from 0.3 m during monsoon to 1.45 m in the pre-monsoon
period (Figure 4).