Bangladesh is a riverine country and even in a normal flood year like 2021, the floodplain of Teesta River and the southern lower part of the country is facing flood. About one-fifth to one-third of the country is flooded every year. The flood mainly occurs during the monsoon period of early July to late September.
Bangladesh ranks high in the list of vulnerable countries in South Asia, and the most vulnerable region of the world to climate change impacts. There are five main types of natural floods occurring in Bangladesh: (i) riverine flood, (ii) rainfall flood, (iii) flash flood, (iv) tidal flood and (v) storm surge flood.
The important elements that determine the extent of flooding are the magnitude, synchronisation of peaks and duration of floods. Smaller differences in peaks of major floods can make a big difference in terms of flood affected area, since it is the spreading of floodwater evenly over a wide and flat floodplain. As all the flows are drained in the Bay of Bengal only by the lower Meghna River, it takes time and lengthens the duration of flood. Also, the synchronisation of peak flows in the Brahmaputra and the Ganges is a major determinant of the extent of flooding in the country. When the peaks of the two rivers coincide, severe flooding occurs as it was the case in 1988, 1998 and also 2004. The country has experienced floods since ancient times but there is an increasing trend in year-to-year variability in the annually flooded areas from the mid-1970s. Some very severe floods were experienced in 1987, 1988, 1998 and 2004, and some medium severe floods in 1991, 1993 and 1995.
There are a few geographical, physiographic and hydro-meteorological factors responsible for floods in Bangladesh. The country is surrounded by hills on its three sides, Rajmahal hills in the west, the Himalayas and the Meghalaya Plateau in the north, and Tripura–Chittagong hills in the east. The rainfall-runoff from this vast hilly area coupled with snowmelt in the Himalayas brings a huge inflow of water to Bangladesh during the monsoon season. About 80 per cent of the rainfall occurs during the months from May to September.
The country is located at the lower parts of the basins of the Ganges, the Brahmaputra and the Meghna but only seven per cent floodplain lies within Bangladesh which drain about 91 per cent of runoff of the basin. In this process the country’s land consisting of 80 per cent of floodplain gets inundated by floodwater. The impacts of this type of flood are on the rise because of a change in hydrological regime in the floodplains due to unplanned construction of different types of infrastructures, such as dams, barrages, roads, bridges, culverts, etc. Also, the siltation of riverbeds and encroachment of wetlands in the floodplain is one major reason for unexpected flooding. Another cause of concern is the damage caused by sudden floods due to failure of flood control embankments.
Anthropogenic activities in the form of construction of roads without sufficient drainage capacity through them, road alignments transverse to the main drainage paths, blocked drainage channels due to siltation, cross-dams or fishing activities and inadequately sized drainage sluices are increasing urban floods.
Recently the flood events are becoming more severe because of the river-bed siltation, river encroachments and inadequate drainage capacity. Because of mass urbanisation, people are filling up the wetlands (ponds, beels) which previously functioned as a reservoir for the rainwater in the floodplain. People throw a large volume of solid wastes in the waterbodies and so the water carrying capacity of the rivers and lakes are decreasing. These are leading to flooding in the localities causing enormous distress to human life.
The writer is a Research Assistant,
Institute of Water and Flood