As stormwater management systems increasingly focus on strategies that emphasize conservation and use of natural on-site methods, pervious concrete pavements
As stormwater management systems increasingly focus on strategies that emphasize conservation and use of natural on-site methods, pervious concrete pavements are becoming a popular solution.
A new publication jointly sponsored by the Portland Cement Association and the National Ready Mixed Concrete Association, Hydrologic Design of Pervious Concrete, provides an overview and discussion of hydrologic design techniques. Pervious concrete is recognized by the U.S. Environmental Protection Agency (EPA) as a best management practice for stormwater management. However, the hydrologic performance of the pavement is the key parameter for effective use.
The publication considers the relationship between runoff characteristics, material characteristics, and site geometry. It also provides guidelines for the preliminary selection of appropriate pervious concrete characteristics for specific applications and environments.
Written by Michael L. Leming, H. Rooney Malcom, and Paul D. Tennis, the report provides civil engineers, landscape architects, and other design professionals guidance in the selection and design of an appropriate pervious concrete pavement system. It also assists permit-granting agencies in the review and acceptance of proposed pavement systems. Developers and owners interested in a more complete technical review of pervious concrete pavement systems also will find the publication a valuable tool.
Pervious concrete pavement systems can be an important part of context-sensitive construction and Low Impact Development, designed to meet a number of goals related to the function of the site and structure. Goals include the ownerÎs objectives and societyÎs requirements, both of which are site specific. The needs of society are often at least nominally addressed through permitting requirements, including land use restrictions, zoning limitations, and fees, and reflect increasing concerns related to control of surface runoff associated with development. These needs can vary by community, location, and application and include both flood control and water quality.
Although pervious concrete has been used in some areas for decades, recent interest in sustainable development or green building and recognition of pervious pavements by the EPA as a best management practice (BMP) for storm water management have heightened interest in its use throughout North America. Its use supports national initiatives, such as EPA’s Heat Island Reduction Initiative and Low Impact Development, and provides a potential for credit in the LEED (Leadership in Energy and Environmental Design) rating system for sustainable building construction. LEED includes provisions for control of both water quantity and water quality in storm water management.
The hydrological performance of the pervious concrete pavement system is usually the characteristic of most interest to agencies with permit-granting authority. In some cases, limits are placed on the percentage of land that may be developed for a given site without the use of specified remedies including structural BMPs, such as detention or retention ponds. Alternately, limits on the quantity of runoff after development may be specified. Pervious concrete pavement systems can be an important part of a sustainable site, while simultaneously providing access and parking space.
Unlike many other structural BMPs, pervious concrete paving systems can be used effectively and economically to retrofit existing built-up sites to attain desired hydrologic performance. The EPA’s Preliminary Data Summary of Urban Storm Water Best Management Practices (1999) notes that retrofitting to correct or attain specific runoff limits for an existing, built-up site can be extremely expensive. Pervious concrete pavement systems can be the exception to this rule since they can reuse existing parking areas for impoundment purposes. For example, analysis of one potential parking lot the size of a football field indicated that the runoff from a reasonably urbanized, nine-acre area over a sandy silt subgrade would be the same as that from grassy pasturage in the area.
Pervious concrete can be used to mitigate problems associated with surface runoff through several mechanisms. Hydrologically, pervious concrete paving systems can:
Capture the first flush of runoff from the surface so the pollutant load including trash, floatables, and other debris in overland surface runoff and, ultimately, streams and rivers, is reduced; and,
Create short-term storage detention of rainfall, which reduces the volume of surface runoff and provides for additional infiltration, thereby recharging groundwater and increasing base flow. Also reducing the velocity of water in both natural and constructed drainage channels, storage detention minimizes surface runoff, which can lower the sediment load carried into receiving waters, since both the erosion of channels and the quantity of materials carried into those channels are reduced.
The material characteristics of the pervious concrete and other elements of the system significantly affect the final design. The porosity of pervious concrete affects both hydrologically important properties (permeability and storage capacity) and mechanical properties (strength and stiffness). Pervious concrete used in pavement systems must be designed to support the intended traffic load (axle loads and repetitions) and contribute positively to local storm water appropriate material properties, the appropriate pavement thickness, and other needed characteristics, including the absence or presence of features such as base course, filter fabric, or geotextile reinforcement, to meet the hydrological requirements and anticipated traffic loads simultaneously.
The PCA/NRMCA report provides an overview and discussion of design techniques that can be used for hydrological design needs and concepts, considering the inter-relationship between runoff characteristics, material characteristics and site geometry. It also provides guidelines for the preliminary selection of appropriate pervious concrete characteristics for specific applications and environments.
HYDROLOGICAL DESIGN METHODS
The methodology used in the hydrological design of pervious concrete pavement systems should reflect the level of detail needed to satisfy the agency specifying, permitting or regulating the use of pervious pavement. The methodology also should be sufficiently rigorous to meet the needs of the design professional, and should reflect the behavior of the system in service within the limits of accuracy needed. Computational efficiency is a desirable, although not compelling, factor; and, model complexity may not necessarily improve model accuracy. A model that is simultaneously simple to use and captures the essential elements of behavior is useful and important to the design professional, even when advanced analysis is not required by local regulations.
Site conditions and regional needs can vary significantly. Local regulations can range from simple to complex, depending on the needs and characteristics of the area and the objectives of the regulatory agencies. Solutions and approaches suitable for one area may be overly restrictive and prescriptive in another, or provide insufficient protection in a third. The method recommended in the PCA/NRMCA report is the Natural Resources Conservation Service (NRCS) Soil Conservation Service (SCS) Method, or Curve Number method, which:
Is well established and widely used by many design professionals involved in managing runoff;
Captures the essential elements of pervious concrete pavement system behavior;
Is appropriate for the design of a structure intended to capture and hold some portion of the runoff in a small urban watershed (such as a retention or detention feature);
Is flexible and easily adapted to a site with several types of surfaces contributing to runoff;
Is easily implemented by adapting well-known stage-storage-discharge principles to the simple geometry of a pervious pavement system; and
Can be used to analyze systems intended to function within the constraints of many different regulatory requirements.
Users of this report who are unfamiliar with hydrological design methods should be aware that there is no nationally accepted standard design technique for estimating total runoff; and, preferred techniques vary with region and application. Techniques favored in the western United States are generally those of the Bureau of Reclamation, while those favored in the eastern two thirds of the country are often those of the NRCS. Results with these methods are similar enough that the techniques presented in this document based on the original NRCS (SCS) methods can be adapted for most applications.
More information on the report (including ordering details) can be found at the PCA Bookstore, www.cement.org/bookstore; or by calling 800/868-6733.