Stormwater Management Series


August 21, 2009

NPDES Update
Guidance and updates about stormwater management
By John Kosco, P.E., CPESC
*Originally published in CE News July 2009 and reposted with express permission of John Kosco*
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
In 2008, the National Research Council released the report, “Urban Stormwater Management in the United States”. The report reviews EPA’s stormwater program and provides recommendations for improvement, including the following findings on EPA’s stormwater management approaches:

  1. Individual controls on stormwater discharges are inadequate as the sole solution in urban watersheds;
  2. Stormwater control measures, such as product substitution, better site design, downspout disconnection, conservation of natural areas, and watershed and land-use planning, can dramatically reduce the volume of runoff and pollutant load from new development; and
  3. Stormwater control measures that harvest, infiltrate, and evapotranspire stormwater are critical to reducing the volume and pollutant loading of small storms.

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 ( 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
There are some very good reasons why design standards vary state by state and why EPA has not developed a nationwide post-construction design standard. A few of the challenges associated with developing post-construction design standards include the following:

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
A good reference describing the process of developing a stormwater management approach and design standards was developed by the Center for Watershed Protection. “Managing Stormwater in Your Community: A Guide for Building an Effective Post-Construction Program” (Hirschman and Kosco, 2008 available at was created to help municipal stormwater permittees develop a comprehensive, post-construction program. Chapter 4 of the guide describes a recommended hierarchy of stormwater treatment methods that a municipal stormwater program should incorporate:

  • First, reduce runoff. Use site planning and design techniques to reduce impervious cover, disturbed soils, and stormwater impacts.
  • Second, reduce pollutants carried by runoff. Use source control and pollution prevention practices to reduce the exposure of pollutants to rainfall and runoff.
  • Third, capture and treat runoff. Design stormwater best management practices to collect and treat the stormwater that is generated after applying the site design and source control methods described above. Some stormwater collection and treatment can be in small-scale, distributed practices close to the source of runoff.
    The guide also describes the development of stormwater design criteria to address water quality volume, flood control, groundwater, impaired waters, and other critical issues.

The science of stormwater control is rapidly evolving. As described in the National Research Council report findings above, stormwater management is moving toward the use of controls that minimize runoff (through reduced impervious surfaces) and infiltrate runoff from small storms onsite using distributed practices. However, many states and local governments are still working to define the specific standards used to achieve these goals.

John Kosco, P.E., CPESC, is a principal engineer with Tetra Tech, Inc., in Fairfax, Va. He can be reached via e-mail at