Permeable pavers offer us the ability to create a hardscape surface that allows water to run back into the water table. We thought it would be nice to to share some specific information based on our familiarity with permeable pavers, which will allow you to highlight the important pros and cons related to this product for your customers, specifically in terms of its role in the arena of sustainable design. Included is an outline of the main benefits of permeable pavers, which should allow you to have some basic information and facts at the ready. Please feel free to make a copy for yourself to use as a guide.
The outline describes, in general, the basic problems with impervious surfaces, what Low Impact Development (LID) is, and how it seeks to address the problems with impervious surfaces, the role of permeable pavers in Low Impact Development, some specific details about how permeable pavers function in eco-friendly construction, how they conform with ADA standards, and some pointers on how to use permeable pavers to score points in LEED projects.
Problems with Impervious Surfaces:
Surfaces that do not permit the penetration of surface water cause two main problems:
1. Overworked storm drains, causing excess, untreated water to run to rivers and lakes.
2. Suspended sediments and heavy metals run directly into storm drains without being filtered by the natural system.
Low Impact Development (LID): A design strategy with the goal of "maintaining and enhancing the pre-development hydrologic regime of urban and developing watersheds." (Taken from the Low Impact Development Center website: www.lowimpactdevelopment.org) One of the primary objectives of LID is reduce water runoff by decreasing permeable surfaces.
The Role of Permeable Pavers in Low Impact Development: Permeable pavers allow water to pass through the upper layer of the pavement and then enter the sub-base reservoir, which acts like a sub-surface infiltration basin. At this level, water is filtered and undergoes natural biological re-processing, as opposed to moving directly to storm drains, and off to our oceans. Water is filtered slowly, and can be used on-site.
Iniltration: There is some disagreement about how much water infiltrates through permeable pavers. The percentage of water that actually moves through the pavers must be figured considering the lifetime of the paver surface, and the type of soil beneath the surface. One study showed a mean infiltration rate of 3.5 inches of water per hour, for clean pavers (free of debris.) This study is here:
Some suppliers and manufacturers will claim much higher infiltration rates, including the rate you would expect from the pavers alone (before they are set on soil.) Beware of unreasonably high infiltration claims.
Exfiltration: Rainfall may exceed the exceed the infiltration rate of the soil below. For different densities of soil, different exfiltration methods may be utilized in order to completely re-route water:
Full exfiltration: For use when the soil is expected to fully exfiltrate all of the surface water. All of the water exfiltrates into the soil below, with no additional sub-surface drainage.
Partial exfiltration: For use when the soil may not exfiltrate all the surface water. In this case, a sub-surface perforated pipe is used to redirect the water.
No exfiltration: For use when the soil below has low or no permeability. In this case, an impermeable membrane is used at the bottom and sides of the system, in addition to a sub-surface perforated drain pipe.
Reduction of Pollutants: The aggregate filters and sub-grade soils the allow sedimentation to occur, and contribute through bacterial treatment of the pollutants and cation exchange. Good bacteria are found on well established sub-soil bases. Also, because rain water enters through the paver surface almost immediately, it is not unnaturally heated, which means that it enters our strem beds and oceans at a lower temperature.
Where Not to Use Permeable Pavers: Pavers should be installed at least 100 feet from water supply wells, streams and wetlands. They should not be installed in facilities where the by-products of products may contaminate groundwater, such as industrial facilities that store
hazardous wastes, or gas stations.
Pavers can clog when exposed to fine sedimentation (As a side note, permeable pavers should be cleaned on a recurring basis in order to maintain maximum permeability.)
ADA: Generally, to comply with ADA rules, the spacing of pavers should be a maximum of 1/2".
LEED Credits:
Credit 6.1 and 6.2: Stormwater Design - 2 possible points (for achieving the appropriate percentage of permeable surface.)
Credit 7: Heat Island Effect Non-Roof - 1 possible point (for using light colored, high albedo materials)
MR Credit 4: Recycled content - 2 possible points (when using pavers that contain fly ash or blast furnace slag)
MR Credit 5: Regional materials - 2 possible points (when using materials that are extracted, processed, and manufactured regionally.
This article is intended as a general overview only. There are a myriad of other factors that we can explore in relation to permeable pavers, and we have included some useful links for those of you who wish to explore the subject in more detail. We always welcome your comments and feedback.
