Providence Stormwater Project
(Conservation Currents, Northern Virginia Soil and Water Conservation District, December 2005)
Many of us give little thought to stormwater. However, stormwater-induced erosion and water quality deterioration are pervasive problems in Fairfax County's streams. Rainwater, as well as snowmelt and even excess water from lawn watering, picks up pollutants as it drains from our driveways, yards and parking lots to our streams via the storm drain system. Additionally, paved or impervious surfaces such as rooftops and roads prevent precipitation from infiltrating the soil. As a result, torrents of untreated water rush into our streams during storms causing flash flooding and erosion.
An innovative stormwater retrofit and demonstration project completed this summer at Merrifield Fire Station 30, site of Providence Supervisor Linda Smyth's office, is collecting and filtering stormwater runoff from paved surfaces and roofs at the complex. In the process, the retrofit is reducing erosion and pollution of Accotink Creek. The Providence Supervisor's office, Fairfax County Department of Public Works and Environmental Services— Stormwater Management, and NVSWCD partnered to complete the project, which was funded in part by a grant from the Virginia Department of Conservation and Recreation.
The first stormwater retrofit of its kind in Fairfax County, the Providence project integrates three emerging low impact development (LID) technologies: a green roof, permeable pavers and a rain garden. LID technologies like these manage stormwater by mimicking natural landscape functions, specifically those that infiltrate, filter, store, evaporate or hold runoff. By allowing time for water to filter into the soil, LID techniques treat stormwater at the source rather than rushing untreated runoff into the nearest storm drain and then, to the nearby stream. LID projects restore, to the greatest extent possible, the hydrologic conditions that existed on the site prior to development.
The LID design for the Providence retrofit takes advantage of a drainage pattern that used to direct water towards the storm drain inlet in the facility's central parking median. Now, when it rains, water flows through curb breaks in the parking median to enter the landscaped rain garden, or percolates through the permeable pavers that have replaced asphalt in the adjacent parking stalls. Water percolating through the pavers or overflowing from the rain garden enters an 8-foot deep gravel infiltration trench beneath the rain garden and paver complex. Hollow Stormtech® storage chambers installed in rows within the trench increase the volume available for retaining and storing stormwater. Nearby, the green roof installed on an existing concrete storage shed absorbs water and reduces rooftop runoff at the property.
Over the 48 hours following a storm, the water held in the rain garden, the gravel trench and the green roof will infiltrate into the soil where natural processes will strip it of pollutants and it will slowly release into local waterways. During periods of very high runoff, stormwater overflow will enter the pre-existing storm drain inlet, which now serves as an emergency drain for the complex.
Together, the three LID structures treat stormwater from 0.83 acres of the 1.8 acre property and can store more than 5,000 cubic feet of runoff, roughly the amount of water generated by 2 inches of rainfall within the drainage area in a 2-hour period. The infiltration trench with its resident Stormtech® chambers holds the majority of the stormwater collected and is a key design feature.
The Providence project has already received national recognition for its innovative design and visionary use of LID to improve water quality in the county's Accotink Creek watershed. Supervisor Linda Smyth's office earned a 2005 National Association of Counties Achievement Award and a 2004 commendation from Delegate Jim Scott for the project entitled "Demonstrating Innovation: A Stormwater Retrofit at the Providence Supervisor’s Office."
Supervisor Smyth's office initiated the retrofit as a way to demonstrate that LID is a viable, cost-effective alternative to traditional stormwater management. "Many developers and many in the county have heard about LID," emphasizes Smyth, "but have never seen it. Now, the county can say we're doing it [LID], and it's working." Because several technologies are showcased at the Providence site, the versatility of LID is also demonstrated. "This project gets across the idea that LID provides a menu of possibilities for all sites, from large developments to the single-family home," Smyth continues. "We're at the point with stormwater, where every little bit helps. We can improve the situation one yard at a time. This site is our backyard."
Exactly how effective the project is at improving quality and reducing the quantity of stormwater entering Accotink Creek continues to be evaluated. Funding through the Department of Conservation and Recreation grant is enabling ongoing monitoring of the LID project. "The great thing about this project,"asserts NVSWCD's Asad Rouhi, "is the monitoring system." John Palmer, a Fairfax County Public Works intern and a University of Maryland landscape architecture student explains, "Most LID designs are based on theory at this point. The incorporation of monitoring enables us to actually measure water quality improvements and retention resulting from this retrofit." Flow and quality measurements for water coming out of the storm drain were taken before construction of the project. These observations will be compared with current quantity and quality measurements for water leaving the site. Additionally, rain gauge and water depth sensors installed in the rain garden and infiltration trench are measuring how well water is filtering into the surrounding soil. Kathryn Moore, the supervising construction engineer and a staff person with Fairfax Countys Stormwater Management, reflects the feeling of most of the project team. "I have never seen this technology used before and I'm looking forward to seeing the monitoring results. I want to know how effective this project is."
Educational signs will be installed this spring and the public is encouraged to visit to see the retrofit firsthand. Ron Tuttle, a Fairfax County landscape architect and a proponent of LID technologies, hopes the project inspires more like it throughout the county. "The Providence project is an example to everyone, local government, developers, and the public, that LID can effectively improve water quality and protect downstream resources."
Supervisor Smyth's office is already fielding questions from visitors about the attractive new pavers and rain garden in their parking lot. Like Tuttle, Supervisor Smyth is hoping that LID will become more prevalent because of the Providence example. "We hope what we have here is a pattern that will be replicated. We are starting in a new direction with LID, and it will take more projects to convince folks that LID is here for the long term." Adds Smyth, "We need LID to be here to stay. Our old stormwater management strategies haven't really worked, we need to do better."
For more information about LID practices, see NVSWCD's Rain Gardens, Green Roofs, and Other LID Practices.
For more information about this project, visit the Providence District Supervisor's Department of Conservation and Recreation Grant Project Update.