WASHINGTON — Construction on a new barrier to protect downtown Washington, D.C., and the National Mall from Potomac River flooding during a massive storm will soon be underway.
Officials said Monday that the $9.6 million levee project, which has been several years in development, will begin in late November or early December.
Built on the banks of the Potomac River, the District of Columbia has always been under threat of river flooding from a major storm. In a review done after Hurricane Katrina in 2005, however, the U.S. Army Corps of Engineers concluded that the city’s flood-control system — built some 70 years ago — was inadequate to protect the city during a major storm.
An updated map of potential flooding drafted by the Federal Emergency Management Agency said museums such as the National Gallery of Art and federal buildings like the Commerce Department could be under as much as 10 feet of water if the current flood-control measures failed. That triggered planning for a better system.
Downtown Washington is now protected by a levee that has been in place since the late 1930s. The earth barrier runs parallel to the Lincoln Memorial’s reflecting pool and prevents water from creeping north into the city. When officials are concerned about flooding, a sandbag barrier is also placed across 17th Street, completing the barrier. Without the sandbags, water could get funneled up the street and then flow down Constitution Avenue onto the Mall. If more serious flooding is projected, the existing plan calls for erecting an 8-foot earthen embankment with dirt taken from the Washington Monument grounds.
The new levee will replace the sandbag barrier with a combination of permanent stone walls and a 12-foot tall removable metal wall. A permanent wall will be constructed on both sides of 17th Street. The removable part of the barrier — posts and metal panels — will normally be stored but will be set up by the National Park Service to block 17th Street if flooding is expected. The levee is only useful, however, to prevent flooding from the Potomac — not if heavy rains hit the area like those in 2006 that flooded the National Archives and other buildings.
Officials last set up the sandbag barrier in 2003 when Tropical Storm Isabel flooded parts of the Washington area, and severe flooding is not unheard of in the region.
“There were floods up through the 1930s and 1940s that literally extended all the way to Pennsylvania Avenue,” said Julia Koster of the National Capital Planning Commission, which approved the levee’s design.
The bulk of the construction on the new barrier is expected to take place in March. The project is expected to be finished by summer 2011. D.C. officials originally pledged to build an improved levee system by the end of 2009.
FEMA’s updated flood map of the D.C. area went into effect in September, affecting who has to purchase flood insurance. Once the levee is completed, however, the map could be revised.