Municipalities have been designing conveyance systems and controls to address stormwater for flood control for the past 50 to 100 years or more, but it is only within the past 20 years with the U.S. Environmental Protection Agency’s (EPA’s) National Pollutant Discharge Elimination System (NPDES) stormwater permit program that most municipalities have begun to address water quality impacts from new developments. At first, most communities focused on controlling peak flows by requiring extended detention (resulting in the typical stormwater detention basin). More recently, communities are trying to reduce stormwater runoff from new developments and use controls that promote infiltration, evapotranspiration, and beneficial uses of runoff instead of just temporary detention. EPA stormwater regulations require regulated municipalities to develop and implement a program to control stormwater runoff from new development and redevelopment projects that disturb greater than one acre. Such regulations require a combination of structural or nonstructural controls (or both), a local ordinance to address post-construction runoff, and procedures to ensure long-term operation and maintenance. EPA also recommends in the stormwater Phase II regulations that such programs “attempt to maintain pre-development runoff conditions,” but EPA has not set a national design standard to address post-construction runoff. More specific guidance needed
The lack of a nationwide standard has resulted in a wide disparity in how states and local governments regulate post-construction stormwater. Several years ago, the Center for Watershed Protection (www.cwp.org) conducted a survey of state stormwater manuals. The center found that although 80 percent of the states had stormwater manuals, only about 25 percent of those manuals were complete and updated with the most recent research and knowledge related to stormwater. It also found a variety of stormwater design standards being used, with about 25 percent of the manuals using the unified sizing criteria first developed by Maryland. (Unified sizing criteria set standards to address pollutant removal goals, maintain groundwater recharge, reduce channel erosion, prevent overbank flooding, and pass extreme floods). More recent standards are focused on keeping stormwater onsite (at least for most storms), rather than discharging the stormwater to a storm sewer and eventually to a receiving water. Section 438 of the Energy Independence and Security Act of 2007 requires most new federal facilities to “maintain and restore ... the predevelopment hydrology of the property with respect to the temperature, rate, volume and duration of flow.” Design standard challenges Alternative design methods — There are a number of different methods that can be used to size stormwater treatment controls, including volume-based methods, flow-based methods, and the unified sizing criteria discussed above. Each has its advantages and disadvantages, so selecting a method and standard to be achieved can be difficult with the many stakeholders involved. Rainfall patterns — Different standards might need to be developed to take into consideration the rainfall patterns (seasonality) and climate (arid, semi-arid, and humid) for each region. For example, Washington state has developed two different stormwater manuals: one to address the arid, eastern half of the state and one for the humid, western half. Addressing local conditions — Design standards might need to be adjusted or include exceptions to address local conditions such as limited infiltration of soils, high water table, cold climate, karst topography, or other issues. Local goals and concerns — The setting of design standards can be complicated by the need to address local goals such as protecting beach water quality or a drinking water source, or addressing specific pollutants affecting an impaired waterbody. Guidance on developing post-construction standards
Summary John Kosco, P.E., CPESC, is a principal engineer with Tetra Tech, Inc., in Fairfax, Va. He can be reached via e-mail at john.kosco@tetratech.com. |
Stormwater Management Series
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