Porous asphalt reduces storm water runoff

Storm water managers worry about water runoff from parking lots and commercial lots carrying residue into streams and aquifers. And they worry about water runoff raising the level of local streams and tributaries, and contributing to potential flooding.

Storm water management and sustaining the quality of the environment are strong incentives to use porous asphalt pavement. A key factor that has been ignored in the past is that land development alters the hydrologic cycle. Land development reduces water infiltration in natural vegetative areas and increases direct stormwater runoff. It also increases the amount of pollutants in the water.

Impervious surfaces like parking lots and paved roads are often the causes of stormwater runoff. Detention basins are often used to collect and slow the rate of runoff from these impervious surfaces. Although detention basins are effective, they require additional land.

20-Year Pavement
A properly designed porous asphalt pavement can significantly reduce storm water runoff by absorbing rainwater into the pavement structure. High voids in the porous asphalt pavement cause the water to filter through the pavement structure into the stone recharge bed underneath the pavement, then into the water table.

Properly designed and installed porous asphalt parking lots are cost effective and attractive, with a life span of 20 years or more. At the same time, porous asphalt parking areas can provide effective stormwater management that promotes infiltration, improves water quality and eliminates the need for detention basins.

What Is Porous Asphalt?
Porous asphalt pavement is comprised of a permeable asphalt surface placed over a granular working platform on top of a reservoir of large stone. The asphalt surface is made permeable by building it with an open-graded friction course. The layer underneath the porous asphalt has the storage capacity to hold the collected water.

In the late 1960s, the concept of porous asphalt was proposed to “promote percolation, reduce storm sewer loads, raise water tables and replenish aquifers.” Throughout the 1970s, the concept was refined to a point where the Environmental Protection Agency sponsored projects to “determinethe capabilities of several types of porous pavements for urban runoff control — in terms of cost and efficiency.”

Where Does It Work?
To find out if porous asphalt will work on a proposed project, designers must consider some key factors. Soil characteristics, local topography and climate are physical factors that must be used in designing porous asphalt pavement.

The proposed uses of the site and the traffic loading factor are also essential in determining the applicability of porous pavement. Government regulations are also a factor. Storm water regulations, site runoff and storm water quality must be considered.

Some designers recommend that porous asphalt pavement be used on sites with gentle slopes, permeable soils and relatively deep water table and bedrock levels. They recommend that soils be well drained or moderately well drained. Cahill Associates, who have designed many infiltration systems in the Mid-Atlantic States, say that infiltration systems that allow drainage over a two- or three-day period are good candidates.

Environmental Factors
Climate and environmental factors influence the design of both the reservoir course and the porous asphalt pavement course. Frost penetration depth is also a factor in determining reservoir course thickness.

Dust is a critical environmental factor to consider. Continuously blowing dust tends to clog the pores of the porous asphalt surface and severely restrict percolation through the top layer of the system. For this reason, porous asphalt pavement systems are not recommended for barren areas with moderate to high winds.

The typical use of porous asphalt pavements is parking lots and recreational areas like basketball or tennis courts or playgrounds.

Design Detail
A typical porous asphalt pavement consists of a porous asphalt course, a top filter course, a reservoir course, an optional bottom filter course, filter fabric and existing soil or subgrade material. The porous asphalt course consists of a 2- to 4-inch thick open-graded asphalt layer. The pavement should be a mix containing little sand or dust, with approximately 16 percent voids.

Designers recommend a top filter course, 2 inches thick, using 1/2-inch crushed stone aggregate. The filter course provides filtration and also protects the reservoir course during placement of the asphalt mix.

One of the main purposes of the filter course is to provide a firm paving platform. Without this layer, the single-size, open-graded aggregate may be unstable under the paver.

The depth of the reservoir is determined by the storage volume, structural capacity, or frost depth, whichever requires the greater thickness. The minimum depth of stone within the reservoir is often 8 or 9 inches. Aggregates between 1.5 and 3.0 inches in size are recommended. This size stone typically yields approximately 40 percent voids, which provides the runoff storage.

Cahill Associates says that porous pavement does not usually cost more than conventional pavement. On a yard-by-yard basis, the cost of a porous pavement installation is about the same as the cost of conventional asphalt.

The underlying stone bed is usually more expensive than a conventionally compacted sub-base, but that cost is usually offset by the reduction in storm water pipes and inlets. Also, because porous pavement is designed to fit into the topography of the site, there are fewer earthworks.

Experience shows that sand, ash or even salt should not be used on snow or ice because they may clog the pores within the pavement. De-icing compounds work well. A liquid de-icer drains out with the snow and ice as they melt, leaving the porosity of the pavement intact.

Porous pavement should be inspected regularly for surface clogging, especially after large storms. The pavement surface can be flushed or jet-washed to help maintain surface porosity. Damage to the porous pavement can be repaired by using non-porous patching mixes.

Information for this article came from the Porous Asphalt Pavement seminar sponsored by Flexible Pavements of Ohio, the Plantmix Asphalt Industry of Kentucky and the National Asphalt Pavement Association.