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From the Frying Pan into the Flood: Pakistan’s Worst Natural Disaster Unfolds (Pt II) Jesse Dykstra Sep 03

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Unprecedented Precipitation?

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 29 July, 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 4 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.

 

Source: Unitar/Unosat

Source: www.allvoices.com

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 1 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 worlds’ premier 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 1 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.

From the Frying Pan into the Flood: Pakistan’s Worst Natural Disaster Unfolds (Pt I) Jesse Dykstra Sep 02

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Source: nationalgeographic.com

Source: nationalgeographic.com

“Amid Drought, Pakistan Prays for Rain”. That was the title of a National Geographic Daily News story published on 1 July, 2010. Since then, Pakistan’s scorching drought has indeed been remedied. Heavy monsoon rains over the past month have instigated widespread flooding, leading to the worst natural disaster in Pakistan’s history. As flood waters finally show signs of receding, the exceptional toll of the disaster is becoming clear:

  • over 17 million people displaced (approximately 10% of Pakistan’s population)
  • 1600 lives lost
  • 4.6 million people left homeless
  • Up to one fifth of the country inundated
  • Initial relief efforts will cost ~$460m US dollars
  • Recovery costs will be in the billions (US dollars)
  • Devastation of millions of hectares of fertile crop land and grazing land will have long term effects on Pakistan’s ability to produce food when they need it most
  • The food shortage in Pakistan will also have a global impact, as Pakistan is one of the world’s largest agricultural exporters
pakistan 2

Source: Unitar/Unosat (click on image to enlarge)

The people of Pakistan have been having a rough go of it in recent years. Parts of northern Pakistan and Kashmir are still rebuilding following the 2005 Kashmir earthquake which resulted in over 80,000 deaths, and left 3.5 million people homeless. Many of these deaths occurred after the initial earthquake, as aftershocks, heavy rain, steep mountainous terrain and landslides ensured that critical transport links were blocked, slowing relief efforts. As is all too often the case with vulnerable populations, a lack of clean water, food, emergency shelter and medical supplies eventually took the greatest toll.

Earthquakes and flooding are simple facts of life in northern and western Pakistan, where the greatest mountain range in the world, the Himalaya, are actively being thrust up by the collision between the Indian subcontinent and the Eurasian plate. More than 50 million years of uplift has created the great topographic barrier of the Himalaya, which dramatically affects the climate of the region. Dry, cold Arctic winds are prevented from crossing the Himalaya into South Asia, keeping the Indian subcontinent relatively warm. During the northern hemisphere summer, moisture laden winds from the Indian ocean are gradually drawn northwards over the warm plains of southern India. Eventually these warm moist air currents collide with the Himalaya and cool as they are forced upwards, generating the summer monsoon rains that nourish the agricultural regions of India, Bangladesh, Nepal and Pakistan. The summer monsoon season in South Asia lasts for only a few weeks, but can generate some of the highest precipitation rates on earth. Some areas near the southern boundary of the Himalaya receive over 10 m of annual rainfall, the vast majority of which falls during the summer monsoon.

The Great Rivers

Several of the world’s great rivers, including the Ganges, Indus, Brahmaputra, Yangtze and Mekong originate in the Himalaya region. Approximately half of the world’s population (3 billion people) live within river drainages that are nourished by snowmelt and monsoon rains in the Himalaya.

A Hazardous Place

Scientists know that flooding, drought, earthquakes, and landslides have been occurring in South Asia for millions of years. There is nothing about these events that should catch us by surprise. The evidence of past events is recorded in the very landscape itself. Before our eyes is the greatest mountain range on earth, still being rapidly uplifted by astounding tectonic forces where two continental plates collide. Some of the world’s greatest rivers systems cut entirely through the immense topographic barrier of the Himalaya, in areas subject to some of the highest seasonal rainfalls on earth. Great gorges and rugged mountains give way to the largest river deltas in the world, reminding us that phenomenal volumes of sediment are being actively moved from the mountain highlands to nourish great floodplains, where half the world’s population lives, and makes a living off of the fertile land.

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Pt II will be posted tomorrow

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