Permeable Pavers: An Innovative Stormwater Management Multi-Tool

Permeable Pavers

Stormwater runoff is a result of rain and snowmelt occurring on impervious surfaces such as roads, rooftops, driveways, and parking lots. In urban environments stormwater runoff often picks up trash, fertilizers, metals, sediment, and other pollutants on the ground. After flowing over impervious surfaces, stormwater typically enters street inlets and travels through a network of underground pipes before flowing into adjacent rivers and lakes. This water is often untreated and filled with pollutants that harm surface waters (above).

In the last 15 years, local, state, and federal agencies have significantly increased attention, public education, and regulations applied towards stormwater because of its growing threat to surface waters as development footprints increase nationwide. These actions are spurring the innovation of new tools to reduce runoff and remove contaminants from stormwater. One example is the emerging use of permeable paver systems, which offer a variety of aesthetic, water quality, and flood reduction benefits.

Permeable paver systems are an engineered stormwater management system comprised of interlocking concrete blocks placed on top of an aggregate sub-base that acts as an underground water storage reservoir. Their primary benefit is that they collect, store, and facilitate infiltration of stormwater, which mimics the natural environment and is a stark

contrast to asphalt and concrete surfaces that shed water. Individuals can use permeable pavers in various residential and commercial applications, including driveways, patios, sidewalks, and parking areas.

Permeable paver systems have numerous environmental, aesthetic, economic, and public safety benefits when designed and constructed by professionals. Environmentally, they mimic natural runoff processes, reduce discharges of polluted stormwater to surface waters, eliminate the need for toxic sealants and deicing salts, and facilitate groundwater recharge. Aesthetically, the variety of paver colors and shapes provides diverse options for designing attractive alternatives to traditional pavement. Economically, they have a 30-year or more lifespan (think Roman roads), reduce the need for expensive underground infrastructure (pipes, inlets, and utility holes), and increase buildable area. Finally, from a public safety standpoint, they eliminate wintertime melt and re-freezing hazards, comply with the Americans with Disabilities Act, and mitigate property flooding.

Three primary elements make up a permeable paver system: the underlying soil, an aggregate layer, and concrete blocks. Individuals considering the use of permeable pavers must carefully consider these and other site conditions to ensure a successful project. First, a thorough understanding of the underlying soil structure and its infiltration capability is necessary. Soils with high infiltration rates, such as sand and gravels, are ideal because they can facilitate infiltration of high volumes of water. Slowly draining soils with low infiltration rates, such as silts and clays, are challenging and necessitate additional design considerations such as an underdrain system to prevent system overflows. Second, permeable paver systems require a customized aggregate profile designed by a professional engineer. Aggregate depths and types vary depending on the systems’ planned use (e.g., patio vs. driveway), drainage area, underlying

soils, and targeted storage volume. Finally, permeable paver systems require a concrete block. Concrete blocks are typically much stronger than standard patio pavers and require interlocking side-tabs that bind the finished surface together. The concrete block should have beveled edges to facilitate shoveling and snow plowing.

Installation processes for permeable pavers vary depending on project-specific considerations. Contractors who possess a concrete paver installer certification should be used to avoid construction-related issues that could drive up costs and impact system function. Construction means and methods are the responsibility of individual installers, but generally include the following sequence: (1) removal of topsoil to clear the project area, (2) removal of soil for the aggregate area (3) construction of a solidified perimeter to establish the project boundary, (4) placement of a porous geotextile fabric that eliminates underlying soil transfer into the aggregate layer, (5) installation and compaction of aggregate to create the storage reservoir, and (6) setting of permeable pavers to finish the surface. Once installed, annual maintenance is critical for preserving desired storage volumes and infiltration rates. Yearly maintenance activities include vacuuming the interlocking joints and fall cleanup to remove leaf debris.

Proper design, installation, and maintenance of permeable paver systems are critical to realize their water quality and flood reduction benefits. They are an excellent option for individuals looking to make their properties more aesthetically appealing and environmentally friendly. Although installations of permeable pavers are not yet widespread in Montana, they are becoming increasingly popular in urban centers throughout the state. For example, three years ago no projects existed in the City of Bozeman, but today there are six installed projects within the city and many more are planned. When adopted on a widespread scale, these systems will result in a more sustainable and healthy future for Montana’s water resources.

Additional information related to the design, construction, and maintenance of permeable pavers is available at www.bozeman.net/ government/stormwater. �