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The Sims Bayou Federal Flood Damage Reduction Project, with one of its detention basins shown here, started in 1990 and is scheduled for completion in 2009. Total project cost is estimated at $345 million; the federal government will pay $220 million and the Harris County Flood Control District will cover the remainder. Photo: Harris County Flood Control District.
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Chapel Hill, N.C., is in its final stages of building a large public works operations facility, which includes geothermal systems; rainwater collection systems; active and passive solar, bioretention, and pervious asphalt; with the latter two shown here. Photo: Town of Chapel Hill

Stormwater detention (temporary storage) and retention (long-term storage) basins are cropping up across the nation, and there's no end in sight.

Harris County, for instance, is a heavily populated Texas county with 3.8 million people, and its No. 1 natural hazard is flooding. The Harris County Flood Control District has stopped many potential floods in their tracks, however.

“During a rain event on Oct. 16, 2006, about 1500 homes did not flood along White Oak Bayou because of an ongoing project to widen and deepen the bayou and excavate 10 detention basins that combined hold more than 1 billion gallons of stormwater,” says Heather Saucier, spokesperson for the district.

“Bayou City” is the nickname for Houston, the major city within Harris County. The county boasts more than 2500 miles of natural bayous and manmade channels that drain into Galveston Bay. Many of these channels were originally lined with concrete in an effort to move storm-water quickly; however, high flood levels often washed over their banks and detention basins became necessary.

The county's flood control district has been using detention basins since the 1980s to reduce flood damage. The 50 regional basins store billions of gallons of stormwater.

The county is relatively flat, gets an average annual rainfall of 45 inches, has impermeable clay soils, and is close to the Gulf of Mexico's tropical storms—all ingredients in the recipe for flooding disasters.

Each year, the Harris County Flood Control District submits an annual five-year capital improvement program. This sliding window advances into the future as each year ends. It encompasses all of the current and estimated capital improvement activities that the district might implement throughout Harris County. The department oversees an operations budget of $102 million each year. The district's $975 million, five-year capital improvement plan just ended; this year's budget is $195 million.

While the price of land is a consideration for the district, it realizes that a basin's effectiveness in reducing flooding for local homes and businesses is the key factor in determining its placement. The district has a planning department that determines the most effective sites for these basins. There are eight basins under construction right now, with several more in the planning stages. Harris County funds these projects with ad valorem property taxes and federal funds through the U.S. Army Corps of Engineers.

Some improvement projects may include channel modifications (modifying of existing streams, bayous, or tributaries), new channel construction, excavation of large stormwater detention basins, or voluntary buyout of homes that have experienced repetitive flooding or are located hopelessly deep in the regulatory floodplain.

Harris County is forward-thinking, too. “There are strict standards in place for private developers and public agencies when they build projects,” says Saucier. Developments cannot exacerbate current flooding conditions, and all development is responsible for additional stormwater runoff it creates. As a result, hundreds of smaller detention basins are scattered around the county.