Porous asphalt pavements with stone recharge beds are rapidly gaining popularity as major components of a storm-water management system. During a storm, water drains through an open-graded asphalt mix into a stone infiltration bed, which provides stormwater storage volume similar to a detention basin. But unlike in a detention basin, water also infiltrates through the soil, removing many of the pollutants commonly found in pavement runoff and providing groundwater recharge. These pavement/stormwater systems are designed to reduce peak and total volume of runoff and also are effective in removing total suspended solids.
These systems are appealing for a number of reasons. First, they are using valuable land for two purposes—pavement and stormwater management—while they reduce or eliminate the need to use more land for stormwater facilities.
A typical porous asphalt pavement consists of a 2½ inch open-graded asphalt surface, a top filter course of small aggregate, a deep stone reservoir course, and a bottom of filter fabric placed on uncompacted natural soils. The depth of the stone recharge bed is determined by the soil infiltration rate and by the total impervious area drained to the bed, which may include rooftops and adjacent impervious roadways.DESIGNING A SYSTEM
Porous pavement design starts early in the project process. One of the keys to porous pavement design is gauging the soil's infiltration rates. Recommended minimum infiltration rates are 1/10 to ½ inch per hour; slower infiltration rates require a thicker stone recharge bed. The bottom of the bed also should be located at least 3 feet above the seasonal high water table and 2 feet above groundwater. In general, infiltration systems perform best on well-drained upland soils where buildings and parking lots often are located.
The evaluation begins with a study of the site's underlying geology, soils, hydrologic soil groups, topography, and drainage patterns; location of streams, wetlands, and wells; and land use history. This will help determine if the site is suitable for infiltrations and the best locations for infiltration.
Next, a soils investigation is conducted that normally consists of test pits, 6 to 8 feet deep, in which soil conditions are noted. Simple percolation tests can be used for the initial evaluation, which are supplemented with infiltrometer readings for the final design. A general recommendation is to conduct four to six tests per acre. This will vary depending on soil variability, topography, geology, and land use.
An engineer proficient in hydrology and stormwater design should design the stormwater component of the system. Basically, the bed acts like an underground detention basin during extreme storm events, with the added benefit of allowing infiltration. Routing the storm through the infiltration bed can be done using the same calculation methods used for detention basins to confirm peak rate mitigation.
A “belt and suspenders” approach is recommended to ensure that stormwater will be able to reach the infiltration bed even if the surface becomes clogged or sealed. An unpaved stone edge or catch basin that discharges into the bed may be used. The infiltration bed typically is not designed to detain and infiltrate all storm events. Therefore, it will be necessary to design and build outlet structures to prevent this excess water from saturating the pavement surface.
MAKING THE BED
The stone recharge bed is the heart of porous pavements. It provides temporary storage of stormwater falling directly on the pavement and from other impermeable surfaces if desired. It uses uniformly graded 1½ to 2½ inch clean-washed crushed stone, such as an American Association of State Highway and Transportation Officials No. 3 stone.
It is important to determine what aggregates are available in your area. Both larger and smaller stones can be used as long as they are crushed, uniformly graded, and clean. The void space between the stones provides the critical storage volume for the stormwater and, therefore, the void content of the aggregate should be confirmed. Stones that are dusty or dirty may clog the infiltration bed and must be avoided. The depth of the stone reservoir should be such that it drains completely within 72 hours. This allows the underlying soils to dry out between storms (improving pollutant removal) and also preserves capacity for the next storm.