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Forecast for the Floodplains

  • By Rachel Nuwer
  • Posted 11.15.12
  • NOVA

For retirees and second homeowners, Manatee County, Florida, seems to have it all. Miles of beaches beckon with powdery sand, nearly 50 parks offer unlimited picnicking options, and a smattering of local shops create a buzzing economy. But paradise is not the only thing on offer in this low-lying, hurricane-prone community. One after another, tropical storms and hurricanes—first Gabrielle, then Bonnie, followed by Charley, Frances, Ivan, and Jeanne—pummeled the residents in the early 2000s, costing millions in damages.

It wasn't just winds that wreaked havoc, either. Around the same time, the county had been busy altering and draining 150 wetlands to accommodate a boom in new development projects. As the rains came, the waters rose.

A Broken System

Had Manatee County's wetlands and other natural areas remained intact, rains from those hurricanes likely would have been less of a problem. Water craves nothing more than to flow downstream, whether that path leads to a river, stream, ocean, or wetland. Along its journey, soil soaks the water up like a sponge. Trees and grass sip at it and form natural speed bumps that slow its downhill flow. Wetlands act like swampy sinks, collecting and storing any remaining precipitation. Then—slowly and gradually—the soil, vegetation and wetlands release the excess water.

A house flooded by Hurricane Katrina. Enlarge Photo credit: Courtesy U.S. National Park Service

Frequently, when humans enter the scene they tamper with this balance. Forests are felled to build impermeable roads. Agricultural fields are carved out of grasslands, exposing soil to runoff and erosion. Wetlands are drained to make way for subdivisions and super center parking lots. These decisions come with costs, both environmental and human. In the Gulf Coast counties like Manatee, for example, wetland alteration translates into approximately $1.5 million per year in additional flood damage.

That's because when heavy rain falls on manmade structures, runoff gains more volume and velocity than it would in nature. Upstream developments can place existing downstream communities at greater risk for flooding. "When we pave, we take away the all-important function of naturally-occurring wetlands," says Samuel Brody, a professor of marine science and urban planning at Texas A&M University. "We create a dual effect of compromising the system and putting people in harm's way."

Of Rain and Runoff

In addition to coastal flooding, hurricanes and tropical storms often bring severe rains that extend well inland. In 2001, Tropical Storm Allison dumped 30 inches of rain on the Houston area—one of the largest continuous swaths of impervious surfaces in the country—turning the entire region into a stagnant bathtub. Elsewhere, flooding can occur even in areas that aren't covered in concrete. With enough water, natural systems can be overwhelmed. Hurricane Irene, for example, poured nearly 9 inches of rain into the Northeast and parts of the Carolinas, causing significant downstream flooding that contributed to around $15 billion in damages.

"When a storm like Sandy rolls over a region and dumps 8 to 12 inches of rain on the landscape, you're going to have flooding, that's just how it is," says Derek Booth, a geologist with a joint professorship at the University of California, Santa Barbara, and the University of Washington. The difference between altered and natural watersheds, he says, counterintuitively becomes most apparent in small to moderate storms that bring half an inch or so of rain. "The more we pave, the more frequent the flooding events will be," he explains. "Storms that would have produced very little change in river or stream levels before will now be very influential because so much more of that water is going directly into the stream channel."

Historically, much of our development occurred in floodplains—or places susceptible to flooding—that normally act as buffers for managing and distributing floodwaters. In response, we've built structures to protect our investments located in flood plains. But that's now always a good thing, says Wesley Highfield, an assistant professor of urban and regional sciences at Texas A&M University. "When we build levies, we create a false sense of safety," he says.

Dams, levy and dykes—which cost tens of millions to build—are only as good as their design and construction. In the past, the U.S. tended to prefer these strategies over non-structural mitigations, such as discouraging floodplain development or preserving wetlands, but the events of the past decade are slowly shifting that attitude. Eventually, manmade fixes break or are topped by rising waters, as Hurricane Katrina so disastrously showed in Louisiana.

Gambling with Disaster

For people living in flood-prone areas, high waters take a toll both emotionally and economically. Flooding costs the U.S. $2 billion each year, yet long-term hazard mitigation hasn't been a priority. Houston, for example, took some measures to improve drainage and bolster flood resistance for some of their buildings after Allison struck, but Brody still doesn't thinks the area is adequately prepared. Per capita, the greater Houston area remains one of the most flood-damaged places in the country.

The U.S. is not alone. Three-quarters of the global population lives within a coastal zone, and worldwide more than half a billion people live within flood-prone river deltas. Models of future climate change impacts predict a rise in severe weather events. "There's no doubt that the number of people who are impacted by these flooding events will increase," says Marcel Marchand, an expert in coastal flood risk and management at Deltares, an independent research institute in the Netherlands.

Houses in Edgecombe County, North Carolina, after Hurricane Floyd. Enlarge Photo credit: Courtesy NOAA

Figuring out who those people are is a top priority for emergency management officials. The standard in the U.S., for example, is the 100-year floodplain, or any area that has a 1 percent chance in any given year of succumbing to a flood of equal or greater magnitude to previous ones. But that concept is problematic: the popular interpretation is that 100-year floodplains will only flood once every hundred years. People become complacent, assuming they won't have to worry about a disaster within their lifetimes when in fact 100-year floods can and do occur in sequential years.

Unfortunately, it's hard to say just who is and isn't vulnerable to floods these days. Floodplain maps are years out of date as a result of rampant development combined with the effects of climate change. "Floodplain maps should change," Highfield says. "As impervious surfaces increase, floodplains will extend and those fringes will spread out." Though floodplains may be expanding, the process of continuously surveying and updating maps is so time consuming that we've quickly fallen behind. The lack of up-to-date information impacts the ability of communities to develop flood mitigation plans that reflect reality.

Meanwhile, taxpayers shoulder much of the post-disaster cost burden for damaged communities, says Philip Berke, a professor of city and regional planning at the University of North Carolina, Chapel Hill. The National Flood Insurance Program covers 5.5 million residential and commercial policyholders for coverage totaling about $1.2 trillion. Since 1978, the program has paid more than $38 billion in claims. Although the program is meant to be self-supporting, as of 2004, repetitive-loss properties cost taxpayers around $200 million annually. "How much is the public going to keep spending on this?" Berke wonders.

Receding Waters

Though natural disasters bring tragedy and loss, they also present an opportunity. They open the window for policy reform, and communities are more willing to consider proactive change, ranging from more extreme measures such as relocation to smaller steps like improving warning and preparedness systems.

In an unpublished analysis Highfield and Brody conducted of 18 different FEMA flood mitigation strategies, the researchers identified the best strategies for mitigating damage. They found that, on average, nation-wide, communities that elevate structures above the projected flood levels save $960,800 in expected flood damage per year. Protecting open space in floodplains ranked second in their analysis, saving $547,000 per year per jurisdiction.

The U.S. could take a hint from Holland for implementing this strategy. The Netherlands—some parts of which lie nearly 20 feet below sea level—are renowned for their centuries-old dyke system, but the country recently adopted a program called Room for the River to address their freshwater flooding concerns. The project attempts to relocate people out of the Rhine river floodplain in order to return the area to its historic function of flood mitigation. "I understand that if you live in a floodplain area maybe the last thing you want to do is move out, but sometimes that's the only way," Marchand, the Dutch coastal flooding expert, says. "We just cannot afford any flooding."

Restoring wetlands and natural floodplains, even if it means moving some families away from the water, would help the situation in the U.S., too. "In some of the coverage of Sandy, reporters are saying things like, 'Wow, people are going to have quite a job of rebuilding homes here,' " says Nancy Grimm, a biogeochemist and ecologist at Arizona State University. "But nobody is saying that maybe they should not rebuild at all." It is an extreme option, but is only one of many.

"I think there's going to have to be some hard looks at how we actually make decisions about the use of land," she continued. "We need to think about repurposing land in ways that are not only economically beneficial but also beneficial for an environmentally and socially fair future."

The Sunshine State

Ultimately, the public and the decision-makers who represent them must decide how to respond to disasters like floods. Back in Manatee County, residents and the local government took action.

Restoration efforts in Manatee County, Florida. Enlarge Photo credit: Courtesy NOAA

To prepare for future floods, decision-makers changed their zoning laws to preserve natural areas and worked hard to maintain their storm water drainage system. The county also committed itself to outreach, offering workshops in wind mitigation to harden homes against storm damage, for example, and providing centers dedicated to hurricane readiness and flood protection. Though developers filled in some wetlands, the ecosystem still comprises around 35 percent of Manatee County's total area. The county government also tries to limit development in flood-prone locations. Because of that commitment to flood mitigation, losses remain comparatively low despite numerous severe storms that continue to pound the area.

"When a big disaster hits, there's an opportunity to learn," Brody says. "Usually those town and cities that are more adaptive and open to learning, sharing information, and thinking systematically tend to do really well."

Rachel Nuwer is a freelance science journalist who has writted for the New York Times, Scientific American, and Smithsonian. She lives in Brooklyn, New York.

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