- 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 meetingstormwater permit requirements for volume, total suspended solids, and totalphosphorus. The section on credits included in this document provides guidanceon the implementation of permeable pavements that may be utilized to meetvarious credit goals.
- Retrofit Suitability – In most cases, existing impervious surfaces can easily bereplaced with permeable pavements to achieve improved runoff conditions. Retrofit requires the removal of the oldpavement and subgrade and the installation of the underlying reservoir layerand the permeable pavement. Whenpossible, compacted subgrade soils should be removed or loosened to achieve themaximum infiltration rate possible.
- Cold Climate Suitability - The effective use of permeable pavement has beendocumented in a variety of climates. However,special consideration is necessary for cold climates, arid regions, or areaswith high wind erosion (California 2003). Dramatic reductions in life span ofthe infiltration properties of the pavement may occur in these areas due toparticulate clogging. In cold climateslike Minnesota, the most notably special consideration is regarding the applicationof sand in the winter for added traction. This is not recommended. Fortunately,permeable pavements require significantly less use of de-icing sand andchemicals to maintain a safe walking or driving surface.
- Special Receiving Waters Suitability - Many of the same design considerations andlimitations 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 andin many cases avoided. Specialconsideration also needs to be taken near wellhead areas and basement foundations. Some designs may require considerationof storms in excess of the infiltration capabilities of the pavement. For these situations the design should ensurethe 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 andother pollutants through processes similar to other infiltration BMPs. However, permeable pavements are not suggestedfor areas that may receive high loading rates of TSS due to their propensity toclog. The expected volume and pollutantreduction for designs without an underdrain equals approximately 100% of theunderlying reservoir storage volume. For designs with underdrains, the reductions are less. Of the water intercepted and draining throughthe underdrain, 45%of the total phosphorus and 74%of total suspended solids can be expected to be removed. When replacing impervious areaswith permeable pavement, the MPCA allows for the reduction in water qualityvolume sizing for up to a maximum of ½ acre of new impervious surface. TheMinnesota Stormwater Manual notes that the MPCA will not allow perviouspavements as a replacement for existing water quality treatment BMPs. Morediscussion 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. Thevolume 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.