Design criteria for permeable pavement

Hydrologic design considerations

Permeable pavement is subject to the following design considerations, including benefits and constraints.

• Available space – A significant advantage of permeable pavement is its ability to combine detention/ infiltration and pavement, thereby reducing or eliminating land required for detention facilities. This is especially important in urban areas with high land prices and highly developed sites with little or no space for stormwater detention.

• Soils – Soil conditions and infiltration rates determine the use of an underdrain. (NRCS Hydrologic Soil Group (HSG) C or D soils usually require an underdrain, whereas HSG A and B soils often do not.) Designers should evaluate existing soil properties during initial site layout with the goal of configuring permeable pavement that conserves and protects soils with the highest infiltration rates. In particular, areas of HSG A or B soils shown on soil surveys should be considered as primary locations for all types of infiltration practices.

• Geotextiles - In the absence of full-depth concrete curbs or impermeable liners, geotextiles are recommended on the (vertical) sides of permeable pavements to separate the reservoir layers from the adjacent soil subgrade. Horizontally placed geotextiles between the aggregate base and soil subgrade are at the option of the designer. Geotextile use should be carefully evaluated and selection should be guided by AASHTO M-288 Geotextile for Highway Applications (AASHTO 2010). Specific selection guidance is provided under the Subsurface Drainage section. Class II geotextiles are generally used.

• Contributing drainage area – Permeable pavements sometimes capture runoff from adjacent areas, pavements, and roofs. Runoff from permeable areas is not recommended due to potential clogging of the permeable pavement. The at-grade contributing drainage area into permeable pavement should generally not exceed twice the surface area of the permeable pavement. This guideline helps reduce the rate of surface sedimentation. The 2:1 ratio can be increased to no greater than 5:1 if at least one of these conditions exists:
  • permeable pavement is receiving runoff from roofs as it tends to be very low in sediment; or
  • runoff from adjacent impervious surfaces remains unburdened with sediment due to effective pre-treatment prior to entering the permeable pavement.

• Soil subgrade slope – The slope of the soil subgrade should be as flat as possible (i.e., less than 1 percent longitudinal slope) to enable even distribution and infiltration of stormwater. Lateral slopes should be less than 1 percent. Steep slopes can reduce the stormwater storage capacity of permeable pavement. Designers should consider using a terraced subgrade design for permeable pavement in sloped areas, especially when the subgrade slope exceeds 3 percent.

• Soil subgrade compaction – This should be avoided wherever possible to maximize infiltration. In some situations, compaction may be needed for supporting vehicular loads. In such cases, compaction density and subsequent soil infiltration should be assessed in a test pit(s) on the site to determine an acceptable soil density and its contribution to soil strength and infiltration. The measured infiltration rate for use in hydrologic calculations may be reduced by the designer to compensate for long-term sedimentation on the soil subgrade.

• Excavation methods - Excavation should be conducted in a manner that minimizes soil subgrade compaction. Tracked rather than wheeled equipment is recommended working from the sides of the excavation. For larger projects, excavation can create cells and berms where equipment removes soil from one area or cell while positioned on higher soil around each cell (see Construction specifications for permeable pavement). Other techniques include ripping or loosening soils compacted by construction equipment. This can be done with the teeth on excavation equipment buckets. Compaction of the aggregate base into these areas is especially important since scarified soil can settle and be reflected on the surface.

• Surface slope – Surface slopes for all permeable pavement types should be at least 1 percent to provide an alternate means for drainage should the surface become completely clogged due to lack of maintenance. Designs should provide an alternate means for stormwater to enter the aggregate reservoir if the pavement surface should ever become clogged, or for extreme storm events. For pervious concrete and porous asphalt without curbs, this can be a 2 foot wide stone edge connected to the reservoir. For curbed pavements, inlets may be used.

• Overflow structures – Permeable pavements are not designed to store and infiltrate all stormwater from all storms. Therefore, an outlet or outlets are required to prevent water from rising into and over the surface. One type of outlet control would be a catch basin with an internal weir and low-flow orifice. The catch basin can also handle runoff from the surface should it become clogged.

• Minimum depth to seasonal high water table – A high groundwater table may cause seepage into the bottom of permeable pavement and prevent complete drainage. Also, soil acts as a filter for pollutants between the bottom of the pavement base and the water table. Therefore, a minimum vertical separation of 3 feet is required between the bottom of the permeable pavement reservoir layer and the seasonal high groundwater table. For systems with impermeable liners, a minimum of one foot clearance is Highly recommended between the liner and the seasonal high water table.

• Setbacks – To avoid harmful seepage, permeable pavement should not be hydraulically connected to building foundations unless an impermeable liner is placed against the foundation or basement wall. Even under these circumstances, great care should be taken to avoid creating a wet basement problem. If there is no liner, the permeable pavement base should be 10 feet or greater from structures (EPA recommends a minimum setback from building foundations of 10 feet down-gradient and 100 feet up-gradient. See EPA factsheet “Storm Water Technology Fact Sheet: Porous Pavement,” EPA 832-F-99-023). Again, it is the designer’s responsibility to avoid creating a wet basement problem. Likewise, permeable pavement bases should be hydraulically separated from adjacent road bases

Limitations – There are several limitations for use of permeable pavement, as summarized below.

• Permeable pavements should not be used in high pollutant loading sites. High pollutant loading sites are those that receive constant sediment or trash and/or debris. Places where fuels and chemicals are stored or handled can be potential stormwater hotspots and permeable pavement should not be constructed in these places. Likewise, areas subject to wind borne dust and sediment should not use permeable pavement unless the pavement can be vacuumed regularly. The following limitations should be considered before utilizing permeable pavements in any design.
• Permeable pavement is suitable for pedestrian-only areas, low-volume roads, low speed areas, overflow parking areas, residential driveways, alleys, and parking stalls. These can be residential collector roads or other applications with similar traffic loads.
• Permeable pavement can be prone to clogging from sand and fine sediments that fill void spaces and the joints between pavers. As a result, it should be used carefully where frequent winter sanding is necessary because the sand may clog the surface of the material. Periodic maintenance is critical, and surfaces should be cleaned with a vacuum sweeper at least two times a year.
• Fuel may leak from vehicles and toxic chemicals may leach from asphalt and/or binder surface. Porous pavement systems are not designed to treat these pollutants.

Conveyance and overflow

Permeable pavement designs should include methods to convey larger storms (e.g., 2-year, 10-year) to the storm drain system. The following is a list of methods that accomplish this.

• Place a perforated pipe horizontally near the top of the reservoir layer to pass excess flows after water has filled the base. The placement and/or design should be such that the incoming runoff is not captured (e.g., placing the perforations on the underside only). Pipe placement should be away from wheel loads to prevent damage.
• Increase the thickness of the top of the reservoir layer.
• Create underground detention within the reservoir layer of the permeable pavement system. Reservoir storage may be augmented by corrugated metal pipes, plastic or concrete arch structures, etc.
• Route excess flows to another detention or conveyance system that is designed for management of extreme event flows.
• Set the storm drain inlets level with the elevation of the permeable pavement surface to effectively convey excess stormwater runoff past the system. The design should also make allowances for relief of unacceptable ponding depths during larger rainfall events.

Maintenance

Proper maintenance of permeable pavement is crucial for ensuring its longevity and functionality. Some portions of the maintenance plan require planning during the design stages. These items are noted below.

• Observation Well – Typically this consists of a well-anchored, six-inch diameter perforated PVC pipe that extends vertically to the bottom of the reservoir layer. This is installed at the down slope end of the permeable pavement. The observation well should be fitted with a lockable cap installed flush with the ground surface (or under the pavers) to facilitate periodic inspection and maintenance. The observation well enables visual monitoring of drawdown within the reservoir layer after a storm.
• Overhead Landscaping – Some communities require a certain percentage of parking lots to be landscaped. Large-scale permeable pavement should be carefully planned to integrate landscaping in a manner that maximizes runoff treatment and minimizes risk of sediment, mulch, grass clippings, crushed leaves, nuts, and fruits inadvertently clogging the surface. Prior to construction, owners should commit to a vacuuming plan that includes vacuuming frequency and equipment needs. The vacuuming frequency typically depends on the time of year. In the spring, tree buds and seeds necessitate frequent vacuuming. In the fall, tree leaves and acorns necessitate frequent vacuuming. In the summer, vacuuming frequency depends on permeable pavement exposure to organic material from trees and nearby vegetated areas. Vacuum equipment and methods for sediment removal are provided in the section addressing operation and maintenance.