The District faces serious potential problems from the Potomac River coming over its bank, failing urban drainage and storm surge flooding, according to a National Capital Planning Commission report.
A worst–case–scenario flood could turn the Jefferson Memorial into an island and flood the National Mall up to the Reflecting Pool, the report said. A warning came last June, when flooding from more than a foot of rain shut down five U.S. government buildings, forced the closure of several Smithsonian museums and left major roadways through the National Mall either off limits or in gridlock.
The capacity of the sewer system in the downtown business district is unknown even to the D.C. Water and Sewer Authority, according to the NCPC summary, but the flooding started “before the rainfall should have exceeded the sewer’s capacity.
“In summary, the flooding may have been caused by the extreme intensity of the rainfall over a very short period of time, but no one can be sure,” the NCPC said.
“Flooding is a risk to the national cultural and historic resources in the area, a financial risk for the property damage and a security risk given the concentration of key federal functions,” the report states.
The June flooding damaged the IRS headquarters, the Commerce Department, Justice Department, National Archives and Environmental Protection Agency. Five Smithsonian institutions were shuttered. The National Zoo banned cars from its parking lot due to high water. Constitution Avenue was closed, and Rock Creek Parkway became impassable.
Washington’s interior flooding problem is rooted in the burial of three major streams. By 1870, Tiber Creek, James Creek and Slash Run, which ran along the Mall, were more open sewer than waterways and werecovered over. But the old creek beds still allow water to flow into sewer pipes and building foundations.
The U.S. Army Corps of Engineers refers to the flood–prone National Mall area as “the bathtub,” said Barry Cortright, acting chief of the civil engineering section in the Corps’ Baltimore office.
Solutions to flooding, the NCPC reported, are complex and will require multijurisdictional coordination. Repairing the District’s combined sewer system, implementing green building standards or scrutinizing developments in floodplains “are costly or beyond the ability of a single jurisdiction to require.”
One option is to close several gaps in the National Mall levee, a subtle flood control system built in 1940 that runs from the Lincoln Memorial to the Washington Monument. The $7 million project was first authorized by Congress in 1946, but never fully funded.
Cortright said permanent improvements at 23rd Street, 17th Street and Fort McNair must be more dependable than the sandbags and jersey walls put up temporarily in a flood, but also must blend in aesthetically with the Mall.
