Washington DC Utilizes Green Infrastructure to Manage Stormwater

Washington D.C. Utilizes Green Infrastructure to Manage Stormwater

Project Summary

Under a consent decree from the U.S. Environmental Protection Agency to reduce combined sewer overflows, the District of Columbia Water and Sewer Authority developed a plan to construct three large holding tunnels to provide extra capacity during high precipitation events. Upon further consideration of the uncertainties regarding future precipitation extremes and the costs associated with developing three large infrastructure projects, the Water and Sewer Authority requested to revise the agreement.

The revised plan replaces one tunnel with green infrastructure projects to reduce the amount of stormwater runoff that the system has to handle. A capacity metric (i.e., amount of stormwater runoff managed) associated with the green infrastructure projects was adopted rather than an initial plan requiring a defined financial commitment ($90 million) to better ensure the expected stormwater reduction improvements.

The Water and Sewer Authority reviewed the National Climate Assessment projections for the Northeast to better understand potential future conditions, however no such projections were included in the capacity agreement. While no climate projections were included, the agreement does provides the District of Columbia greater adaptive flexibility to scale and increase green infrastructure to accommodate future precipitation extremes.

With the installation of green infrastructure projects in the Rock Creek Park corridor, the revised plan provides substantive environmental, economic, and health benefits as early as 2017 as compared with the original project projection of 2024. The revised plan, upon completion in 2030, is expected to reduce combined sewer overflow releases by 96% based upon current precipitation levels. 

Implementation

When the Water and Sewer Authority realized its current approach was insufficient to meet current and future climate extremes, the entity re-evaluated its plan to address combined sewer overflows and recognized that the financial, engineering and environmental commitments under the original consent decree agreement were not likely to yield the results it needed given uncertainties of current and future extreme weather events. The Water and Sewer Authority assessed alternative strategies (PDF) (13 pp, 233 K, About PDF) that provided greater flexibility and adaptive capacity to provide for substantive combined sewer overflow reduction benefits both now and in the future.

To select a unique adaptation option specific to local conditions, the Water and Sewer Authority developed and proposed a plan utilizing a mix of grey and green infrastructure. The District had identified a variety of green infrastructure and low impact development measures with potential for use, including rain barrels, grassed swales, cisterns infiltration trenches, permeable pavements, increased tree cover and rooftop greening.

The Water and Sewer Authority defined performance-based metrics rather than financial ones. They committed to installing Green Infrastructure (PDF) (890 pp, 40 MB) to absorb “…1.2 inches of rain falling on 365 impervious acres of land that currently does not absorb stormwater…” If this commitment proves infeasible, the plan will revert to using underground storage. This plan is projected to reduce combined sewer overflows by 96% and is expected to be capable of handling more than 90% of storms under baseline climate conditions. The Water and Sewer Authority has already completed preliminary green infrastructure demonstration projects and is expected to complete an additional 44 acres of by June 2019.

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Similar Case Studies

  • Increased precipitation events may lead to increased sewer overflows as well as threaten the water or wastewater utility facilities themselves. For more information on what a Washington, D.C. wastewater facility is doing to adapt to climate change and the threats from flooding, view the Blue Plains Wastewater Facility Case.
  • For information on a city that is moving wastewater services away from an area vulnerable to flooding view the case study about the Iowa City Riverfront Master Plan.
  • For additional examples of how cities use green infrastructure to manage stormwater, view the Chicago and Gary, Indiana case studies.