On Water

Dams and levees heighten flood danger in a warming world

Patrick McCully

Sunday, July 29, 2007

Floods are the most destructive, most frequent and most costly natural disasters on Earth. And they're getting worse. Large parts of central and western England are underwater in the worst flooding in 60 years. Insurers estimate the damage could reach $6 billion -- on top of the $3 billion in flood losses suffered in northern England in June.

Over the past two months, the monsoon season in Bangladesh, China, India and Pakistan has, conservatively, claimed hundreds of lives. Texas has suffered major flood damage, as have Nebraska, Oklahoma, Kansas, Missouri and North Carolina. Although California's primary water worry right now is drought, increasingly serious floods lie in store for us, too.

Flood damages have soared around the world in recent decades for a variety of reasons. Global warming is worsening storms; we've deforested and paved over watersheds; and more people are living and working on floodplains (there are few better examples of this than the fast-sprawling cities of California's Central Valley). But a key factor behind the spiraling flood damages is the very flood-control measures supposed to protect us. Flood damages soar when engineering projects reduce the capacity of river channels, block natural drainage, increase the speed of floodwaters and cause the subsidence of deltas and coastal erosion. In addition, "hard path" flood control based on dams and levees can ruin the ecological health of rivers and estuaries.

Dams and levees are not fail-proof, and when they do fail, they do so spectacularly and sometimes catastrophically. Worse, they provide a false sense of security that encourages risky development on vulnerable floodplains. When New Orleans was devastated in 2005, the primary cause was not Hurricane Katrina, but the failure of the city's poorly conceived and maintained flood defenses. Sacramento lies at the confluence of the Sacramento and American rivers behind a network of aging levees. California's capital is widely regarded as second only to New Orleans among major U.S. cities in the risk it faces from major flooding.

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