Benefits of Vegetated Permeable Pavement

Photos courtesy of Soil Retention Products, Inc.

When permeable pavement is vegetated with turfgrass or groundcover, the overall effect can be stunning, and serves to integrate a project into its environment. Vegetation over pavement has the ability to absorb carbon dioxide, emit oxygen, and biodegrade pollutants. As a living plant material, its evapotranspiration naturally makes it cooler than inert surfaces such as concrete, reducing albedo and the Urban Heat Island (UHI) effect. The turfgrass surface reduces glare and absorbs noise, while adding to green open space on a developed site.⁴ In addition to this comfort factor, there is a distinct design advantage to vegetated permeable pavement systems since hardscape can be disguised and better integrated into the project's environment and ecology. Using vegetation or “soft” materials such as sand, gravel, or decomposed granite, for instance, the otherwise overwhelming effect of parking lot concrete or asphalt can be mitigated. Another advantage is that valuable space can now be considered multifunctional, creating a better aesthetic appeal and often a better neighbor without sacrificing buildable land (see photo 1 above).

Suitable for a variety of scales, vegetated permeable pavement is typically not used for major streets, except perhaps for parallel parking spaces. These pavements' ability to add vegetation into the voids, and even to cover the paved area, creates site area that becomes more a part of the landscape. Many applications are perfect for site areas infrequently used, such as fire lanes, utility easements, and drainage ways. Areas like these which use large amounts of space, but are seldom used, leave an under-utilized vacant area in a project. Vegetated permeable pavement is especially helpful to the designer and developer when site design or city code dictates accommodating these facilities. Critical when site area is limited, vegetated permeable pavement can add green space, giving additional landscape and usable area back to the project (see photo 2).

In one of the most recent examples of the effectiveness of vegetated permeable pavement, a 2008 study responded to the need to clean up beachfront runoff. An Oceanside, California, fire station tested the viability of using vegetated permeable pavement for washing fire trucks. The fire station is near one of the most polluted beach outlets in southern California. The test was prompted by a mandate of the San Diego Regional Water Quality Control Board to clean runoff from the washing of fire trucks several times a day. The trucks were washed on the asphalt driveway in front of the station, which drained directly into the San Luis Rey River just upstream. The installation of a vegetated flexible concrete mat was used to resolve both polluted runoff and sustain daily truck loads. Placed over a bed of granular infill and base material, the site experiences no runoff, storing up to 0.40 inch of water at the surface and infiltrating at a rate of more than 3.0 inches per hour (see photo 3).

Design Considerations for Vegetated Permeable Pavement

Design of a vegetated permeable pavement system for any site is a multidisciplinary effort. Once a project is envisioned, important site planning factors must be considered for building layout, access, circulation, and parking, not to mention federal, state, and local code requirement compliance. Vegetated permeable pavements can satisfy several objectives for stormwater management, while adding value and aesthetics to the project.

Structural and Stormwater Design
In one of the most concise summaries to date, a 2008 Australian conference paper, by engineering professors at the Universities of New South Wales and South Australia, lays out several distinct objectives to ask early on: “Flood mitigation/stormwater retention or detention? Water quality improvement, whether filtration or retention? Water conservation for collection and reuse? And ability to carry the intended site traffic.” In the chart below, a design decision flowchart clearly illustrates the process for designing a vegetated permeable pavement. A key design consideration is the composition of the subgrade (native soils below the paving section) and their infiltration rates. Depending on the composition of the subbase (structural base material), in some cases enough rainfall can be collected to offset and store a percentage of the increased runoff from site development. For some projects, this may eliminate an expensive and separate “hard” drainage system. For other projects, with native soils with low infiltration, excess water could be detained and stored. Use of this excess stored water may have to be considered. For example, this excess water could be harvested for reuse, or alternatively, piped away with an elevated underdrain. The paper points out another key design question, in addition to pavement system design life, rainfall absorption, infiltration, and retention: how thick the pavement should be to carry the intended traffic. Permeable pavement thickness may be slightly thicker for managing stormwater than for load bearing, but there is usually an associated economic benefit for its use.