Being a riverine country, about one-fifth to one-third of Bangladesh is flooded every year. The flood mainly occurs during the monsoon period of early July to late September. Even in 2021, when rainfall throughout Bangladesh is below average, floods are being experienced in some floodplains of Teesta, Jamuna River as well as the southern lower portion of the country.
Bangladesh is one of the most susceptible countries to flood in South Asia, as well as the world's most vulnerable area to climate change consequences. 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 principal sources of floods are the river floods from the overbank flows of the major river systems, the Brahmaputra, the Ganges, and the Meghna, in the monsoon months. Local rainfall floods frequently accompany river floods caused by runoff from Bangladesh's high-intensity, long-duration precipitation. This huge discharge of water cannot be drained to the Bay of Bengal because of high outfall water levels. The northern and north-eastern transboundary hilly rivers are susceptible to flash floods from the adjacent hills in India in the pre-monsoon during the months of April and May. The areas adjacent to estuaries and tidal rivers in the southwest and south-central parts of the country experience tidal floods twice a day due to astronomical tide from the Bay of Bengal. Tropical storms in the Bay of Bengal trigger cyclonic storm-surge floods once in a while, affecting around 12,000 km2 of coastal land between April and November.
The important elements that determine the extent of flooding are the magnitude, synchronization of peaks, and duration of floods. Smaller differences in peaks of major floods can make a big difference in terms of flood-affected areas 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 the flood. Also, the synchronization 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 area 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 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 % 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 7% floodplain lies within Bangladesh which drains about 91% of runoff of the basin. In this process, the country’s land consisting of 80% of the 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 the failure of flood control embankments.
Anthropogenic activities in the form of construction 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, encroachment to rivers, and inadequate drainage capacity. Because of urbanization, 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 waterbody and so the water carrying capacity of the rivers and lakes is decreasing. These are leading to flooding in the localities causing enormous distress to human life.
With the increase of population, more and more people are settling in flood-prone areas, making them more vulnerable to floods. An analysis conducted with 2001 population census data revealed that some 45.5 million people were exposed to severe and moderate floods. Flood-prone zones are the worst off among different disaster-prone areas in terms of food shortages, the incidence of extremely poor, insufficient income, illiteracy, and a high concentration of wage laborers. Proper management of floods in a country like Bangladesh is a crying need and the integration of all stakeholders in the management process can achieve a fruitful result.
Writer: Ashik Iqbal, Research Assistant, Institute of Water and Flood Management, BUET. Email: [email protected]
BDST: 1405 HRS, AUG 16, 2021