Floods
are an annual occurrence in Bangladesh. The
four main kinds of natural floods affecting
Bangladesh are flash floods, river floods, rainwater
floods, and storm surges. Many floods are attributed
to human activities too.
Flash Floods
are caused by run-off during exceptionally heavy
rainfall occurring over neighbouring upland
areas. They occur most frequently, sometimes
several times a year, at the foot of the northern
and easter hills. They are also common along
the Teesta, Atrai and little Jamuna rivers in
the northwest and in valleys within upland regions.
Flash Floods rise and fall rapidly, usually
within a few days. They may also flow rapidly
along river channels and over the land. Water
levels in some eastern rivers can rise by several
meters within 24-28 hours. They do not however
cause extensive damage to crops and property.
River Floods
result from snowmelt in the high Himalayas and
heavy monsoon rainfall over the Himalayas, the
Assam and the Tripura Hills the upper Brahmaputra
and Ganges floodplains, outside Bangladesh.
River floods extend beyond the active floodplains
and damage crops on parts of the adjoining meander
floodplains mainly alongside distributary channels.
However the timing and duration of the floods
are important determinants of crop damage. Sediments
deposited in channels reduce the drainage capacity
of minor rivers, road and railway bridges as
well as irrigation and drainage canals. Severe
floods, which cause extensive damage to crops
and property, occur at an average intervals
of 7-10 years.
Rainwater Floods
are caused by heavy rainfall occurring over
floodplain and terrace areas within Bangladesh.
Heavy pre-monsoon rainfall between the months
of April and May cause local run-off to accumulate
in floodplain depressions and in the lower parts
of valleys within the Madhupur Tract. Later
between the months of June and August, local
rainwater is increasingly ponded on the land
by the rising water-levels in adjoining rivers.
Rainwater flooding is characteristic of meander
floodplains, major floodplain basins, and old
estuarine floodplains. Early floods cause damage
to rice and jute crops.
Storm Surges
are raised sea levels caused by a combination
of low barometric pressure and strong onshore
winds associated with tropical cyclones. They
cause sudden but temporary, flooding of coastal
areas with seawater or brackish estuarine water
for a few kilometers inland during the passage
of cyclone and are responsible for most of the
casualties caused by cyclones. Floods caused
by cyclones and the mitigation measures they
require differ from that occurring on the river
floodplains.
Man-made Floods
are caused mainly by the sudden breaching of
an embankment at a time when there is a difference
in elevation of several meters between the external
river level and the land inside the embankment.
It can also be caused by the release of water
from dams at high rates, failure of major dam
or barrage on one of the major rivers or their
tributaries and ponding of water behind road,
railway and flood embankments following heavy
rainfall.
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