Introduction
The process of urbanization has significant adverse effects on the volume and quality of stormwater runoff that enters our lakes and streams (Davis, 2005; Wang et al., 2001; Williamson, 1993). When natural land covers, such as grasslands and forests, are replaced with impermeable surfaces like streets and parking lots, the ability of the soil and vegetation to retain water is lost. This results in increased runoff, leading to hazardous floods, erosion of stream channels, reduced groundwater recharge, and degradation of aquatic habitats. Moreover, impervious surfaces transport various pollutants found in urban areas, including nutrients, sediment, bacteria, pesticides, and chloride. In severe cases, the pollutant levels in urban runoff make it unsafe for swimming or fishing in local waters.
To mitigate the impact of urban runoff, efforts have been made at the federal, state, and local levels. The Clean Water Act (CWA), through the National Pollutant Discharge Elimination System (NPDES) program, regulates water quality in the United States, imposing pollution limits on entities discharging into water bodies, including cities. Wisconsin, for example, issues NPDES permits through the Wisconsin Department of Natural Resources (WDNR), requiring cities to create management plans that employ best management practices (BMPs) to meet the prescribed limits. Permeable pavement is one such BMP believed to enhance water quality and reduce the consequences of urban runoff.
What is Permeable Pavement?
Permeable pavement is a porous surface typically made of open pore pavers, concrete, or asphalt, with an underlying stone reservoir. This type of pavement captures precipitation, rain, and surface runoff, storing it in the reservoir, where it slowly infiltrates the soil below or discharges through a drain tile. Permeable pavement is commonly used in parking lots, low-traffic roads, sidewalks, and driveways.
What are the Potential Benefits of Permeable Pavement?
General Hydrologic benefits:
- Permeable pavements help reestablish a more natural hydrologic balance and reduce runoff volume by trapping and slowly releasing precipitation into the ground instead of allowing it to flow into storm drains and out to receiving waters as effluent. This same process also reduces the peak rates of discharge by preventing large, fast pulses of precipitation through the stormwater system.
- Permeable pavement can reduce the concentration of some pollutants either physically (by trapping it in the pavement or soil), chemically (bacteria and other microbes can break down and utilize some pollutants), or biologically (plants that grow in-between some types of pavers can trap and store pollutants).
- By slowing down the process, permeable pavements can cool down the temperature of urban runoff, reducing the stress and impact on the stream or lake environment.
- By controlling the runoff at the source, such as a parking lot, permeable pavement can also reduce the need for or the required size of a regional BMP, such as a wet detention pond, which saves money and effort.
Cold-Weather Benefits:
- Another benefit of permeable pavement is the reduced need to apply road salt for deicing in the winter time. Researchers at the University of New Hampshire have observed that permeable asphalt only needs 0 to 25% of the salt routinely applied to normal asphalt (Houle and others, 2009).
- Other researchers have found that the air trapped in the pavement can store heat and release it to the surface, promoting the melting and thawing of snow and ice (Roseen and others, 2012).
Concerns People Have About Using Permeable Pavement
Here are some of the concerns and questions about permeable pavement:
Durability – Will permeable pavement last as long as traditional pavement?
Upkeep and Maintenance – Permeable pavement can clog with sediment and pollutants, reducing its permeability and beneficial productivity.
- How much effort is required to keep permeable pavement functioning?
- What is the frequency of maintenance needed to maintain design life of the pavement?
Water Quality – How much pollutant reduction can be expected? Of particular interest, low reductions have been observed for nutrients (phosphorus and nitrogen). This concern has two implications:
- What is the potential for groundwater contamination by infiltrating water treated by permeable pavements?
- What is the quality of water discharged from a permeable pavement drain tile?
Temperature – What temperature reductions can be expected with permeable pavement?
Residence Time – How long does the runoff need to stay in the storage layer to adequately treat the runoff?
Model Accuracy – How well can existing urban runoff models predict the water quality benefits of permeable pavement?
Study Design
To address these concerns and determine the benefits of permeable pavement, a study has been designed with the following objectives:
- Assess how quickly permeable pavement clogs with fine particulates, affecting infiltration rates over time.
- Calculate the residence time of runoff collected in the storage layer during different precipitation events.
- Investigate the capacity of permeable pavement to cool heated runoff during summer months.
- Evaluate the potential for permeable pavement surfaces to reduce the need for road salt.
- Improve permeable pavement routines in the Source Load and Management Model (WinSLAMM*) using data from the study.
- Provide results to support amendments to WDNR Conservation Practice Standard 1008 for Permeable Pavement.
Study Site
A small portion of green space, adjacent to the overflow parking lot serving the Madison Streets Division’s East Office, in Madison, Wis., has been designated as the study location (fig. 3 and 4). The white study area shown in figure 4 is split equally into three smaller study plots, each receiving similar volumes of runoff from the adjacent parking lot. These plots will test three types of pavement: permeable pavers, permeable concrete, and permeable asphalt (fig. 5). Each plot is equipped with instrumentation to measure reductions in runoff volume (water quantity) and pollutants (water quality).
Measuring Water Quantity
Runoff from the parking lot flows toward an existing curb cut, which is equipped with a calibrated flume. Runoff enters the flume and drains into a concrete structure that divides the runoff into three equal portions, each draining to one of the three test plots (fig. 5). The runoff either infiltrates into the permeable subsurface or exits the plot as overflow runoff. Each test plot is lined with an impermeable membrane, which captures and routes infiltrated runoff through a buried drain tile (fig. 6). Runoff that does not infiltrate into the permeable surface is captured by an overflow surface grate. The test plots are constructed to prevent cross-contamination from adjacent test plots and surrounding soils.
Both the drain tile and surface grate are routed into a monitoring facility, where the volume of infiltrated and overflow runoff is captured separately (fig. 7). The monitoring facility accurately measures all inputs and outputs of water using calibrated flumes.
Measuring Water Quality
Water-quality samples will be collected from seven locations:
- The original runoff draining from the parking lot (1)
- The runoff that infiltrates through each test plot (3)
- The runoff that by-passes each test plot as overflow runoff (3)
Water Quality samples will be tested at the Wisconsin State Lab of Hygiene, a certified USGS analytical laboratory. Samples will be tested for concentrations of the following pollutants:
Nutrients
- Total Phosphorus
- Dissolved Phosphorus
Solids
- Total Suspended Solids
- Suspended Sediment
- Particle Size Distribution
Other
- Temperature
- Chloride
Ancillary data will also be collected, including, but not limited to: precipitation, sand/salt application during winter months, runoff temperature at depth, and a record of maintenance.
This permeable pavement test site will be operated and maintained through 2018.
References
Davis, A.P., 2005, Green engineering principles promote low-impact development: Environmental Science and Technology, A-pages, v. 39, no. 16, p. 338A–344A.
Houle, K., Roseen, R., Ballestero, T., Briggs, J., and Houle, J., 2009, Examinations of Pervious Concrete and Porous Asphalt Pavements Performance for Stormwater Management in Northern Climates: World Environmental and Water Resources Congress 2009: p. 1–18.
Roseen, R., Ballestero, T., Houle, J., Briggs, J., and Houle, K., 2012, Water Quality and Hydrologic Performance of a Porous Asphalt Pavement as a Storm-Water Treatment Strategy in a Cold Climate: Journal of Environmental Engineering, vol. 138, no. 1, p. 81–89.
Wang, L., Lyons, J., Kanehl, P., and Bannerman, R., 2001, Impacts of urbanization on stream habitat and fish across multiple spatial scales: Environmental Management, v. 28, no. 2, p. 255–266.
Williamson, R. B., 1993, Urban runoff data book: a manual for the preliminary evaluation of urban stormwater impacts on water quality. Water Quality Centre, Ecosystems Division, National Institute of Water and Atmospheric Research
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