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Posted at 12:45 PM ET, 11/19/2010

Is D.C.'s planned flood wall enough?

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As reported recently in the Washington Post, a flood control plan is in the works to protect the Washington Mall and large sections of downtown DC by constructing large earthen berms and eight-foot high aluminum panels just west of the Mall near the Washington Monument (see enlarged map).

At present flooding poses hazards to the security and infrastructure of federal buildings and treasured historic sites. Not incidentally, the plan was largely formulated to avoid the Federal Emergency Management Agency (FEMA) declaring many sections of the District a flood zone, which would require government and individual property owners to purchase expensive flood insurance.

The project, which will start next month and be completed next summer, is designed to hold back rising waters of the Potomac River during flood events. The rational and details of the plan are described in the FEMA Report on Flooding and Stormwater in Washington, D.C..

But will this project alone offer anything close to a guarantee that DC's flood problems will become something of the past? No way!

Flooding in the nation's capital is nothing new nor especially rare, and can result from multiple causes. The FEMA report (in Appendix A) lists and offers comment on some of the major flood events in Washington through 2006.

The berm/wall project addresses only the threat posed by flooding of the Potomac River. That can occur separately or in combination of the two main scenarios contributing to a flood potential.

In the first scenario, the Potomac River overflows its banks as a consequence of river channels receiving more rain and/or snowmelt than they can handle.

In the second, a tidal storm surge arises from an abnormal rise in water level forced by winds and/or the low pressure of intense coastal storms, usually but not always hurricanes. A prime example cited in the report is the flooding associated with Hurricane Isabel in 2003. (Note: the project would not have protected the waterfront area of southwest D.C. shown in the bottom two photos in this link).

Flooding in the Mall and downtown area of Washington can occur independently of the Potomac River, with or without the planned berms and walls. The threat arises because heavy rain and/or snowmelt can readily overwhelm the drainage capacity of the inadequate aging storm sewer system. The principle example described in the FEMA report occurred when intense tropical downpours over a short period inundated the District in June 2006. Similar events occurred in June of 2007 and 2009. At this time there is no long term federal or local management plan to alleviate addressing this critical problem.

By the way, there now is a small berm just north of the Reflecting Pool in the shadow of the Lincoln Memorial. The berm is part of an amazingly inconspicuous 70 year old levee system designed pretty much for the same purpose as the new project (anyone ever notice it?). However, it has not been maintained and likely would fail in a major storm. Ironically, this system could contribute to flooding problems by hindering flood waters originating from the Potomac draining back as river levels lowered, as well as drainage into the river from excessive rainfall.

The FEMA report also mentions that flooding in Washington is exacerbated with higher relative sea levels (net of earth subsidence and rising water levels). According to the report, the severe flooding in the region associated with Hurricane Isabel (2003) was, in part, the result of a relative sea-level rise of one foot since 1933. The report notes severity of flooding events of this sort could increase further with rising sea levels associated with the prospects of continued melting of glaciers and ice sheets (though hopefully not as bad as I once imagined ).

Not mentioned in the report is what appears to be an increasing threat of flooding in D.C. from breakage of the city's water mains. As recently as October 17th this year a break of a large water main near the Mall shut down a sizable portion of Constitution Avenue resulting in considerable disruption of normal Saturday activities and forced closings of the Smithsonian's Natural History Museum.

It's important to note that flooding of the Potomac, especially when due to a tidal storm surge, is often associated with flood waters from the Anacostia River inundating sections along its banks. However, it is nominally viewed as a lesser threat than the Potomac to the Washington Metro region because of its far greater size, reach of its watershed and much larger volume of water. Nevertheless, the FEMA report acknowledges that the Anacostia watershed is becoming increasingly more urbanized, at least on the western bank (think National's stadium and redevelopment northeastward to and including the Washington Navy Yard). As a result, the real estate of this area is vulnerable to major losses from flooding of the Anacostia. Unfortunately, there does not appear to be any "shovel ready" projects to address the problem.

By  |  12:45 PM ET, 11/19/2010

Categories:  Floods, Floods, Floods

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