Pinpointing the causes of Pakistan's devastating floods

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This post originally appeared on Sciblogs.co.nz.

So why has this monsoon season caused the worst flooding in Pakistan's history? The overall impression given by the media is that this year's flood is unprecedented. But is it?

On July 29, 2010, nearly 300 mm of rain fell in parts of the upper Indus catchment. As should be expected during the summer monsoon season, this very heavy rainfall was followed by additional precipitation in the headwaters of Indus catchment. Over one month later, flood waters in the lower reaches of the Indus (where most people live), have only begun to recede.

So is such a wet summer monsoon season unheard of? Knowing that it is not unheard of for 10,000 mm of rain to fall over a period of approximately four weeks during July and August in some parts of the southern Himalaya, a back of the envelope calculation suggests an average daily rainfall of nearly 170 mm, for 60 days straight! So it would appear that at least in the wettest regions of the Himalaya, 300mm in 24 hours is a somewhat regular occurrence.

Science Provides a Warning

Looking for more concrete evidence, I did a quick search of the literature, which brought up a recent paper appropriately titled "Flood risk assessment of the River Indus of Pakistan" (Khan et al., 2010, Arabian Journal of Geosciences). The authors estimated the future risk of flooding in the Indus valley by expanding upon the mathematical distribution of the last 60 years of historical river gauge data for peak river discharge, in order to simulate future flood events. The Indus river system is the primary source of hydroelectric generation in Pakistan, with several crucial dams and reservoirs in the lower catchment.

The authors' results showed that the design capacity of many of the various dams and spillways on the River Indus were barely sufficient to handle a one in 20 year flood event. For example, at the GUDDU dam site, which is designed to handle a maximum discharge of 1.2 million cubic feet per second (cfs), the authors estimated that the 1 in 18 year flood event would have a peak discharge of nearly 1.1 million cfs. The authors concluded that "there is an urgent need to construct new dams/barrages on the River Indus and to increase the spillway capacity of reservoirs".

A few months on, these warnings have a prophetic air. On the 8th of August, 2010, the gauge at the GUDDU dam site peaked at 1.16 million cfs, or approximately a 1 in 20 year flood event. Other gauges on the Indus tell a similar story - this does not appear to be an exceptionally large flood event for the Indus River. In fact, based on historical evidence, we should expect similar peak discharge at least once every 20 years. Despite this, the results of the latest flooding have been catastrophic. Why?

A Country of Extremes

Pakistan is a country of geographical extremes, from the extensive deserts and plains of the Indus valley delta in the south, to the lofty heights of the Karakoram Himalaya in the north. With an area of approximately 800 thousand square kilometres, Pakistan encompasses an area nearly three times the size of New Zealand. The River Indus and its' tributaries are the lifeblood of Pakistan and 170 million people depend upon the river for clean water, agriculture and hydroelectricity. The River Indus provides the water to irrigate vast tracks of agricultural land that would otherwise be parched for 9-10 months of the year.

All this irrigated land requires a massive system of dams, reservoirs, levees and canals. Now one of the world's premiere agricultural producers, Pakistan has the largest contiguous irrigation system in the world. All these structures have been engineered to capture, store and divert the precious waters of the Indus and its tributaries. The resulting conversion of flood channels, grasslands, wetlands and even desert into arable land has fundamentally changed the natural flow regime of the Indus.

Quite simply, the river system no longer has the capacity to convey significant flood events out to sea. As has been tragically illustrated over the past few weeks, even a one in 20 year flow events can result in catastrophic flooding. Unfortunately, this means that the increasing millions of people who occupy and farm the floodplains of the lower Indus river valley will continue to be vulnerable to future flood events.

Jesse Dykstra is a PhD student in the Natural Hazards Research Centre at Canterbury University. View his work and that of 30 other scientists and science writers at Sciblogs, New Zealand's largest science blogging network.

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