‘Linnean Playground’ Stream To Be Restored

by Suzanne Wald
District Department of the Environment

This summer, the National Fish and Wildlife Foundation (NFWF) and the District of Columbia are restoring a perennial stream in Linnean Playground using an innovative technique called a Regenerative Stormwater Conveyance (RSC).

This waterway is fed by a storm sewer outfall that conveys stormwater runoff from 24.5 acres of urban residential properties with 10.3% impervious surfaces. The stream is disconnected from the floodplain, and has steeply-cut, actively eroding ravines that are not able to support aquatic life. For these reasons, downstream water quality is very much degraded.

Linnean Playground sewershed is the area of streets and rooftops and sidewalks that drain
rainwater into this stream. It is shaded blue in this picture:

What is a Regenerative Stormwater Conveyance (RSC)?
RSCs are a special type of stream restoration designed to treat headwater streams degraded from stormwater outfalls.

RSC systems use a series of shallow pools, grade controls, and native vegetation to treat and slow storm flow. RSCs reconnect tributaries with their historical floodplains and convert stormwater to groundwater via infiltration. RSC’s physical features have potential to improve water quality by removing large amounts of pollutants from the water.

Why Use RSCs?
The District Department of the Environment (DDOE) believes that RSCs are an important tool for several reasons:

• RSCs help stabilize actively degrading streams and support aquatic habitat;
• RSCs are favored by the National Park Service (NPS) because RSCs do not require the removal of numerous trees;
• RSCs slow stormwater flows and encourage infiltration, recharging the water table;
• RSCs can protect the sewer pipes buried in District streams to prevent damage and leaks; and
• RSCs may improve water quality by removing large amounts of pollution from the water.

 

Monitoring for Pollutant Removal and Habitat Improvement
The University of Maryland’s Chesapeake Biological Laboratory (CBL) is providing monitoring, analysis and synthesis of the data collected in an effort to assess the value of applying RSCs as a stream restoration technique. Some questions include:

• Are RSCs as effective in the Piedmont and mountain regions as they are in the Coastal Plain?
• How well do RSCs perform at stream stabilization?
• Are RSCs effective at restoring aquatic habitat?
• How effective are RSCs at reducing nutrient and sediment loads?
• How well do RSCs increase groundwater recharge, baseflow and wetland habitat?
• Do RSCs remove stormwater pollutants?

 

In January 2013, CBL began pre-installation monitoring for concentrations of nutrients, sediments, metals, bacteria, flow volume and velocity, water temperature and habitat health. DDOE staff will perform photo surveys to document RSC system stability over time. Monitoring is scheduled for one year before the stream is restored and at least one year afterward, but will continue as long as necessary to meet the requirements of our restoration permit. This schedule allows researchers to compare the results and determine whether or not the RSC improves wildlife habitat and water quality as expected.

Construction Plan and Timeline
Biohabitats, Inc., www.biohabitats.com, will design the restoration and work with a long-time partner, Underwood and Associates, to complete construction. Biohabitats is coordinating the construction with DC Water so that they can support an undermined sewer line and also slip-line the entire length of sewer pipe to make sure they won’t need to return for future repairs. This work will occur during construction to ensure that the stream will only be physically disturbed once.

Restoration work will begin this summer at the bottom of Linnean stream, near Harrison Street. This portion of the work is necessary to control stormwater flow entering the Broad Branch daylighting project (see the Forest Hills Connection article about the plans). Construction of the remainder is set to begin in December of 2013, and should last no more than two months. For this work, trucks will enter the restoration site from the top of the stream on Broad Branch Terrace, travel down to the bottom of the stream across from Harrison street and work their way back out. When work is complete, the open, grassy area that neighbors see at the top of the system will retain a park-like character. The portion that neighbors see near Harrison Street will retain its natural look, but will have more traditional wetland elements.

Soon after construction, a rainy-day walk down the length of the Linnean tributary stream may look like this recently completed RSC at Milkhouse Run in the Rock Creek Watershed:

Updates on the status of this project and additional information will be shared with The Forest Hills Connection as the restoration progresses. Please send questions and information inquiries to: suzanne.wald@dc.gov or steve.saari@dc.gov.