Permeable pavers, porous asphalt, and bioretention cells at the Silver Lake beach parking lot, Wilmington (GeoSyntec Consultants)
Permeable Paving Materials and Bioretention in a Parking Lot
Location: Silver Lake Beach Parking Lot, Wilmington,
- Reduce the quantity of stormwater runoff and nonpoint
source pollution to Silver Lake and maximize infiltration
- Demonstrate the use and performance of different types of permeable
paving materials to infiltrate stormwater.
- Demonstrate the use of bioretention
cells to reduce runoff and pollutants from impermeable areas.
- Assess and characterize any potential impacts to groundwater
quality that might result from the use of permeable pavement.
Description: Silver Lake is an important recreational
resource that supports swimming, fishing, wildlife viewing, and
boating. However, the lake was degraded from nutrients, sediment,
and bacteria from the surrounding conventional stormwater system
(which includes several direct discharge outlets to the lake) and
nonpoint source runoff. Beach closures due to high bacteria counts had occurred repeatedly for many years prior to this demonstration.
This project incorporated three LID practices designed to reduce
runoff volume, improve water quality, and enhance groundwater recharge:
- Permeable paving materials
- Bioretention cells
- Vegetated water quality swales
Four types of permeable paving materials were used to repave about half of the parking lot. The parking spaces themselves, consisting of about 8,000 square feet, were paved with porous pavers, which allow stormwater to soak into the ground between the paving units. The parking aisles, consisting of 16,600 square feet, were paved with porous asphalt, a special mix that allows water to pass through the pavement surface. Underneath the porous pavers and asphalt is an infiltration bed of crushed stone that filters contaminants and allows water to slowly percolate into the underlying soil. Additionally, a porous paving system called Gravelpave, consisting of interlocking plastic cells filled with gravel, was installed in a portion of the overflow parking area. Finally, a porous material called Flexi-Pave, made from recycled tires, was installed in another small section of the overflow parking lot. For comparison purposes, one half of the existing main parking lot was resurfaced with standard asphalt.
(click image for larger view)
Surface runoff from the standard pavement and any overflow from
the permeable paving surfaces was designed to drain to bioretention cells,
which will provide treatment as water percolates through the soil,
and nutrients and contaminants are processed by the plants. The
parking lot design included two center bioretention islands and
seven bioretention cells around the perimeter.
Two ten-foot-wide vegetated water quality swales were installed
to replace existing piped stormwater outfalls at either end of the
beach. As with the bioretention cells, these swales were designed to improve
water quality through the physical, chemical, and biological processes
that occur as runoff is exposed to sunlight, percolates through soil and is taken up by
plants. The northwestern swale replaced a grassed area that
formerly attracted geese and other waterfowl, leading to the buildup
of droppings that contributed to bacteria loading and closure of
the public swimming beach area. The planting plan for this swale
includes native shrubs that are not attractive to grazing waterfowl.
Click here for project photos.
Data Collection and Analysis: The U.S. Geological
Survey (USGS) installed seven wells in the parking lot. Four wells
provided data on groundwater levels, and three provided data on groundwater
quality. USGS monitored preconstruction conditions after five storms over four months. Following construction, USGS monitored groundwater levels and collected samples monthly. Samples
were analyzed for a range of parameters, including conductivity,
temperature, pH, nitrogen, phosphorous, metals, and total petroleum
hydrocarbons. Sampling was designed to detect any changes in groundwater
quality associated with recharge from the parking lot.
This project was closely coordinated with additional LID
work occurring across the lake from the town beach. For details,
see Demonstration 4: LID retrofits in a lake-side neighborhood
- Construction of pavement and installation of planting materials completed May 2006
- Monitoring of groundwater quality conducted by USGS:
- Preconstruction: sampling of five storm events, July - October 2005
- Postconstruction: monthly sampling, June 2006 through June 2007
- Project dedication ceremony, June 2006
- Maintenance of planted areas performed spring and fall, 2006 through 2008
Design, engineering, permitting, and construction management (combined costs for Demonstration Nos. 3 and 4): $92,000
Construction, and three years of maintenance (combined costs for Demonstration Nos. 3 and 4): $355,000
Key Results and Conclusions:
- Infiltration tests of the permeable paving materials, conducted after construction, indicated that infiltration rates met or exceeded specifications; the average observed infiltration rates were:
exceeds 5,000 in./hr
- Results of USGS monitoring show no indication of groundwater impairment beneath the areas with pervious paving.
- Reports from the town Board of Health show no closures of the swimming beach as a result of E. coli bacteria in the four years following installation of the LID features. For eight years prior to installation, beach closures due to E. coli occurred one or more times each summer.
- Since the installation of the LID features, the beach had one closure due to cyanobacteria, an algal bloom often associated with in influx of nutrients.
Publications and Related Materials:
Project partner: Town of Wilmington
Design/Engineering/Construction oversight: Geosyntec Consultants
Construction and three years of maintenance: Cali Corporation
Monitoring and Analysis: U.S. Geological Survey
Ongoing Maintenance: Wilmington Department of Public Works
Acknowledgements: DCR would like to acknowledge contributions to this project from Invisible Structures, Inc. (porous asphalt), K.B. Industries (Flexi-Pave) and Oldcastle/Foster-Southeastern (permeable pavers)