Flood affect about 80% of land in Bangladesh. Four types of flooding occur in Bangladesh.

• Flash floods caused by overflowing of hilly rivers of eastern and northern Bangladesh (In April - May and in September - November).

• Rain floods caused by drainage congestion and heavy rains.

• Monsoon floods in the flood plains of major rivers (during June - September).

• Coastal floods due to storm surges.

Pattern of flood cycle in Bangladesh is shown below

The following factors have been identified as being responsible for flood in Bangladesh:

• Heavy and incessant rainfall in the upper catchment of the cross - boundary rivers as well as inside the country.

• Heavy snowmelt in Himalayas

• Topographically about half of the country is situated below 7.60 meter above mean sea level (MSL). Floodwater spills from the rivers and accumulates on these low- lying areas.

• Bangladesh and surrounding catchment areas lie in the heavy monsoon rainfall area that leads a concentrated large discharge in the rivers. As a result, this channel is frequently overloaded and spills over the banks and cause flood in the plain.

• There have been significant changes in the behavior of off-takes of the main rivers - . distributaries of the Ganges - Padma and the Brahmaputra after the Assam earthquake of 1950, which caused the Brahmaputra River to become heavily silted. Due to blockage at confluence and off-take of distributaries, there has been a concentration of water in the main river system.

• Simultaneous high discharge of the upland streams and high tides in the estuaries, low topography of the country and river bed siltation all contribute to the over bank spilling of the rivers.

• More than 80% of the annual precipitation occurs during the monsoon. This localized heavy rainfall and runoff coincide with the high discharge in rivers resulting in drainage congestion. Locally concentrated rainfall of more than 300 mm in 2 - 6 hours leads to severe flash / local floods.

• Spring tides of the Bay of Bengal retards drainage of floodwater into the sea and locally increases monsoon flooding. A rise of MSL at times during the monsoon period due to effect of monsoon winds also adversely affect the drainage and rises the flood level along the coastal belt.

• High water level of the main rivers slows down the flow from the tributaries. In particular, the flow of the Dharla and the Teesta being backed up by the Brahmaputra and mutual backing up by the Ganges and the Jamuna affect the drainage of the Hurasagar. This increases the flood intensity in the adjoining areas. The backing up the drainage of the Sylhet - Mymensingh haors through the Meghna due to high water prevailing in the lower Meghna at Chandpur is a well known phenomenon.

• A series of monsoon depression passing over Rajshahi, Pabna, Bogra and the Sylhet - Mymensingh haor area. These areas become inundated with the monsoon activities and remain water logged until November. Generally, the Brahmaputra records its peak flood during the month of June - July and the Ganges in the August - September, while the Meghna remains high from May to September. Some years it may happen that these rivers reach peak-flood level simultaneously and aggravate the flood situation in the country.

• Flooding in the coastal areas due to spring high tide (May - November), tidal and cyclonic storm surges.

Analysis of past floods suggests that about 26 percent of the country is subject to annual flooding and an additional 42 percent is at risk of floods with varied intensity. According to government sources, the 1998 flood inundated about 100,000 km2. In contrast, the 1987 flood had inundated about 57,000 km2 and the 1988 flood inundated 89,000 km2. The 1998 flood affected 68% of the country, and seriously impacted the livelihoods of 30 million people. Overall damage was estimated at two to three billion US dollars. Final estimates showed that 51 districts (out of a total of 64) and 307 Upazilas were inundated, about 1400 people were killed, 1.77 million houses were damaged, and 23,458,713 Bangladeshis became homeless. While the 1987 and 1988 floods inundated three-quarters of the country, and killed more people, they receded after three weeks. The 1998 floods lasted for over 10 weeks.

A 10 percent increase in monsoon precipitation in Bangladesh could increase runoff depth by 18 to 22 percent, resulting in a sevenfold increase in the probability of an extremely wet year. Since it is provided that monsoon precipitation will increase by 11 and 28 percent  by 2030 and 2050, surface runoff will increase by 20-45 percent, respectively. Alam et al, (1998) reported that, by the year 2030, an additional 14.3 percent of the country would become extremely vulnerable to floods, while the already flood-vulnerable areas will face higher levels of flooding. It is also reported that, even if the banks of the major rivers are embanked, more non-flooded areas will undergo flooding by the year 2075. Mirza and Dixit (1997) estimated that a 20C warming combined with a 10 percent increase in precipitation would increase runoff in the GBM rivers by 19, 13 and 11 percent respectively. Increased depth of flooding will be pronounced in the lowlands and depressions in the Faridpur, Southwest Dhaka, Rajshahi-Pabna, Comilla and Sylhet - Mymensingh greater districts and more areas are likely to be flooded by the year 2030, even after completion of about 60 percent of the flood protection schemes considered under the Flood Action Plan (Alam et al., 1998).

Changes in Flooded Area and Land Inundation Classes

There were two important outputs from the flood calculation in BDCLIM, the change in total flooded area and the change in land inundation classes. Land use in Bangladesh corresponding to flood inundation classes is presented in Table-4.6. Changes in total flooded area, and area of each of the land inundation classes were calculated for both the mean and the 20-year flood. Results are provided for the mean flood only Table-4. 7. For comparison the 1990, 20-year mean values are also presented in the Table.

By 2020, most of the scenarios indicate that the total area flooded approaches the amount of area flooded under the current 20-year flood event. However, there are differences in, the depth of flooding (and thus the flood inundation classes). Most of the 2020 increases in flooded area are in the shallowly flooded land (FO). The higher rainfall GCMs (GFDL and UKTR) coupled with a climate sensitivity of 4.5°C show a proportionately higher increase in the more deeply flooded land. However, the area of deeply flooded land under these two high scenarios is still much smaller than the area of deeply flooded land for the current 20 year flood event.

It has been found that the rainfall in the pre-monsoon and monsoon period has been increasing during the recent decades. As per the present trends and the projected scenarios, the monsoon rainfall will increase in the future. This increase will be more over the northern part of Bangladesh and in the adjacent territories of India, Nepal and Bhutan, which would drain over Bangladesh. As a result more frequent and severe floods are feared to occur in Bangladesh in changed scenario of climate. The floods in Bangladesh cause injuries and deaths due to drowning. The sources of the potable water are contaminated by the floodwater. As a result, the waterborne diseases like cholera, hepatitis-A spread very fast due to ingestion of contaminated water. The skin disease also spread due to contact with the contaminated water. Besides, the respiratory diseases also prevail due to overcrowding in the shelters. A study in populations displaced by catastrophic flood of Bangladesh in 1988 found that diarrhea was the most common illness, followed by respiratory infection. Watery diarrhea was the most common cause of death for all the age groups under 45 (Siddique et al., 1999). The number of severely malnourished children had increased after flooding (Choudhury and Bhuiyan 1993). In the aftermath of flooding, molds and fungi may grow on interior surfaces, providing a potent stimulus to allergic persons (American Academy of Pediatrics. 1998). In the future, since more frequent floods of much bigger scale than those occurred in 1987, 1988 and 1998 might occur as per the present trend, the impacts of the floods on human health would be more severe.

The humidity is expected to increase because of high temperature in association of higher rainfall and probable higher frequency of severe floods. The enhanced humidity is likely to increase the occurrences of heart and respiratory diseases and mortality rate from these diseases. The impact of extreme summer heat on human health may be exacerbated by the increase of humidity (Gaff en and Ross, 1998; Gawin et al., 1999).

The excessive precipitation and the resulting floods can transport terrestrial microbiological agents in the drinking water sources. Significant correlation between the cumulative monthly distribution of cholera cases and the monthly distribution of precipitation has been observed in Guam (Borroto and Haddock, 1998). In many of the countries including Bangladesh, handling of the sewage is not separate from the drainage system for storm and flood water. As a result, the drain water occasionally overflows the drainage system and flows over the surface contaminating the surface water and drinking water and their sources.