Home > Issue by Date > June 2008

Anne Vazquez

Porous Paving Provides "An Out" For Stormwater

Stormwater management is an essential part of resource conservation.

By Anne Vazquez

Most facilities have paved areas on their property, whether in the form of a parking lot, sidewalk, plaza, or all of these. And, all facilities must deal with stormwater management. The rainwater not absorbed by soil on a site must travel somewhere, and, in most cases, this overflow goes into sewer systems or directly into natural water sources, such as a nearby stream or lake.

Stormwater management demands that the facility manager (fm) pay attention to where the rainwater is going. An immediate concern is flooding, so fms work to ensure that the infrastructure is in place on their properties to handle this. But besides this issue, stormwater management brings environmental and financial issues to the forefront. From an environmental standpoint, there is concern about chemicals and debris being "picked up" by stormwater and ending up in natural water sources. Financially, as public infrastructure in many locations is coming due for upgrades, municipalities and other water management entities have begun to increase the fees associated with handling stormwater runoff from properties.

If an fm is facing challenges with the environmental and/or financial issues related to stormwater, porous paving may offer a solution. Also referred to as "permeable" or "pervious," this type of paving enables rain to flow through openings in its construction. The soil below then has the ability to absorb the water, returning it to the ground. Depending on the rainfall that occurs at a location, this mechanism can eliminate stormwater runoff. However, in most cases, the soil does not absorb all of the rain, and the facility will still need to maintain a stormwater management system.

While many paving surfaces could be considered porous, Invisible Structures, a porous paving provider located in Golden, CO, defines this type of paving as "that specifically designed and constructed to encourage rapid infiltration and percolation of rainfall and stormwater through the entire pavement cross section."

The U.S. Green Building Council (USGBC) includes stormwater management credits in its LEED rating systems, and "permeable alternatives" to leaving a site untouched are suggested as ways to reduce the impervious spaces on a site.

Explaining the motivation for offering credits for stormwater management (of all kinds), Corey Enck, manager, LEED certification at the USGBC, says, "The main reason is to limit the disruption of the natural stormwater flows. Typically, where there are pervious areas, stormwater is absorbed into the earth and used by the vegetation. It also recharges the underground aquifers. With impervious paving, that doesn't happen, and it creates runoff, which can lead to negative effects of excessive stormwater runoff. That is what the LEED rating system is trying to address."

Not all areas of a facility site may be appropriate for porous pavement. Each project must be considered in terms of its specific requirements. But, in many scenarios, such an installation can benefit a facility. There are appropriate applications for each, and the designer must determine which is best suited to each project, or portion of a project.

In his LEED certification activities, Enck says he sees many more new construction projects specifying porous paving options compared to existing buildings. "Typically with an EB [existing buildings] project, there would be more cost to tearing up their hardscape and replacing it," he says. "We see it more often in a new construction project where instead of paving a parking lot, they would go with pervious paving."

In terms of where on a project site porous paving is more likely to be used, Enck continues, "We see it everywhere on a site. However, it is more common for use on smaller areas, such as pedestrian ways. Or instead of doing their entire parking lot with impervious pavement, some projects will do a portion with pervious and a portion that's not."

Porous pavements are generally appropriate for applications where traffic moves slowly (less than 30 mph) and/or infrequently, such as with fire lanes, service drives, overflow or all day parking, and pedestrian areas. Enck notes the majority of projects he inspects for LEED certification (that are pursuing credits for stormwater management) do not use porous paving in that pursuit. However, he estimates that about 20% do, and those "that are going the pervious paving route are typically installing open cell paving systems-a concrete grid with vegetation in between."

Porous paving is competitive with asphalt paving and has a lower life cycle cost due to reduced maintenance costs and surface replacement needs. According to literature from Invisible Structures, asphalt paving requires resealing every two to four years, while resurfacing requires this maintenance every eight to 15 years, depending upon traffic weight, traffic volume, and climate.

Maintenance costs for porous pavements over a 12 to 15 year period (asphalt life cycle) will generally cost between 60% to 90% of asphalt maintenance/repair costs, states literature from Invisible Structures. And, the company notes that a properly installed pavement can be expected to perform over a life cycle of 50 to 100 years. The upheaval to an existing facility, along with the initial costs, can deter fms from pursuing this course for their sites already in operation. However, with environmental concerns growing and stormwater related fees rising in many areas, this type of installation may become an increasingly attractive option.

Research for this article included information from Enck, along with literature from Invisible Structures, Inc.

Have you installed porous paving at your facility? Share your experiencing by sending an e-mail to avazquez@groupc.com.

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