Porous pavement: A not-so-new low-impact design technique

That childhood saying “when it rains, it pours” may be getting an update to “when it rains, it’s porous.” Using low-impact design techniques can reduce stormwater runoff and protect local water resources.

In many urban and suburban communities, buildings, roads and sidewalks are replacing open land where stormwater used to soak into the ground. To replace this natural infiltration, communities have built storm sewers. The sewers collect the runoff; channel it into underground pipes where it gathers speed as it moves to the local stream, river or lake. This fast-moving water empties out of the pipe eroding streambanks and damaging aquatic habitat. In addition, this stormwater may carry sediment, chemical pollutants and higher water temperatures.

Many communities are looking at porous pavement as a compromise between nature and development. Porous pavement is a special type of pavement that allows rain and snowmelt to pass through it, thereby reducing the runoff from a site into surrounding areas. In addition, well-maintained porous pavement filters pollutants as it soaks in.

This technique has been around since the mid-1970s, but it is now being looked at as a way to assist with new stormwater regulations. The porous pavement provides a place for the stormwater to go rather than allowing it to run off. Porous pavement is made up of two layers. The surface layer looks like traditional asphalt but actually is made without the “fine” materials that close all the pores in the asphalt. This change provides pore space in the pavement which allows water infiltration. Below the pavement, there is a stone bed layer. This layer is typically 18 to 36 inches deep depending on the use for the pavement. The stone bed layer must be deep enough to ensure that the water level never rises up back into the asphalt. As the water drains through the pores in the pavement, it soaks through the stone layer into the soil beneath cleaning it. This infiltration recharges local groundwater resources.

According to Michigan State University Extension, some benefits of using porous pavement in developed areas are reduced run off into local waterways, increased base flows of waterways, less pollution of local water resources, increased groundwater recharge, reduced flooding and less land needed for detention ponds.

The main challenge in using porous pavement is location. This pavement is best-suited to low traffic areas. However, many areas have used this pavement successfully, including at the Ford Rouge Plant in Dearborn, Mich. Porous pavement should not be used in areas that may generate contaminated runoff, nor should they be used near drinking water well supply areas. Finally, road salt and sand should not be used on these surfaces. The salt will soak into the soil below and the sand will clog the pore space, reducing the porosity.

The initial cost of porous pavement is usually higher than traditional asphalt but when the cost of land for detention ponds needed in traditional pavement is factored in, porous pavement can be significantly less. Longevity of porous pavement when properly constructed and maintained can be twenty years or more.

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