Broad Branch

Broad Branch

Daylighting & Restoration of Linnean Park Tributary, Washington, DC

Broad Branch

The restoration of the Linnean Park tributary and daylighting and restoration of Broad Branch are two linked projects located in Northwest, Washington DC.  The goal of this effort was to restore in-stream habitat and improve the urban hydrology of these two tributaries that flow to Rock Creek through a combination of bioretention, RSC design, and stream daylighting.

One portion of the project involved daylighting a part of a stream that was put into a pipe in the 1930s. Daylighting this section of the Rock Creek watershed recreated riparian habitat in an area that had not had surface stream for eighty years and created at least half an acre of new wetlands.  In addition to creating habitat and improving the hydrology and hydraulics of these streams by raising the local groundwater, lowering the peak discharge and in-stream velocities, and increasing the duration of flow, the project will also improve water quality at the location and downstream water quality by exposing this previously piped water to sunlight, air, soil, and vegetation, all of which help process and remove pollutants.  The project also involved directing stormwater from adjacent streets, alleys, and rooftops into bioretention cells by creating curb cuts and redirecting storm sewers.  These bioretention cells slow, cool, and filter that stormwater which then recharges the groundwater table thereby providing an additional source of clean baseflow to the new stream.

Two of the three RSCs included in this project were built to treat and safely convey stormwater in highly eroded gullies with no baseflow.  The third RSC restored a perennial stream that originates at a 60 inch stormwater outfall and ends downstream when it enters into a culvert and storm drain. This $1.2 million project was constructed by U&A between February and October 2014. Through the project, 1,900 linear feet of stream were daylighted, 1,000 linear feet of existing incised stream were restored using RSC techniques, 500 linear feet of stormwater gullies were stabilized, 30,000 square feet of wetland were created, and nine bioretention cells treating 86,300 square feet of impervious surface were installed. The stream restoration work will reduce loads to the Chesapeake Bay by 580 pounds of nitrogen, 197 pounds of phosphorous, and 162,400 pounds of sediment annually. This project also created connected habitat and produced several new vernal pools for a previously isolated spring that supports a spotted salamander population, a keynote species of a healthy habitat.  

In addition to the in-stream habitat created, the project removed invasive plants. Construction work was carefully designed to remove and in some cases deeply bury the invasive seed bank in the project area.  The invasive plants were replaced with hundreds of native trees, shrubs, grasses, wildflowers, and aquatic emergents.  Additionally we were able to save twenty-one trees that were originally marked for removal.

This project was constructed over approximately five and half months in 2014. To date the project partnership has completed a full year of pre-restoration monitoring and has begun the first year of monitoring the post-restoration condition.  Early post-restoration data has shown that the restoration work has resulted in an almost immediate four foot increase in groundwater around the project area and improved the volume and duration of perennial stream flow.  The monitoring effort is extensive and includes monitoring habitat, flows, stability, and pollutant removal efficiency for the eight pollutants required to be monitored through the District’s Municipal Separate Storm Sewer permit.  

The site is now and will continue to be a teaching tool through an innovative online teaching tool (http://www.cacaponinstitute.org/Benthics/vss6.html) and field visits with students.  Photo monitoring points have been installed at key locations throughout the project area where visitors can take photos with their phones and upload them to a photo sharing site where they will aid the District in monitoring the site over time.  Furthermore the site was featured as a part of a National Geographic series on stream daylighting (http://news.nationalgeographic.com/news/2014/11/141125-dc-daylighting-broad-branch-stream-restoration-science). Interpretive signage will be installed in the coming year. This project was the overall winner of the 2015 Best Urban BMP in the Bay Award (BUBBA).