Federal Triangle Stormwater Study
The Federal Triangle Stormwater Drainage study analyzed the causes of a 2006 flood event that put the Federal Triangle Area of Washington D.C. under up to 3 feet of water, including several major traffic arteries, tunnels, and subway stations. The Study analyzes possible alternatives to reducing the risk of interior drainage flooding, including permeable pavements and other Low Impact Development (LID) techniques. In considering the causes of flooding in the area, the Study uses the 200-year flood as a proxy for assessing how climate change may increase flood risks and the frequency of severe storm events.
The Federal Triangle study area is in the NW quadrant of Washington DC between 15th Streets to the west, Madison Drive to the south, 3rd Street to the east, and Pennsylvania Avenue to the North. This area is home to many federal government buildings. In 2006, Washington DC experienced a 200-year storm event, which dropped more than six inches of rain in six hours. As the lowest point in the District, the Federal Triangle area experienced the greatest flooding. The existing stormwater drainage infrastructure (a combined sewer system) could not handle the large volume of rainfall over such a short duration, and it overflowed. The storm caused up to three feet of flooding in City streets in the Federal Triangle area, and flooded the 9th and 12th Street tunnels under the mall and two Metro Stations. The flooding also caused extensive property damage, power loss, and equipment damage, costing tens of millions of dollars. In response to the flood, the affected agencies initiated this Study to evaluate the cause of flooding and analyze possible alternatives to reduce the likelihood of future flooding.
To evaluate the viability of alternatives to mitigate flooding in the study area, a detailed model of the terrain and sewer systems was developed building upon a GIS-based model already in use by DC Water (the District’s water utility company). The model was analyzed under conditions of the 15-year (the current design storm that DC uses to construct new sewer facilities), 50-year, 100-year, and 200-year storm events. The 200-year storm event was modeled to account for potential effects of climate change recognizing that more severe weather events may become more common in the future. The model was calibrated using data from conditions during the 2006 storm event. The 2006 flooding was due solely to interior drainage, not from overbank flooding from the Potomac River. Washington’s outdated combined sewer overflow system was incapable of handling the large volume of rainfall over such a short duration.
The Study considered nine possible alternatives but quickly eliminated two options that were not physically possible. The remaining alternatives were evaluated based on costs, benefits, and technical factors. The study found three viable alternative solutions to Federal Triangle flooding: a drainage storage system beneath the National Mall; a new pumping station serving the National Mall area; and a new tunnel to the existing Main and O Street Pumping Station. Each of these solutions was projected to cost between $300-500 million.
Several of the flood mitigation strategies that were initially rejected as infeasible would have affected transportation infrastructure, such LID or green infrastructure strategies. LID includes approaches that attempt to maintain or recreate the natural hydrological conditions of a site by reducing impervious cover. They often include changes to transportation assets, which make up most of the impervious coverage in urban areas, and include techniques such as vegetated swales and permeable pavements. The Study found that LID would be useful for preventing runoff from small storm events. However, the Study concluded that, given the small scale of the Federal Triangle area, LID strategies could not be implemented on a scale necessary to capture rainfall from medium- or large- storm events to prevent flooding, even in conjunction with existing sewer systems. Thus, the Study concluded that LID would not make an effective stand-alone strategy for addressing flooding in the Federal Triangle region, but could be used to augment other strategies. The Study recommended further evaluation of hybrid strategies using a combination of LID, a small-scale storage system under the mall, and use of the existing sewer system to prevent flooding during smaller storms.
The Study was led by the District of Columbia Water and Sewer Authority (DC Water), through a consultant, with guidance and funding from partner agencies in the Federal Triangle Stormwater Study Working Group: the General Services Administration; Federal Emergency Management Agency; National Archives; National Capital Planning Commission; National Gallery of Art; National Park Service; the Smithsonian Institution; US. Department of Justice; U.S. Environmental Protection Agency; the Washington Metropolitan Area Transit Authority; and the District of Columbia Office of Planning, Department of the Environment, and Homeland Security and Emergency Management Agency.
This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on January 29, 2016.
Publication Date: October 2011
- U.S. Department of Justice (DOJ)
- District of Columbia Water and Sewer Authority (DC Water)
- Washington Metropolitan Area Transit Authority
- District of Columbia Office of Planning
- District of Columbia
- Federal Emergency Management Agency (FEMA)
- U.S. General Services Administration (GSA)
- National Capital Planning Commission (NCPC)
- District Department of Energy and the Environment (DDOE) - Washington DC
- U.S. Environmental Protection Agency (EPA)