Bangladesh: Monsoon Seasonal Floods - July / August 2004
Causes:
1. Climatic
Bangladesh has a Monsoon climate. In summer the EurAsian
continental land mass warms up faster than the Indian Ocean. the
warm air rises creating a Low Pressure. Moist air blows from the
relative High Pressure of ocean bringing rain. This is the Monsoon
which brings important annual flooding that irrigates crops and
deposits fertile silt and which has operated for thousands of years.
Most places in Bangladesh get over 1500mms of rainfall a year.
In some years the monsoon rains are heavier and the flooding is
greater leading to damage to property and lives. Disastrous flooding
has occurred in 1988, 1998, 2002 and 2004.
In 2004 heavy rains in early June led to the Meghna flooding
bursting its banks in north east Bangladesh by early July. Further
heavy rains caused flash flooding in the north on the Brahmaputra in
July. The floods spread and eventually impacted on the capital
Dhaka in mid August. A further wave of flooding was caused by
more intense thunderstorms in September in this event four days of
heavy rain averaged 300mms a day - the highest rate since 1952.
Every summer the warming leads to snowmelt in the high
Himalayan rivers and glaciers that feed the major rivers like the
Ganges and Brahmaputra that flow down to Bangladesh. This
snowmelt adds to the risk of flooding. Global warming has increased
the rate of melting (human activity).
2.Terrestrial
processes
Whilst the low ground in Bangladesh has about 1500mms of rain
annually the higher ground exceeds 5000mms. There is a definite
relief rainfall (climatic) effect to the monsoon rains and this
increases the flooding.
Bangladesh is the floodplain and deltas of the Ganges and
Brahmaputra and as such is liable to flood easily. About 30% of the
country floods annually. About 80% of Bangladesh is alluvial
floodplain.
Bangladesh is very lowlying and flat with most of the country under
10 metres asl making it susceptible to both river and sea flooding.
3. Human
activity
Global warming is making sea level rise and this will flood low-Lying
coastal areas but also the higher sea level restricts river flow and
leads to more flooding of the flood plain land.
Extraction of groundwater for irrigation via tube wells has led the
compaction of the alluvium and ground subsidence by up to 2.5
metres. This lowering of the land makes it more vulnerable to
flooding and the flooding more devastating.
In upstream areas on the major rivers many dams have been built.
These dams trap silt and thus it is no longer deposited on the flood
plain in Bangledesh. Bangladesh is densely populated with over 140
million people and all land areas are occupied. This includes
sandbanks called Chars in the river channels and without additional
silt they are subject to erosion and flooding.
Increased urbanisation has led to higher peak flows and shorter lags
with more flooding. Dhaka now has over 6 million inhabitants.
In Himalayan countries like Nepal rapid population growth puts more
pressure on the land. Deforestation and terracing of steep hillsides
occurs to make more farmland. This deforestation has led to
increased run-off and discharge in the rivers flowing to Bangladesh.
With no leaves to intercept rainfall and with no roots to hold the soil
in place rapid soil erosion and landslides occur in the Monsoon. The
soil is transported by the rivers and is deposited on the lower ground
blocking river channels and decreasing their capacity to store water
so increasing the likelihood and depth of flooding
The 2004 flood on the Brahmaputra was intensified because the
Kurichu Dam on the Tsatitsu Lake in Bhutan was breached by a
Glacial Lake Outburst Flood. This dam burst added to the monsoon
rain discharge intensifying the early July floods.