The Drainage Basin - Bangladesh

WATER – HYDROLOGY CASE STUDY

RIVERS OF BANGLADESH

Meghna, Ganges, Brahmaputra

The Drainage Basin

Extensive, covering much of Bangladesh, parts of India, Nepal and China. The Himalayas and Tibetan Plateau form the rivers’ watershed. The river Brahmaputra detours over a thousand kilometres as the river has not been able to cut a valley through the resistant rock of the Himalayas. The flood plain covers about 50% of Bangladesh’s land area. The river erodes material in its upper course in China, India and Nepal, carrying huge amounts of sediment to be deposited as the river slows crossing the floodplain and delta of Bangladesh. The river enters the Bay of Bengal through its distributaries in the delta.

How and why does the river flood ?

MONSOON & SNOWMELT : along the delta, huge tidal waves can be whipped up in the storm conditions of the Monsoon season (May to October). These waves can reach 7 metres in height and therefore have very destructive effects. Islands in the delta such as Sandwip island house the very poorest of people in Bangladesh. They are forced to live on the stretches of land with greatest flood risk. They argue that no one in government is interested in protecting their land.

DEFORESTATION: some scientists argue that large scale clearance of forests in the Himalayas for firewood, furniture and sporting equipment for other countries is one cause. Forest clearance exposes the soil to rainsplash which loosens soil particles and surface run off.

OVERFLOW: The increase in soil erosion adds to the silt / sediment load of the river. The rivers in Bangladesh carry 1600 tonnes of silt per kilometre. River’s channel fills with debris & there is less room for water which overflows.

CHANNEL STRAIGHTENING / CANALISATION: increasing control of the river through concreted ...

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DEFORESTATION: some scientists argue that large scale clearance of forests in the Himalayas for firewood, furniture and sporting equipment for other countries is one cause. Forest clearance exposes the soil to rainsplash which loosens soil particles and surface run off.

OVERFLOW: The increase in soil erosion adds to the silt / sediment load of the river. The rivers in Bangladesh carry 1600 tonnes of silt per kilometre. River’s channel fills with debris & there is less room for water which overflows.

CHANNEL STRAIGHTENING / CANALISATION: increasing control of the river through concreted sections & straightening can actually increase the river’s speed (smooth sides reduce friction). This means more water is carried faster downstream. This can be controversial - building up concrete or stone levees can prevent floodwater draining away quickly onto the floodplain when the floods do come.

Are floods all bad news any way ?

Scientists are again divided on this. Many people who live & farm on the floodplain believe that the floods are “BARSHA” - good floods. For them, flooding brings disaster, yes but also free fertiliser (silt) for crops, they wash out salt from the delta, they can flush out disease, irrigation water for crops. As well as the benefits, there are disastrous effects such as death, destruction of homes or even whole islands of the delta, death to fish populations(over 80% of the protein in people’s diets is from fish and 70% of the national fish catch is from rivers., sewage and excess fertilisers / pesticides from upstream. The worst floods have killed hundreds of thousands of people and animals. They have destroyed homes, transport, communications, industry, crops.

Are flood control management strategies the answer ?

Scientists are again divided, particularly learning from the Mississippi flood of 1993 where even in one of the richest and most technologically advanced nations, a flood occurred despite millions of $ being spent on control and it being made “the river which could not flood”.

What schemes are used ?

RIVER STRAIGHTENING & LEVEES:- raise banks but make water difficult to control if it cannot drain away onto flood plain naturally ( high concrete banks stop this). Straightening can increase river’s power to erode and carry floods even faster to lower reaches of floodplain.
DAMS: these hold water back in the upper course but may alter the river’s flow so that poor farmers are faced with irrigation reductions and silt gets trapped behind the dam rather than on their land.
TIDAL BARRAGE ACROSS BAY OF BENGAL: this has been proposed but is an expensive option. It would protect land of the poorest people but would prevent river water draining freely out to the Bay but it would reduce salt in the delta and make its crops more reliable. However a build up of chemical e.g. pesticides or fertilisers may occur.
URBAN FLOOD PROTECTION LEVEES - these would benefit the richer communities of the urban areas but would help reduce problems related to breakages in sewage and communication systems and reduce spread of disease.

Is money better spent on aiding people to live with rather than control floods ?

Killas can house up to a thousand people. They are concrete shelters for people and animals built on stilts.If people get enough warning and 5,000 killas can be

built, most of the people at risk could shelter for the days when floodplain land is under water. Benefits: cheap, easy to build, can help poor areas as well as richer areas.

Embankments – Strengthens river banks along the stretches most at risk. Problem is cost (1000kms of river costs £20 billion to protect). Structures are up to 7 metres high. No natural supplies of rock in Bangladesh – expensive to import. Another problem – restricted access to river for fishing people. Embankments stop water draining naturally into fields either side of the river.

Satellite Technology for warning systems can be used with increasing accuracy to predict when floods are most likely to occur - satellite tracking can follow storms which bring tidal waves up the Bay of Bengal. Benefits – increasing co-operation with India, improves skill and I.T. expertise.

Urban flood protection targets most populated and costly areas – protecting hospitals, housing, transport networks, industrial complexes. Would benefit most wealthy. Does not necessarily protect from risk of pollution / contamination.