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New Hampshire Department of Environmental Services

Pervious Pavement Pilot Demonstration Site

Pervious PavementBackground
A pervious pavement demonstration pilot project was installed in the summer of 2009 in the parking lot near the entrance of the NH Department of Environmental Services’ building on 29 Hazen Drive, Concord. Three different pervious materials are demonstrated. Pervious pavers, pervious concrete, and pervious asphalt pavement replace conventional paving materials in several walkways and parking spaces.

A cooperative project of the DES and the NH Rivers Council, the purpose of the demonstration site is for people working in and visiting the DES building to learn about the importance of stormwater management and see a real life example of an innovative stormwater management strategy, like pervious pavement.

A typical 10 acre surface parking lot creates 270,000 gallons of stormwater after a 1-inch rain storm. The polluted runoff generally contains high levels of oil and grease, sediment, salt, heavy metals, and bacteria. Pervious pavement allows water from rain or snowmelt to flow through the pavement, into a stone base, and then infiltrate into the native soils below. This reduces the amount of stormwater that runs off the parking lot and protects nearby surface waters from stormwater pollution.

Examples of each material, pervious concrete, permeable pavers, and porous asphalt, were installed by the manufacturers in two parking spaces for a total of six retrofitted spaces. Two outer walkways were installed with pervious pavers and two center walkways were installed with pervious concrete. Excavation, sub-base and the top pervious pavement layer were all installed according to manufacturer’s specifications at the time of installation.

Lessons Learned
Advances and new information in pervious pavement technologies are constantly being developed. As a pilot project, this project has identified several lessons learned for recommendations on future installations. Although the walkways and parking spaces still show some infiltration, puddles have been observed on the edge portions of some of the walkways and very slow infiltration has been noted on the pervious asphalt parking spaces. Some explanations include:

  1. Design of Pilot Installation: Ideally, when retrofitting or installing a parking lot or walkway with pervious materials, the drainage area to the pervious material should be limited to the amount of runoff it can infiltrate. Due to budget constraints and the intent of the project to demonstrate on a small scale, the DES parking spaces and some of the walkways receive more runoff volume from the parking lot than recommended. In traditional installation, a whole parking lot or large portion would contain pervious material and therefore would not need to infiltrate additional run-off from other surfaces.

  2. Installers of pervious parking lots and walkways also recommend allowing several months to cure after installation before applying salt.” This allows the concrete to be more resilient against degradation due to salt application. With installation occurring in September and maintenance staff not aware of suggested reductions in salt, it was determined that some of the crumbling and reduction of performance may have occurred as a result. In addition, due to design of curving on the surface of the walkway, maintenance staff was unable to adequately clear the walkways of snow and compensated with additional salt applications in order to maintain safety.

  3. Winter Maintenance: In addition, installers of pervious parking lots and walkways recommend not to sand on a regular basis during winter maintenance. Although an effort has been made to reduce the amount of sand on the DES walkways while still maintaining safe egress, several of the walkways receive sandy runoff from the parking lot and driveway as the water drains towards the lowest point – the walkways. Therefore some of the capacity to infiltrate has been noted with slow disappearing puddles on the edges of the two lower walkways while the walkway high points in the centers remain porous. The parking spaces have been sanded and salted the same as the rest of the parking lot and have shown some decrease in infiltration.

  4. Overall Maintenance: Manufacturer’s maintenance recommendations include vacuuming and/or power washing the surfaces at least once a year. DES staff and representatives from the pervious concrete industry power washed the pervious areas in the summer of 2010, which resulted in improved performance. The maintenance recommendations have been passed on to Administrative Services for future maintenance.

Next Steps
To reduce the impacts of stormwater runoff, the UNH Stormwater Center has been demonstrating several best management practices (BMPs) including pervious pavements at their demonstration site at UNH in Durham, NH. Several successful pervious pavement installations have occurred throughout New Hampshire before and since the DES parking lot demonstration site. DES is working closely with UNH Stormwater Center to identify recommended changes in materials, specifications, appropriate situations for installation, and maintenance for the most successful pervious pavement installations.

Please be sure to visit the UNH Stormwater Center website for the latest information on pervious pavements and a listing of existing installations sites in New Hampshire. For more information on this project, contact Barbara McMillan, Watershed Outreach Coordinator,

This project was a collaborative effort between the New Hampshire Rivers Council, several DES programs including the Shoreland Protection Program, the Alteration of Terrain Program, and the Watershed Assistance Section and the University of New Hampshire Stormwater Center. Funding for this project was provided by the McIninch Foundation, a State of New Hampshire Conservation Committee Grant (Moose Plate Grant) and donations of labor and pervious paving materials from Nicolock, Pike Industries, and the Northern New England Concrete Promotion Association.

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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