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).