Water: Best Management Practices
Menu of BMP Background
Problems Associated with Stormwater
The stormwater pollution problem has two main components: the increased volume and rate of runoff from impervious surfaces and the concentration of pollutants in the runoff. Both components are directly related to development in urban and urbanizing areas. Together, these components cause changes in hydrology and water quality that result in a variety of problems, including habitat modification and loss, increased flooding, decreased aquatic biological diversity, and increased sedimentation and erosion. Effective management of stormwater runoff offers a multitude of possible benefits, including protection of wetlands and aquatic ecosystems, improved quality of receiving waterbodies, conservation of water resources, protection of public health, and flood control.
In addition to chemical pollutants in stormwater, the physical aspects related to urban runoff, such as erosion and scour, can significantly affect a receiving water's fish population and associated habitat. Alterations in hydraulic characteristics of streams receiving runoff include higher peak flow rates, increased frequency and duration of bankfull and subbankfull flows, increased occurrences of downstream flooding, and reduced baseflow levels. Traditional flood control measures that rely on the detention (storage) of the peak flow (referred to as peak shaving) have been characteristic of many stormwater management approaches, have generally not targeted pollutant reduction and in many cases have exacerbated the problems associated with changes in hydrology and hydraulics. EPA recommends an approach that integrates the control of stormwater peak flows and the protection of natural channels to sustain the physical and chemical properties of aquatic habitat.
For more information on the water quality problems from stormwater runoff, see:
- National Water Quality Inventory Report to Congress (305(b) report)
- Chapter 4 (Environmental Assessment) from EPA's Preliminary Data Summary of Urban Stormwater Best Management Practices (1999) (442 pp, 442K, About PDF)
- Protecting Water Quality from Urban Runoff (2 pp, 238K, About PDF)
- Slideshow on the Impacts of Urbanization from the Center for Watershed Protection's Stormwater Manager's Resource Center
Using BMPs as Systems to Prevent Water Pollution
Effective stormwater management is often achieved from a management systems approach, as opposed to an approach that focuses on individual practices. That is, the pollutant control achievable from any given management system is viewed as the sum of the parts, taking into account the range of effectiveness associated with each single practice, the costs of each practice, and the resulting overall cost and effectiveness. Some individual practices may not be very effective alone but, in combination with others, may provide a key function in highly effective systems. The Phase II rule encourages such system-building by stating the minimum requirements in more general terms, which allows for the use of appropriate situation-specific sets of practices that will achieve the minimum measures.
Prevention versus Treatment
Once pollutants are present in a waterbody, or after a receiving waterbody's physical structure and habitat have been altered, it is much more difficult and expensive to restore it to an undegraded condition. Therefore, the use of a management system that relies first on preventing degradation of receiving waters is recommended. BMPs under each of the minimum measures-particularly the obvious category of pollution prevention, as well as outreach, education, and erosion and sediment control-focus on the prevention of pollutants from ever getting into stormwater. Similarly, some of the practices under the post-construction runoff control minimum measure address site design issues that can result in pollution prevention.
Background on the Stormwater Phase II Rule
Published on December 8, 1999, the Stormwater Phase II Rule generally requires operators of small MS4s in urbanized areas to develop and implement a stormwater management program that addresses six minimum control measures. A series of fact sheets describe the various components of the Phase II Rule.
Implementing these minimum control measures typically requires the application of one or more BMPs. It is important to recognize that there is site-specific, regional, and national variability associated with the selection of appropriate BMPs, as well as in the design constraints and pollution control effectiveness of practices.
EPA has found the practices listed in the menu of BMPs to be representative of the types of practices that can successfully achieve the minimum control measures. The list of BMPs is not all-inclusive, and it does not preclude MS4s from using other technically sound practices. However, in all cases the practice or set of practices chosen needs to achieve the minimum measure.
EPA also recognizes that some MS4s may already be meeting the minimum measures, or that only one or two additional practices may be needed to achieve the measures. Existing stormwater management practices should be recognized and appropriate credit given to those who have already made progress toward protecting water quality. There is no need to spend additional resources for a practice that is already in existence and operational.