Understanding the Capital's Flood Risks

Washington, DC is most at risk from overbank flooding and urban drainage flooding. Overbank flooding occurs when a river receives more rain or snowmelt than it can handle, while urban drainage flooding happens when stormwater runoff exceeds the capacity of the storm sewer to carry it away.

Penelope Gross, Valerie Jarrett, and NCPC Chairman John CogbillOverbank flooding has not occurred recently in downtown Washington, but it remains a potential threat to the city’s monumental core. The U.S. Army Corps of Engineers (ACOE) constructed a levee in 1940 along the north edge of the National Mall between the Lincoln Memorial and the Washington Monument to reduce the flood risk.

The levee’s effectiveness depends upon a system of temporary closures at 23rd Street NW, 17th Street NW, and Fort McNair. ACOE had sought funding since 2000 to improve the system of closures and fortify several low points in the earthen structure. NCPC strongly endorsed this project every year in its Federal Capital Improvements Program. The project finally received funds in 2008 from the District of Columbia and but Congress has not yet funded the work.

Even with an improved levee, many areas, including East Potomac Park, the Tidal Basin, and the National Mall up to the Reflecting Pool are at risk for severe overbank flooding. This area, much of it under the jurisdiction of the National Park Service, contains many significant commemorative works. Cleanup and restoration costs of a single cataclysmic event—or the cumulative effects of repeated inundations—could be substantial.

Urban drainage flooding, like the flood of June 2006, is a chronic risk. One-third of the city, including the downtown business district has a combined sanitary and storm water sewer system. During heavy rains, the sewer system may become overburdened and unable to collect all the water, which then pools up in the streets.

Continuous dewatering may further reduce the capacity of the sewer system. Buildings in low-lying areas and on top of Washington’s submerged streams must dewater their basements and subbasements because water still flows in the old river beds and infiltrates building foundations.

The 2006 flood paralyzed operations at four Federal Triangle buildings—IRS Headquarters, the Commerce Department, the Justice Department, and the National Archives. The National Gallery of Art and several Smithsonian buildings also became incapacitated when water in the basements shut down the buildings’ electrical and climate control systems.

Questions still remain about why the Federal Triangle flooded so badly and so quickly. NCPC—along with other federal, local, and regional agencies—is currently modeling stormwater activity under a variety of conditions and developing strategies to reduce the risk of flooding.

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