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Auf Schlimmeres vorbereiten

by D+C | E+Z
Flutopfer in Norduganda / Flood victim in Northern Uganda

Flutopfer in Norduganda / Flood victim in Northern Uganda

Global warming means that more people will be vulnerable to stronger and more frequent floods. Conventional approaches to flood-control will probably only compound the problem, because they are not fit to protect people from unusually strong floods. As floods will happen, humankind must prepare to cope with such events, rather than to try to prevent them. [ By Patrick McCully and Himanshu Thakkar ]

Floods are among the most destructive, frequent and costly natural calamities on Earth, and they are getting worse. Late summer flash floods drenched a fourteen-country-wide belt of equatorial Africa: Uganda and Ghana were particularly hard hit with 400,000 affected in each nation. Summer monsoons in Asia displaced or affected some 25 million people, and killed over 2,500 people in India alone. Bangladesh, China and Pakistan were also hard-hit.

Other areas around the world were not spared in 2007. The United Kingdom suffered its worst flooding in half a century. Australia, Canada and the USA also saw exceptionally severe rainstorms resulting in numerous deaths and substantial damages.

Although floods have occurred throughout human history, the number and cost of deadly floods have soared in recent decades, despite the expenditure of hundreds of billions of dollars on flood-control structures. The growing flood danger is due, in part, to global warming, which is causing more intense storms, and in part to increasing development on floodplains. Where once there were farmlands, now there are homes and businesses. But there is another reason for more severe flooding. Current flood-control technologies that rely on dams and embankments are often counterproductive. They make floods more powerful and offer a false sense of security to those “protected” by embankments that will eventually fail. Traditional or “hard” flood control can prevent most “normal” floods, but in the long run it tends to increase damages from particularly severe floods – the frequency and dimensions of which climate change is expected to dramatically increase.

A better way to cope with floods than dams and embankments is essential. This better way is called the “soft path” of flood risk management. “Soft path” management aims to understand, adapt to and work with the forces of nature. Instead of spending billions of dollars vainly trying to eradicate floods, we need to recognise that floods will happen and learn to live with them as best we can. This means
– finding ways to reduce floods’ speed and size by restoring meanders and wetlands,
– raising houses on mounds or stilts and only using (well maintained) embankments only to defend built-up areas, as well as
– getting out of the way of oncoming floods by developing early warning and evacuation measures.

A hard problem

Conventional “hard path” flood-control is based on dams and embankments. This approach has three critical weaknesses:
– No complex engineering system is foolproof or fail-proof.
– Measures are all too often based on an incomplete understanding of the workings of rivers and coasts.
– It encourages intensively developing flood-prone areas, while discouraging investments in flood-proofing measures and in preparations for flood evacuations.

The city of Surat in Gujarat, India, was the victim last year of a less-than-foolproof engineering system. The 2006 monsoon was both early and heavy, and by late July the 350-foot-high Ukai Dam was filling rapidly. The dam’s reservoir should have been partly empty ahead of heavy rains. Instead, it was nearly full in the first week of August. On 8 August, the New York Times reported that dam operators “threw open the reservoir’s 21 sluice gates. Water then did what water does. It surged downriver.” Surat went under, at least 120 people died. Inquiries into the incident determined that dam operators had allowed the reservoir to overfill, while failing to take into account the additional monsoon rainwater yet to come. The situation was exacerbated by the fact that the reservoir’s storage capacity was diminished due to sediment build-up behind the dam.

Similar disasters have befallen other nations when dam operators opened sluice gates with limited, or without any, warning. Late August floods in Ghana killed at least 20 people and completely or partially destroyed some 20,000 homes. First reports indicate the situation was exacerbated by dam operators at the Bagre Dam in eastern Burkina Faso who opened a floodgate, and released a surge of water into the White Volta River that flows into Ghana. Burkina Faso informed the Ghanaian government, but it is not known if that warning got to the people living along the river.

In 1992 dam operators at Pakistan’s Mangla Dam opened spillway gates with no warning; 500 drowned. In 2005, at least 62 died in India due to a flash flood created by Indira Sagar Dam operators who opened spillway gates without notice at a time when the banks of the river were crowded with Hindu pilgrims preparing for a festival on the banks of the holy Narmada River.

Then there are the dams that fail. In 2002, two dams broke on the same day in Mexico. Three thousand people were displaced and 21 killed. In 2004, a temporary cofferdam in China failed due to heavy rainfall, 18 died. That same year the Camará Dam in Brazil ruptured, killing five, while 800 were made homeless. In 2005 five dams in Pakistan—one of them over 110 ft. high—burst after torrential rains. Eighty died, many more were injured, and 4,000 were left homeless. That same year heavy rains overwhelmed Afghanistan’s Band-e Sultan Dam. Eight drowned and thousands of acres were flooded.

Floods are often made worse when wetlands, open waters and swamps that once served as storage areas for surplus floodwaters are developed for farming and occupation. For example, India’s Ganga-Brahmaputra floodplain lost over five million acres by the early 1990s due to drainage, irrigation or flood control measures.

Dams and embankments set off profound changes in the ways in which water and sediment flow through watersheds. They increase flood damage by reducing channel capacity through increased sediment deposition. Sediment trapped in reservoirs does not make it to river deltas; this contributes to coastal erosion and delta subsidence. Flood control embankments constrict rivers. When rivers are made straighter, shorter and narrower they move faster and their waters levels are higher, increasing the flood danger.

Finally, “hard path” flood control planning is based on a static climate. Engineers use this fictional static climate to determine the largest likely flood a particular dam or embankment will have to tolerate. But on a warming planet, flood control based on this fictional climate is likely to prove both inaccurate and dangerous.

A soft solution

”Soft path” flood risk management is, by contrast, adaptive and flexible. It seeks to reduce damage from any size of flood. It is adaptive in that it seeks to respond to the hydrological changes caused by alterations to land use and river morphology. Such risk management assumes that floods will happen, that all flood-protection infrastructure can fail, and that these failures must be included in planning models. Such strategies are also based on an understanding that all floods are not inherently bad – and indeed that floods are essential for the health of riverine ecosystems.

There are five key elements for successful flood management in a changing climate. Many of these policies are already in limited use, but they must be scaled up.

Slow the flood: Strategies to reduce the speed and size of floods include moving embankments back from rivers; restoring wetlands, floodplains and meanders, and slowing down urban run-off. Many countries have now adopted such strategies. In China almost 8,000 square miles of wetlands are being restored on the middle Yangtze. In the United States, Florida’s Kissimmee River restoration project is designed to revive some 10,000 hectares of river and floodplain ecosystems damaged by a 1960s flood-control project. In Northern California, a 10-year, $ 220 million project to reduce floods on the Napa River will restore tidal marshlands, remove some buildings in the flood zone and set back levees.

Improve emergency procedures: Possibly the most important life-saving measures that can be adopted are improved flood forecasting, warning and evacuation procedures. In the Ganga-Brahmaputra-Meghna basin a community-based flood watch involving 1,000 people using telephones and email has operated successfully for the past three years. It is also essential to prepare strategies in advance to help households and communities cope with and recover from the impacts of floods.

Move out of harm’s way: A vital part of reducing damage, especially in less densely populated areas, is to discourage people from living in the areas most vulnerable to floods. Floodplain management includes planning and regulations to discourage new floodplain development, and financial incentives for people living in the riskiest areas to move to higher ground. After the disastrous 1993 Mississippi floods in the United States some 10,000 homes and businesses were relocated from the floodplain.

Protect the most vulnerable buildings and areas: Flood risk management includes structural measures such as flood-proofing individual buildings by putting them on stilts or mounds, as is the case in many traditional communities. Other strategies include building floodplain storage and bypass systems – areas of sparsely or undeveloped land that are kept that way – and even the judicious use of well maintained embankments for vulnerable urban areas.

Improve dam management: In many countries, dams aggravate flood damage when they overtop, collapse or are poorly operated. Operating rules for dams should be developed with public input. They should be published and stringently enforced. A safety assessment of existing dams is also critical and plans for removing unsafe dams should be a priority.

While there is a growing consensus that mitigation not elimination is the most realistic flood management policy, powerful interests, including the World Bank and the Indian water establishment, remains devoted to outmoded “hard path” flood control. Indeed, an iron triangle of politicians, bureaucrats and dam builders may promise salvation through embankments and dams after a community is struck by a flood, even though many such floods have been worsened – or caused – by those very dams and embankments.

We can and must do better. Improving our ability to cope with floods under the current, and future, climates requires adopting the more sophisticated “soft path” flood management techniques. Such a path can deliver a powerful one-two punch that not only reduces death and damages, but also restores rivers, wetlands and other valuable ecosystems.