In the early 1990’s, the United States Environmental Protection Agency (EPA) established the National Pollutant Discharge Elimination System (NPDES) stormwater regulations to comply with the requirements of the Clean Water Act. Compliance with federal, state, and local stormwater programs involves the use of “best management practices” (BMPs) to manage and control stormwater runoff.
Permeable Paving System as an EPA
Best Management Practice
The EPA encourages “system building” to allow for the use of appropriate site-specific practices that will achieve the minimum measures under Phase II of NPDES. Governing authorities must develop and implement strategies that include a combination of structural and non-structural BMPs appropriate for their communities. Structural practices include storage practices, filtration practices and infiltration practices that capture runoff and rely on infiltration through a porous medium for pollutant reduction. Infiltration BMPs include detention ponds, green roofs, bioswales, infiltration trenches and permeable pavements. Non-structural practices are preventative actions that involve management and source controls. Many states and municipalities have incorporated the EPA regulations into their stormwater design and BMP manuals as they attempt to deal with increased impervious cover, stormwater runoff and over-taxed drainage and sewer systems.
Effective stormwater management is often achieved through a comprehensive management system instead of a collection of individual practices. Some individual practices may not be effective alone, but may be highly effective when used in combination with other systems. Ordinances or other regulations are used to address post-construction runoff from new development or redevelopment projects. In addition, it is important to ensure adequate long-term operation and maintenance of BMPs.
Permeable Paving Systems are considered structural BMPs under the “infiltration practices” category. From an engineering viewpoint, permeable pavements are essentially infiltration trenches with
paving on top that supports pedestrian and vehicular traffic. By combining infiltration and retention, Permeable Paving Systems offer benefits over other types of structural infiltration systems. Permeable Paving Systems also work well in conjunction with other recommended BMP infiltration practices such as swales, bioretention areas and rain gardens.
PREVENTION VS. TREATMENT
Once pollutants are present in a waterbody, or after its physical structure and habitat have been altered, it becomes difficult and expensive to restore the waterbody to its original condition. Therefore, the use of a management system that relies first on preventing degradation of receiving waters is recommended. This is why BMPs under each of the EPA’s minimum measures focus on preventing pollutants from ever getting into stormwater. Similarly, some of the practices under the EPA’s post-construction runoff control minimum measure address site design features that can prevent pollution.
LOW IMPACT DEVELOPMENT
In addition to the EPA, other agencies and organizations are addressing the issue of land development and the impact of stormwater runoff on the environment and society. According to the National Resources Defense Council, Low Impact Development (LID) has emerged as a favored approach to controlling stormwater pollution and protecting watersheds.
MINIMUM IMPACT DESIGN STANDARD (MIDS)
The Minnesota Pollution Control Agency (MPCA) has developed a performance and design standard providing the tools to enable and promote the implementation of LID and other stormwater management techniques. For the purpose of this section, “low-impact development” means an approach to stormwater management that mimics a site’s natural hydrology as the landscape is developed.
Using the LID approach, stormwater is managed on-site with the goal that the rate and volume of stormwater reaching receiving waters remain unchanged from pre-development levels. The calculation of pre-development hydrology is based on native soil and vegetation. One of the primary goals of LID design is to reduce runoff volume by infiltrating rainwater into groundwater and finding beneficial uses for water as opposed to pouring it down storm sewers.
Some LID runoff control objectives include reducing impervious cover, preserving and re-creating natural landscape features, and facilitating infiltration opportunities. LID principles are based on the premise that stormwater management should not be seen as stormwater disposal, but instead that numerous opportunities exist within a developed landscape to control stormwater closer to the source. This allows development to occur with low environmental impact. LID is much more than the management of stormwater – it is about innovation in the planning, designing, implementing and maintaining of projects. The MPCA lists Permeable Paving Systems as one of the ten common LID practices.
MUNICIPAL STORMWATER MANAGEMENT OBJECTIVES
Many regional authorities, including county, municipal, watershed and conservation districts, manage the preservation of the natural water drainage and purification systems or control the downstream flow, especially if those systems are working at or near capacity. A vital step in controlling the harmful effects of development on urban water quality is managing post-construction stormwater runoff. Stormwater runoff is generated when precipitation from rain and snowmelt events flows over land or impervious surfaces (paved streets, parking lots, driveways and building rooftops) and does not percolate into the ground. Stormwater runoff is the primary source of pollutants found in surface waters and contains a toxic combination of petroleum hydrocarbons, pesticides, heavy metals, nutrients, road salt, bacteria, sediments, and other pollutants detrimental to water quality. A Permeable Paving System will help your project comply with the requirements of local and regional authorities to restrict runoff of these pollutants.