Interior Drainage Flooding in Downtown Washington, DC - A Coupled Modeling Approach Using 2D Urban Flood Modeling

Volker Janssen, Brad Udvardy, John Cassidy and Roger Gans

ABSTRACT

The Federal Triangle area in downtown Washington DC is home to many Federal Agencies, the Federal Archives, National Gallery of Art and other prominent buildings. The area itself is close to the Tidal Basin and the Potomac River, and is the lowest point of an approximately 5.8 square mile urban drainage area.

In June 2006, tropical storm Alberto inundated the District with up to 14 inches of rainfall in some areas. That resulted in street flooding in the Federal Triangle area, with up to 3 feet of water on Constitution Avenue, flooded basements for various Federal agencies and Smithsonian Institution properties, and partial flooding of DC Metro subway tunnels.

Starting in 2009 DC Water conducted a stormwater drainage study for a workgroup formed by the National Capital Planning Commission. A detailed computer model was developed to analyze and dynamically simulate the interchange between the surface flow and sewer system, as well as the performance of inlets and catch basins. DHI’s MIKE URBAN Flood model software, a combination of the surface flow model MIKE 21 and the MIKE URBAN/MOUSE sewer model, was chosen for this task. Both models are dynamically linked with each other to provide the necessary interactions between the surface and the sewers via catch basins, inlets and manholes.

The two-dimensional model grid representing the surface was based on available high density DEM data and additional terrestrial survey data. The grid also included all street and building features. An existing one-dimensional model of the sewer system was extended and the pipe detail increased as necessary. Additional storage elements were used to represent flooded basements and street tunnels. All catch basins and curb inlets within the project area were added into the model.

The model was then calibrated to the events of June 2006 using available empirical data, flooding photos, eyewitness accounts, and sewer metering and pump station data. Based on the calibrated model, the system performance was assessed for various design storm events. Relief alternatives were identified and evaluated that included upstream storage solutions, additional stormwater pumping stations, design of an additional sewer conveyance and storage tunnel, and implementation of LID practices as a secondary measure.


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