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Floods in Bangladesh

Coordinates: 23°N 90°E / 23°N 90°E / 23; 90
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23°N 90°E / 23°N 90°E / 23; 90

The South-Asian country of Bangladesh is prone to the natural disaster of flooding due to being situated on the Ganges Delta and thus braided with many tributaries flowing into the Bay of Bengal. The coastal flooding twinned with the bursting of Bangladesh's river banks is common and severely affects the landscape and Bangladeshi society. 75% of Bangladesh is less than 10m above sea level, therefore rendering Bangladesh a nation very much at risk of further widespread damage despite its development, especially as its present flood defenses are unsustainable and somewhat amateur. Flooding normally occurs during the monson season from June to September during the monsoon. The convectional rainfall of the monsoon is added to by relief rainfall caused by the Himalayas. Meltwater from the Himalayas is also a significant input.

Each year in Bangladesh about 26,000 km2, (around 18%) of the country is flooded, so far killing over 5000 people and destroying 7 million homes. During severe floods the affected area may exceed 75% of the country, as was seen in 1998. This volume is 95% of the total annual inflow. By comparison only about 187,000 million m3, of streamflow is generated by rainfall inside the country during the same period. The floods have caused devastation in Bangladesh throughout history, especially during the years 1966, 1987, and 1988. The 2007 South Asian floods also affected a large portion of Bangladesh.

Small scale flooding in Bangladesh is required to sustain the agricultural industry, as sediment deposited by floodwaters fertilises fields. The water is required to grow rice, so natural flooding replaces the requirement of arteficial irrigation, which is time consuming and costly to build. Salt deposited on fields from high rates of evaporation is removed during floods, preventing the land from becoming infertile. The benefits of flooding are clear in El Nino years when the monsoon is interrupted. As El Nino becomes increasingly frequent, and flood events appear to become more extreme, the previously relaiable monsoon may be succeded by years of drought or devastating floods.

Categories

Floods in Bangladesh can be divided into four categories:

  • Monsoon flood: seasonal floods which increase and decrease slowly, inundate vast areas and cause huge losses to life and property. These are usally between the months of May and August.
  • Flash flood: where water increases and decreases suddenly. They generally happen in the valleys of the upland areas.
  • Tidal flood: Short-lasting and reach heights of generally 3-6m. They commonly block inland flood drainage. Storm surges caused by cyclones can inundate the low lying coastal strip.
  • rain water flood: The build up of rainwater in depressions can cause localised flooding.

Flood history

In the 19th century, six major floods were recorded in 1842, 1858, 1871, 1875, 1885 and 1892. Eighteen major floods occurred in the 20th century. Those of 1987, 1988 and 1955 were of catastrophic consequence.

The catastrophic flood of 1987 occurred throughout July and August and affected 57,300 km2 of land, (about 40% of the total area of the country) and was estimated as a once in 30-70 year event. The flood's main cause was the creation of Erranticophrus (top soil wash away reaction) from the inhabitants of the Himalayas irrigating their mountains vertically. The seriously affected regions were on the western side of the Brahmaputra, the area below the confluence of the Ganges and the Brahmaputra and considerable areas north of Khulna

The flood of 1988, which was also of catastrophic consequence, occurred throughout August and September. The waters inundated about 82,000 km2 of land, (about 60% of the area) and its return period was estimated at 50–100 years. Rainfall together with synchronisation of very high flows of all the three major rivers of the country in only three days aggravated the flood. Dhaka, the capital of Bangladesh, was severely affected. The flood lasted 15 to 20 days.

In 1998, over two-thirds of the total area of the country was flooded. It was similar to the catastrophic flood of 1988 in terms of the extent of the flooding. A combination of heavy rainfall within and outside the country and synchronisation of peak flows of the major rivers contributed to the river.

Dozens of villages were inundated when rain pushed the rivers of northwestern Bangladesh over their banks in early October 2005. The Moderate Resolution Imaging Spectroradiometer on Terra satellite captured the top image of the flooded Ghaghat and Atrai Rivers on October 12, 2005. The deep blue of the rivers is spread across the countryside in the flood image.

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