The Ganges, Brahmaputra and Meghna (GBM) basin and numerous minor rivers provide the main freshwater flow into the northern and central part of Bangladesh (see Fig. 8.1). During the monsoon period (June–October), the volume of water often exceeds the carrying capacity of these rivers, and fluvial flooding occurs.
The number of historical flood events from 1954 to 2014, categorised by extent, are shown in Table 8.1 (BWDB 2015). Extreme, dry and average flood years are defined by the national area flooded. Twenty-five per cent of all flood events occurred during extreme flood years and caused significant damage. Two typical examples of fluvial floods, one for an average flood condition (year 2000) and one for an extreme flood condition (year 1998), are shown in Fig. 8.2. The location of the discharge measurement stations are the entry points of the rivers into Bangladesh from India.
The Brahmaputra water level starts rising from the early monsoon (June–July) and reaches its first peak in the third week of July. It then falls and rises again and, in an average flood year, attains its second peak in the first week of August. The Ganges has a single peak of flood level occurring in the second week of September. For the Upper Meghna, the first flood peak occurs in the second or third week of May, and, in an average flood year, a second peak occurs close to the second peak of the Brahmaputra. The effect of flow from the Upper Meghna on overall flooding is small as its discharge only represents ten per cent of the combined discharges of Ganges and Brahmaputra Rivers (Islam et al. 2010). The different timing of the flood peaks of the major rivers are mainly due to variations in rainfall in the upper catchments and the travel time to reach the discharge measurement points considered. Synchronisation of the peaks across the three rivers is rare, but has occurred: the floods of 1988 and 1998 (see Fig. 8.2) being examples (Islam and Chowdhury 2002). During peak synchronisation, the second peak of the Brahmaputra is delayed or may occur as a third peak coinciding with the single peak of the Ganges. This triggers significant fluvial flooding in Bangladesh.
Most fluvial flow adjacent to the study area occurs through the Lower Meghna estuary (MoWR 2005; Haque et al. 2016). This can cause inundation directly through the Lower Meghna, or via the estuarine networks that ultimately drains the combined flows of the three major rivers (Fig. 8.1). The drainage rate of fluvial flood water is largely dependent on sea levels. An elevated sea level due to either spring tides or a sustained monsoon may slow drainage prolonging flood duration, as happened during the 1998 flood (Haque et al. 2002). Over the longer term, subsidence and sea-level rise will increase mean sea levels (see Chaps. 14 and 16), influencing the fluvial flooding pattern in the coastal region.
There are few channels to take fluvial flow from the upper part of the country into the study area (Fig. 8.1), and most of these channels are restricted due to sedimentation. This limits flow and hence fluvial flooding. In addition, when polder embankments are considered (see Fig. 8.1 (lower)), only a small part of the study area is susceptible to fluvial flooding as they significantly determine flood extents. The maximum, minimum and average polder heights in the study region are 5.75, 4.50 and 4.79 m, respectively. Areas within the embankments are generally therefore not flooded during average fluvial events by overtopping (because flood water levels rarely exceed the embankment height), but there are incidents when areas within the embankments are flooded due to polder breaching (because embankments are not strong enough to resist the thrust of the fluvial flood). During extreme fluvial events (e.g. floods of 1998), both overtopping and breaching of polders