Welcome back to CSN’s Stormwater Spotlight Series! Over the next few months, we will be showcasing some of the top BUBBAs projects from this past year. To kick us off, what better place to start that our GRAND PRIZE winner?! Here are excerpts from the Alger Park Stream Restoration and Upland LID Project’s narrative:
The Alger Park Stream Restoration Project began in 2011, at the request of residents in the Hillcrest neighborhood in SE, DC. Staff from the District Department of Energy and Environment (DOEE) met with community members to tour the stream that runs through Alger Park. The unnamed stream is part of the Texas Ave sub-watershed of the Anacostia River in the District of Columbia. The stream was in a highly degraded state with little base flow, a highly incised channel with vertical stream banks over 20 ft. tall, and few areas for in-stream habitat. The watershed area that drains into Alger Park is 32 acres in total. Overall, 32% of the area that drains into Alger Park is impervious cover. The watershed and Alger Park itself are steeply sloped and with Christiana soils. In all likelihood the park was home to an intermittent stream before the neighborhood was developed in the 1930s, 40s, and 50s. As the neighborhood developed, stormwater from roadways was directed into the park and stream valley via five stormwater outfalls. With the installation of the outfalls and increasing impervious area throughout the drainage basin, erosion rates along the stream banks continued to rise.
Knowing that stormwater was the driving force behind the degradation in
Alger Park, DOEE set about working with community members to ensure they maximized stormwater retrofits on both public space and private space. Over the two year design period DOEE met with civic groups, knocked on every door in the watershed multiple times, and led several community clean-ups to raise awareness about why stormwater capture in public and private space. Over the course of two years DOEE was able to install 29 rain barrels and 7 rain gardens on private property in the drainage areas to Alger Park. Additionally, DOEE transferred funds to the District Department of Transportation (DDOT) to execute design and construction work on low impact development (LID) projects in the upland public space areas. DDOT began constructing the LID work in 2018 and expects to complete them in 2019. When the public space LID work is complete there will be 29 Best Management Practices (a mix of bioretention & permeable pavers) installed treating 200,000 square feet of impervious surface installed in the public space area that drains into Alger Park.
The stream restoration project itself focused reducing in stream bank erosion. DOEE moved forward with two stream distinct stream restoration design methods. First, for the highly incised upper reach of the stream a regenerative stream design (RSC) approach was selected. This approach allowed the highly incised stream gully to be filled upwards of 20ft in some areas with a sandy soil mix allowing for stream wetted width to go from 2-3ft pre-restoration to over 20ft wide in some pools in the upper reaches. This increased channel width allows for high flows to spread out and dissipate energy thus reducing erosive forces. Because the stream valley is very steep boulder cascades were installed to adjust grade changes throughout the reach. Due to the urban setting of the project, steep slopes, and RSC approach DOEE used the channel itself as the haul road with our contractor, Environmental Quality Resources (EQR), spending over 2 months filling the upper channel with a sand and wood chip mix to bring the stream up to the design grade. After filling the channel EQR worked its way out of the upper reach by building riffle, cascades, and burying logs in the floodplain perpendicular to stream flow to both stabilize the floodplain and act as a carbon source for denitrifications. Based on Biohabitats’ designs EQR was able to use the stream channel as the haul road which helped to minimize impacts to the adjacent forest buffer during project work.
The downstream 700+ feet only had a slight grade and to replicate the pre-restoration braided channel complex in this area the designs called for to channels but with buried logs across the valley for grade control and floodplain stability. Behind the installed riffles are a series of varying depth pools and the entire 0.5 acres was turned to into a stream-wetland complex planted densely with native wetland plants. In total, 3,447 native wetland plants, 1,160 native shrubs, 389 native herbaceous plants, 382 trees, and 59 pounds of native seed were planted in the Alger Park stream corridor.
In the project area DOEE was also able to convert three outfalls into bubbler systems so that water from the stormwater pipe was forced vertically before being released into dissipation pools. Converting traditional outfalls into bubblers and installing plunge pools at the other outfalls helped to reduce the velocity of stormwater exiting the storm sewer network in the stream and allowed additional filtration of pollutants.
During the 100% design portion of the work, DOEE also engaged in extensive pre-restoration monitoring that looked at pre-restoration conditions in the stream and in the water entering the stream from several outfalls. Pre-restoration monitoring showed that only a few macroinvertebrates which are highly tolerant of pollution were found. Following the completion of the upland LID work, DOEE will implement a multi-year monitoring project that will compare pre-restoration conditions to post-restoration conditions. DOEE will also monitor in-stream water quality as well as habitat conditions to see if the quality and quantity of macroinvertebrates improve in the stream.
The Alger Park Stream Restoration Project is a model for urban stream restoration because it combined public and private space LID in the upland areas with two different types of stream restorations in the stream valley to maximize the restoration opportunities the site presented to ensure environmental uplift.