Could Hurricane Florence Pose a Flood Risk for the National Mall?

The potential for flooding and high waters caused by Hurricane Florence reminds us here in Washington that the National Mall and surrounding federal buildings are vulnerable to storms and floods.

Mall flooding is not an anomaly. It has happened several times before. The most recent, and the most devastating, was in 2006 when heavy rains overwhelmed stormwater drains and inundated Constitution Avenue, causing millions of dollars in damage to Mall museums, roads, businesses, and federal buildings. The National Archives remained closed for three weeks.

The June 2006 stormwater flooding caused millions of dollars in damage to, clockwise from upper left, federal office buildings, the Department of Justice, the basement theater in the National Archives, and the 12th Street tunnel under the Mall.

As Florence approaches, the Capital Weather Gang warns in The Washington Post, “Our soils are saturated and our rivers, creeks and streams are full, if not already overflowing. If we receive several inches of rain or more, flooding will be inevitable, and it could become severe in a worst-case scenario.”

Bracing for “torrential rain,” on Tuesday, DC’s Mayor Muriel Bowser declared a state of emergency and Metro is putting in place sandbags around vulnerable Metro stations including in the Federal Triangle and the Mall.

And for the first time, the National Park Service is preparing to test its new 17th St levee if the Potomac River threatens to overflow its banks and flood 17th Street at Constitution Avenue.

But even if the levee succeeds in holding back river waters, the Mall is still vulnerable to a different yet equally destructive flood threat, namely stormwater flooding during heavy rains as water flows from higher ground in the city down to low-lying Constitution Avenue and the Mall. This is the type of stormwater or “interior” flooding that caused devastating damage in 2006.

A 2011 government report on the 2006 event raised the alarm of future and more intense flooding. Though the report proposed some viable solutions, no stormwater solution has even been agreed upon, let alone implemented.

So what could Hurricane Florence portend for the Mall? Worst case scenario could bring a repeat of 2006 with millions of dollars in damage to our public museums and buildings and potential loss of invaluable cultural resources.

On a positive note, federal and DC government bodies are beginning to collaborate on addressing the stormwater flooding problem. A September 5th meeting of the Silver Jackets flood group discussed a few solutions, including a stormwater cistern under the Mall. At the slow pace of action since 2006, however, a solution providing reliable relief from threats such as Florence may be years away.

Watch the slideshow of historic floods:

  • Devastating Floods are a Historic and Ongoing Problem for the National Mall (1889 Photo courtesy Library of Congress)
  • Devastating Floods for the National Mall (1930 Photo courtesy Library of Congress)
  • 1942 East Potomac Park and the Jefferson Memorial (Photo courtesy Library of Congress)
  • 1985 East Potomac Park with the Washington Monument in the distance (Photo courtesy Library of Congress)
  • In fact, Washington has two separate flood threats: river flooding and stormwater flooding
  • Potomac River flooding has inundated, clockwise from upper left, Pennsylvania Avenue at 6th Street NW in 1889, the Navy Yard in 1936, East Potomac Park (before the Jefferson Memorial) in 1936, and Washington Harbour in Georgetown in 2010
  • The stormwater flood of 2006, by contrast, was caused by runoff from heavy rains on higher ground into this lowest lying part of the city along Constitution Avenue. (Image courtesy Arthur Cotton Moore)
  • This “interior” flooding caused millions of dollars in damage to, clockwise from upper left, federal office buildings, the Department of Justice, the basement theater in the National Archives, and the 12th Street tunnel under the Mall
  • These flooding problems are the result of Washington’s topography and changes made to rivers and streams over the years
  • This 1793 topographic map by Andrew Ellicott shows the site chosen for the nation’s capital at the confluence of the Potomac and Anacostia (Eastern Branch) Rivers with a broad creek – Tiber Creek – wending its way along the Mall area from the Capitol to the Potomac
  • A satellite view of modern Washington superimposed on Ellicott’s 1793 topographic map shows Tiber Creek (now covered by Constitution Avenue) and the original shorelines of the Potomac and Anacostia (outlined in black).
  • This diagram illustrates that while the levee may prevent Potomac River flooding (the area outlined in red), it will not address the separate interior stormwater flooding problem.
  • No government action has been taken to address the ongoing stormwater flooding threat by implementing the recommendations of the 2011 Federal Triangle Stormwater Drainage Study sponsored by DC Water and the National Capital Planning Commission. Lacking a plan, Metro officials use sandbags to protect the Metro system from stormwater flooding.
  • Washington’s flood threats continue to be well documented in recent government and private studies but no comprehensive plan has been developed
 

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