- Soil infiltration rates of 0.1 to 10 inches/hour work best.
- Minimum depth to bedrock or seasonal high water should be greater than two feet.
- The bottom of the infiltration bed should be flat to maximize the infiltration area.
- Limit the maximum slope of porous pavement surface to 5 percent. For parking areas on steeper slopes, terrace the parking areas with berms between parking areas.
- Look for opportunities to route runoff from nearby impervious areas to the infiltration bed to minimize stormwater structures. Pretreatment may be required.
- Spread out the infiltration. The maximum ratio of impervious to pervious area should be 5:1. For carbonate soils where there is a risk of sinkholes, the maximum ratio should be 3:1. Do not place porous pavements over known sinkhole areas.
- The design should provide for an alternate path for stormwater to enter the stone recharge bed in the event that the pavement surface becomes plugged or experiences extreme storm events.
- An overflow system should be included to prevent water in the stone bed from rising into the pavement surface during extreme storm events.
- The stone recharge bed should be able to drain within 12 and 72 hours.
How thick does it need to be?
The thickness of the stone recharge bed will be determined by the amount of water that needs to be stored, the infiltration rate of the soil, and traffic loading. In most cases, the water quantity and soil infiltration rate will control the thickness of the stone recharge bed and traffic loads will control the thickness of the porous asphalt surface.
Determine location early in the design process
Infiltration systems perform best on upland soils. When a site is being newly developed, the location of the porous pavement should be considered early in the design process. In conventional construction plans, parking lots are often placed at the lowest portion of a site, where high groundwater and poor soil infiltration rates may exist. Soil Series and Hydrologic Soil Group Maps from the Natural Resources Conservation Service at http://websoilsurvey.nrcs.usda.gov/ are helpful in the initial design phase. Custom soil reports can also be generated from the same source.
Infiltrating from other impervious areas on the site
Early in the design process, look for opportunities to use the stone recharge bed to infiltrate stormwater from nearby impervious areas on the site. The stone recharge bed is typically between 12 and 36 inches in depth. With 40 percent voids in stone this would mean that the recharge bed is capable of storing between 4.8 and 14.4 inches of precipitation. This will typically exceed most design storm volumes. Therefore, there may be opportunities to store and infiltrate stormwater from impervious areas at the site to avoid piping water long distances.
Keep the bottom flat
The bottom of the infiltration bed should be flat to maximize the infiltration area and reduce the amount of stone required, as illustrated.
Designing for slopes
Porous pavements work best on flat or gently sloping areas. The surface of the porous pavement should not exceed 5 percent. For parking on sloping areas, consider terracing the parking areas with berms separating the parking bay as shown. These parking areas can be connected with conventional dense-graded asphalt pavement.
Protection from sediment
It is very important to protect porous pavements from sediment during and after the construction process. Therefore, avoid constructing a porous pavement in a location where fine material and silt can wash onto it from other areas of the site.
Many building codes and fire codes require hard surfaces to be in place before structures are built, but, because construction sites are inherently dirty, there are advantages to constructing porous pavements late in the construction schedule. One solution is to construct access roads and driveways of dense-graded asphalt and build the porous pavements later.
It is also possible to construct a porous pavement, either partially or entirely, early in the development process and cover it with a geotextile to protect it from clogging while construction is being completed. The geotextile can then be removed at an appropriate time. This technique was used at the Pringle Creek community in Oregon. See Traffic Will Travel on Green Streets in Oregon.
More information on site design is available in NAPA’s Porous Asphalt Pavements for Stormwater Management (IS-131) .
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