This photo illustrates an example of pervious concrete.
Permeable pavements allow stormwater runoff to filter through surface voids into an underlying stone reservoir where it is temporarily stored and/or infiltrated. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt and permeable interlocking pavers. Permeable pavements have been used for commercial and residential sites to replace traditionally impervious surfaces. These include roads, parking lots, driveways, sidewalks, plazas and patios. While the designs vary, all permeable pavements have a similar structure, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains and geotextile over uncompacted soil subgrade.
From a hydrologic perspective, permeable pavement is typically designed to manage rainfall landing directly on the permeable pavement surface area. While designers and regulators often discourage it, permeable pavement surface areas may accept runoff contributed by adjacent impervious areas such as driving lanes or rooftops, or from adjacent vegetated areas. Sediment in runoff from adjacent areas increases clogging of the permeable pavement, especially at the edges. This should be considered when determining the size of the contributing drainage area. Additionally, the capacity of the underlying reservoir layer will limit the size of the contributing area.
- Pretreatment Considerations – Pretreatment to remove sediment from runoff draining onto permeable pavement from adjacent impervious areas is desirable since sediment tends to clog permeable pavements. This is usually accomplished through the use of a vegetative filter upstream of the pavement. Permeable pavement itself can be considered a pre-treatment device and included in a stormwater treatment train if underdrains are utilized within the storage reservoir. The underdrains will typically be routed to bioretention or a raingarden.
- Permit Applicability – Permeable pavements can be utilized to assist in meeting stormwater permit requirements for volume, total suspended solids, and total phosphorus. The section on credits included in this document provides guidance on the implementation of permeable pavements that may be utilized to meet various credit goals.
- Retrofit Suitability – In most cases, existing impervious surfaces can easily be replaced with permeable pavements to achieve improved runoff conditions. Retrofit requires the removal of the old pavement and subgrade and the installation of the underlying reservoir layer and the permeable pavement. When possible, compacted subgrade soils should be removed or loosened to achieve the maximum infiltration rate possible.
- Cold Climate Suitability - The effective use of permeable pavement has been documented in a variety of climates. However,special consideration is necessary for cold climates, arid regions, or areas with high wind erosion (California 2003). Dramatic reductions in life span of the infiltration properties of the pavement may occur in these areas due to particulate clogging. In cold climates like Minnesota, the most notably special consideration is regarding the application of sand in the winter for added traction. This is not recommended. Fortunately,permeable pavements require significantly less use of de-icing sand and chemicals to maintain a safe walking or driving surface.
- Special Receiving Waters Suitability - Many of the same design considerations and limitations apply to permeable pavement as to other infiltration practices. Infiltration of runoff from hotspots (e.g.,gas stations, chemical storage areas, etc.) should be carefully considered and in many cases avoided. Special consideration also needs to be taken near wellhead areas and basement foundations. Some designs may require consideration of storms in excess of the infiltration capabilities of the pavement. For these situations the design should ensure the excess runoff does not negatively impact special surface waters (e.g.,trout streams) through the implementation of additional BMPs.
- Water Quality - In general, permeable pavement does provide removal of TSS and other pollutants through processes similar to other infiltration BMPs. However, permeable pavements are not suggested for areas that may receive high loading rates of TSS due to their propensity to clog. The expected volume and pollutant reduction for designs without an underdrain equals approximately 100% of the underlying reservoir storage volume. For designs with underdrains, the reductions are less. Of the water intercepted and draining through the underdrain, 45%of the total phosphorus and 74%of total suspended solids can be expected to be removed. When replacing impervious areas with permeable pavement, the MPCA allows for the reduction in water quality volume sizing for up to a maximum of ½ acre of new impervious surface. The Minnesota Stormwater Manual notes that the MPCA will not allow pervious pavements as a replacement for existing water quality treatment BMPs. More discussion on this item is available in the section on credits.
- Water Quantity – The primary advantage of permeable pavements is their ability to provide volume reduction by reducing runoff from a site and/or providing attenuation during runoff events. The volume of water that will be reduced during a given rainfall event will be equivalent to the volume available for storage below the pavement or underdrain (if an underdrain is present). More discussion on this item is available in the section on credits.