Climate change and river flooding in Asia

Most hydrologists and the techniques they have established to measure floods see them as random events in a stationary sense. However, river flooding is increasingly teleconnectable, a climatic event linked to climatic anomalies.

 

Amplification

Floods are a normal occurrence for the people of south Asia. Every year, floods destroy crops and displace the region’s inhabitants. Floods are most common in the lowlands of Bangladesh where the Ganges and Brahmaputra rivers meet. Floods are both a hazard and an asset. While the floods are drowning crops and livestock, and damaging property, they are also fertilising the soil in the form of dissolved and suspended solids.

Projection and observing

Global warming has the effect of increasing temperature difference between the land and sea surface of the Indian subcontinent! The higher winds increase evaporation resulting in the transport of huge quantities of moist air. This leads to a greater number of rainy days during the summer monsoon with 20% – 25% increased rainfall.

Observation and research reports show significantly increased trends of extreme rainfall over Bangladesh, West and Central India, Pakistan, Sri Lanka and China: increases in the order of 5% to 18% have been recorded since 1970. The result of all of this has been an increase in abnormally high floods. In the last decade in 1991, 1993, 1997, 1999 and 2000, disastrously huge floods have inundated Asia. Other ‘global warming linked’ phenomena have also contributed to this pluvial increase, El Niño is one of them, as is increased melting of the Himalayan snow pack. Both affect the magnitude and intensity of the monsoon in the area.

CASE STUDY - Hyderabad - India, late August 2000, flooding from the heaviest rainfall in 50 years, with origins in a fierce summer cyclone, killed 2000-plus residents and destroyed 10 000 homes. At least 150 000 were effectively displaced with winds approaching 200 kph. India’s most fertile rice growing regions had also been destroyed, some 178 000 ha in total. This storm followed in the wake of a huge storm that killed some 350 people earlier in the month. Most fatalities in Hyderabad seem to have fallen victim to poor house construction, as mud-walled houses collapsed around them. Some 20 cm of rainfall was received in about 24 hours, one third of the city area’s annual rainfall receipt. The floods in India followed catastrophic monsoon rainfall in north and north east India, Bhutan, Nepal and Bangladesh. The floods killed several hundred and left one million homeless. Damage in Hyderabad, India’s second city, will cost billions of rupees to put right!

Rebuilding may well necessitate a rethink over building styles, as brick and stone buildings were the only ones to survive the deluge.

CASE STUDY - Gansu - At the same time! In India’s north west province of Gansu, 19 people died and 24 were injured. This latest flooding occurred when high water poured through five cities in Gansu. The water flattened more than 1200 homes and damaged 5520 ha of wheat and other crops. 30 bridges and many water retention facilities were destroyed.

Summary

It has been scientifically proven that as CO₂ concentrations increase so mean annual global precipitation will increase in the order 3% to 15% (by 2070). Consensus suggests that greater amounts, 20% plus, may well fall in areas affected by monsoon rainfall. Heavy rainfall events have increased markedly in their frequency and intensity in the period 1961–2000 in the monsoon region. So if an enhanced greenhouse effect is forcing hydrological cycle changes what future is there for the hundreds of millions of coastal lowland/estuarine dwellers of Asia? Certainly into the new decade much will have to be done to ensure the viability and permanency of the built-up areas. There will be much human intervention in this area to ensure the area’s long term future.