Pervious Concrete Pavement Design for Sustainable Porous & Permeable Stormwater Drainage

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Overview: Pervious Concrete Pavement System Applications

2.1 Water Quality and Water Quantity

Water quantity issues have traditionally dominated hydrologic design decisions. Efforts to control flooding have often consisted of sizing various structures or elements including culverts, open channels, and storm sewer pipe to ensure adequate capacity. Designs are based on estimating the peak flow at the structure in a given design storm. Water quality has become a much more critical concern in many communities and therefore a more critical factor in many permitting decisions or restrictions. While water quantity controls also positively affect water quality by reducing stream flow velocities, it is important to recognize the paradigm shift in thinking about storm water management. Peak flow is only one aspect of design in improving water quality. The behavior of the entire system must be understood and considered when modeling and making decisions regarding suitability.

2.2 Uses and Applications

2.2.1 Detention and Retention Structures

Conventional stormwater management system BMP’s include impoundment structures, that is, ponds or basins designed to capture stormwater runoff for either retention or detention. A retention pond is designed to hold water for infiltration into the soil. Ponds designed for detention purposes are intended to capture runoff for discharge into natural or manmade channels, or a storm sewer system over an extended time so as to reduce the maximum rate of flow.

Pervious concrete pavement systems are often designed as retention structures. A significant advantage of pervious concrete pavement systems is the ability to park on the “pond,” providing a multi-use facility with many additional advantages. Additional design features outside the scope of this publication are generally required when pervious concrete pavement systems are included as part of a larger detention system and when the overflow discharges into the storm sewer system. Output of the National Resource Conservation Service (NRSC) Curve Number (CN) approach described in this document can be used as input into more complex detention system designs.

2.2.2 Passive or Active Mitigation Systems

In many situations, the use of pervious concrete to simply replace an impervious surface may be considered a sufficient regulatory standard to manage runoff. In other situations, the regulations governing development of a site may require that runoff after development not exceed runoff, or some percentage of runoff, prior to development. In the latter case, the pervious concrete pavement system must be designed specifically to handle much more rainfall than that which will fall on the pavement itself. For example, a parking lot can be used to capture excess runoff from rain falling both on itself and on surrounding areas, including, for instance, the rain collected and discharged through roof drains of nearby buildings. These two applications may be termed passive mitigation and active mitigation, respectively.

Figure 2. Pervious concrete permits water to sustain trees to infiltrate and may eliminate the need for irrigation.

A “passive” mitigation element is used only to reduce the quantity of impervious surface in a given area by replacing impervious surface with pervious surface. A passive mitigation element might also capture much, if not all, of the “first flush,” providing additional hydrological benefit, but is not intended to accommodate excess runoff from adjacent surfaces.

An “active” mitigation system, on the other hand, is designed to maintain total runoff at some specified level for a particular site with several types of features. Pervious concrete used in an active mitigation system must capture a sizeable portion of the runoff from other areas on site as well as rain falling on its own “footprint.” Typically, such areas include buildings, areas paved with conventional (impervious) pavement (including delivery areas, trash pick up areas, and bus lanes, all of which may carry significant, heavy truck traffic), and traffic islands and buffer zones, which may or may not be vegetated and which may or may not belong to the owner developing the property.

Active mitigation systems are particularly well suited to rehabilitating existing impervious areas for remedial control of urban runoff since they can be designed to capture runoff from adjacent areas. Depending on the size, geometry, and porosity of the pervious concrete system, the excess surface runoff from the site can be kept at or returned to pre-development levels.

Active pervious concrete pavement systems can also be designed as boundary features used in conjunction with conventional pavement to create a locally active, but site-wise passive feature. For example, the pervious concrete system can be designed to capture and temporarily store much, if not all, of the runoff from a conventionally paved parking area by placing a relatively narrow strip of pervious concrete over a deep, clean stone base along the edges of the parking lot. Pervious concrete borders used for tree wells or vegetated traffic islands can be designed as active elements, helping maintain the net runoff from the entire parking area at desired or permit constrained levels. An important benefit of pervious concrete pavement systems with vegetated islands or tree wells is that adequate moisture may be available with minimal, if any, need for irrigation. This is particularly important when working with minimum tree density requirements or when protecting large, existing trees on the site.

The active mitigation design approach is very flexible and can be used for a variety of applications. Primary applications of pervious concrete in an active mitigation role therefore include commercial parking lots, boundary features of commercial development sites, and containment features designed to intercept at least a portion of overland surface runoff prior to entering drainage channels.

2.3 Effects of Ponds

It is important to consider other, often unintended, effects of various design alternates and BMP's in order to make an informed decision. There are a number of significant consequences of retention or detention ponds which may not be obvious at first. Pervious concrete pavement systems may not be the lowest initial cost option and may only be economically feasible in comparison with other alternatives. In new, or “greenfield” construction, specifications can prohibit runoff in excess of that which would occur prior to development, effectively prohibiting development unless mitigation features are provided. Expansion or changes in ownership or use of existing facilities can trigger regulatory constraints and may limit the usable land area by requiring the installation of an arbitrarily pre-selected alternative, such as a detention or retention pond.

Storm water control devices such as detention and retention ponds in a built up area create a significant maintenance and management expense. Fences are often required around such features to limit liability to the owner by controlling access by the public, especially children looking to play in an “attractive,” but potentially dangerous site. Danger can come from physical injury, drowning, or exposure to certain wildlife, including vermin and disease vectors such as mosquitoes, which must be expected in these areas. All of the maintenance and liability exposure must be handled by the owner in a legally responsible and environmentally acceptable manner. While moving soil is not particularly expensive, the loss of the effective use of the land for the pond and the immediate surrounding area, along with maintenance and safety costs can be very high, making pervious concrete economically advantageous. Pervious concrete pavement systems provide a significant and unique value by simultaneously improving water quality, helping to mitigate flooding, and returning the surface area to commercially productive use.

Reference: Leming, M.L., Malcom, H.R., and Tennis, P.D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2007, 72 pages.

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