The country has experienced 17 large floods in the last 60 years since 1954. The damages caused by the annual floods and particularly by the devastating abnormal ones are enormous. The 1987 flood completely or partially damaged 2.06 million houses (or 16.67% of the national housing stock). The 1988 flood has displaced and affected an estimated 45 million people and destroyed or partially damaged 12.8 million houses of which 5.4 million were completely destroyed and 7.4 million were partially damaged (BBS, 2008). The losses of standing crops and livestock and damage to rural and urban infrastructures have also been colossal. The 1998 flood-affected 30.92 million people in 52 out of the 64 districts (BDM, GOB handout, 1998). An estimated 916,660 houses were totally damaged with other 1,300,000 houses damaged partially, (Alauddin, 2010).
3. Types of Flood in Bangladesh Floods are categorized based on spatiality, timing, intensity and duration into four groups in Bangladesh: viz. Monsoon floods, Flash floods, Rainfall-induced floods and Tidal floods.
4.1. Short-term Causes There are two major causes of the short-term flood stated below
4.1.1. Monsoon Downpour An increased amount of precipitation can cause flooding. An above normal monsoon downpour in the Ganges Brahmaputra-Meghna drainage system is thought to be the primary cause of the 1988 flood in Bangladesh (GOB and UNDP, 1989; Brammer, 1990).
4.1.2. Synchronization of Flood Peaks For the major three rivers the synchronization of flood peaks took place within a two week time period, causing a sudden increase in water level in virtually all areas of the country that reduce the water carrying capacity of the drainage system and decrease land elevation with respect to the base level of the rivers in Bangladesh (GOB and UNDP, 1989; Brammer, 1990)
4.2. Long-term Causes There are several causes of long-term floods and short descriptions about them are below.
4.2.1. Compaction and Subsidence of Sediments Most deltas subside due to the weight of the thick sediment layer. Subsidence along with compaction reduces land elevation with respect to the rising sea level. Even though the rate of subsidence and compaction are not yet well documented, based upon our knowledge about processes active in other deltas it can be assumed that Bangladesh's delta is also undergoing subsidence and compaction (Khalequzzaman, 1989).
4.2.3. River Bed Aggradations: River bed aggradations are most pronounced for the Ganges and its distributaries. The riverbed has aggraded as much as 5-7 meters in recent years from the border with India to the point where the Ganges meets the Brahmaputra. River bed aggradations reduce the water carrying capacity of rivers, causing them to overflow their banks and it is so pronounced in Bangladesh that changes in river bed level can be observed during one's lifetime (Khalequzzaman, 1989).
4.2.4. Accumulation of Inadequate Sediment: The earlier described data showed that the average sediment accumulation rate for the last few hundred years in the coastal areas of Bangladesh is 5-6 mm/year, which is not enough to keep pace with the rising sea level. As a result, net land elevations must have been decreasing over time, resulting in more flood inundations. For the land, the counter effects of a rising sea level are accumulating sediment at a rate that is sufficient to keep pace.
4.2.5. Deforestation in the Upstream Region An extreme amount of deforestation increased in the hills of Nepal due to the rapid increase in population in the Indian Subcontinent over the course of the present century to meet the increasing demand for food and fuelwood. Deforestation of steep slopes is assumed to lead to accelerated soil erosion and landslides during monsoon precipitations. It, in turn, is the result of devastating floods in the downstream regions in Bangladesh.
4.2.6. Rivers Damming River damming reduces the velocity of water flow downstream from the dam. As a result of reduced velocity, the sediments carried by the river start to settle down faster on the riverbed, causing riverbed aggradations and in turn reducing the water carrying capacity of the river. The Farakka Barrage (river barrier between Bangladesh and India) on the Ganges has already caused tremendous damage to the agriculture, navigation, environment, and hydrodynamic equilibrium in Bangladesh (Khalequzzaman, 1989).
4.2.7. Soil Erosion due to Tilling The surface run-off can easily wash away the topsoil from cultivated land. This surface erosion reduces land elevation, which in turn increases flood intensity in an area. The land elevations in Bangladesh must have been reduced over time due to cultivation. Aside from this, the rapid river bed aggradations in Bangladesh eventually increased due to the tilling on the mountain slopes of the Himalayas which is thought to be responsible for massive soil erosion in Nepal (Khalequzzaman, 1989).
4.2.10. Climate Change Recently the extreme flooding events increase around the world. Scientists assumed that the flood increased as the world gets warmer, which is absolutely true as the world is about to enter a period of rapid warming. Rising sea level of even greater magnitude will cause a flood on the low land and slow the speed at which rivers can remove water from the land. Bangladesh will be severely impacted by such an increase in sea level. Besides many other adverse environmental, economic, and climatic consequences, the base level of all rivers will change following any change in the sea level. The greenhouse effect will cause frequent cyclone as a result of high temperature will lead to flood problem and the rainfall could increase by 10% to 15% by the year 2030 (O’Riordan, 2008).
5. Impacts of Flood Flood is always a damaging and devastating disaster for Bangladesh. As it is a very low-lying country with 70% of its land less than 1m above sea level and 80% of its floodplains. The country receives large amounts of water passing through it by two major rivers (the Ganges and Brahmaputra) converging with tributaries on the huge delta formed from silt deposited by the same rivers as they entered the sea. Both rivers have large volumes of water flowing through them to the sea as they have large drainage basins which increase the flood risk, especially on the environment in the long run. There are two types of impacts on the natural environment due to flooding are usually occurred.
6. Flood Management The structural measures and non-structural measures are considered for flood management.