Pakistani Floods

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The Pakistani Floods of 2010
by The British Geographer

The Causes of the Flood
Physical Causes
From its headwaters in the Himalayas of Tibet, the River Indus flows northwest through India before turning sharply south across Pakistan. It finally discharges into the Arabian Sea, a journey of some 3,200km (2,000 miles). The River Indus has an annual flood caused by tropical monsoon rainfall. It’s rich alluvium floodplain led to one of the cradles of civilization, 9000 years ago.  However, this flood’s magnitude was according to Professor Rajiv Sinha, from the Indian Institute of Technology, 5 or even 10 times stronger than normal. The annual monsoon is caused by the movement of warm moisture laden air from the Indian Ocean toward areas of low pressure, marked out by the Inter-Tropical Convergent Zone (ITCZ) over the subcontinent. Here, the subcontinent is superheated, which creates strong rising thermals of low pressure. As warm air moves over the subcontinent it rises and dumps vast quantities of rainfall, which cools the surface and replenishes the vital soil moisture and ground water. In July 2010, more than half the normal rains fell in just one week in an unprecedented sequence of days. Intense rainfall totaling in excess of 200mm fell in a 4-day period from 27th to 30th July along with above average rainfall in August. The recorded monsoon rainfall associated with La Nina was the highest in a 50-year period.

The total area affected by flooding was 796,095 square kilometers, approximately one fifth of Pakistan’s total land area

 

There was much discussion over the exact causes of this level of rainfall. La Nina, which is a coupled ocean-atmosphere phenomenon that impacts southeast Pacific Ocean temperature but is also thought to increase Indian monsoon rainfall was thought to be a contributing factor. The cycle of El Nino and La Nina, which are both important global heat transfers seems to be occurring more frequently and is potentially a consequence of climate change. Climate change scientists are observing both greater spatial variation and severity in the monsoon rains. Scientists have observed in the last 30 years, a 40-60 mile northwest shift in the Pakistan monsoon. A second report in the New Scientist linked the severe monsoon to the affects of a phenomenon that was freezing the jet stream. This had in the same been previously associated with forest fires and heat waves in Russia. A second contributing factor is the alluvial nature of the River Indus. The Indus is choked with vast quantities of sediment supplied by its Himalayan headwaters. When combined with raised levées, the sediment only serve to choke the river further, reducing its capacity and causing the likelihood of floods to increase.

Human Causes
Raised levées and protected banks contain the sediment and reduce the river capacity. In this way river management is seen to exacerbate river floods along the Indus. Western river management systems have been wrongly transferred to Asian rivers. For example, with UK rivers, due to their size and scale they transport far less sediment. Raising the UK riverbanks has relatively little impact on channel capacity and sedimentation. A similar problem is the use of concrete line riverbanks. Often in rich sediment filled riverbanks, the shifting sands lead to concrete banks being undermined and less effective in holding back floods. This problem is more associated with Bangladeshi river management. Deforestation is also considered to be a major cause of the flood, with some commentators suggesting that it was key trigger. Deforestation rates vary in Pakistan from 2 percent to 2.4 percent annual rate. At this rate the country's forest cover would be reduced to half of its 1995 extent by 2019-24, says Pakistan's Food and Agriculture Organisation. Deforestation is known to aggravate flooding by reducing the ability of drainage basins to intercept inputs; consequently run-off rates and discharge...
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