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.