Calculations and Methodologies

In Pennsylvania, the regulatory standards for post construction stormwater discharges state that:

• In Areas Tributary to High Quality and Exceptional Value (Special Protection) Waters—there shall be no degradation of existing or designated stream quality through a change in post construction stormwater runoff volume, rate and quality.
• In Areas Tributary to Waters other than Special Protection—any change in post construction stormwater runoff volume, rate and quality shall not impair existing or designated uses, whichever use is more stringent.

The PA DEP Stormwater BMP Manual offers guidance for achieving these standards by focusing first on prevention and second on mitigation. Its comprehensive, integrated approach is based on ten basic principles:

1. Managing stormwater as a resource;
2. Preserving and utilizing existing natural features and systems;
3. Managing stormwater as close to the source as possible;
4. Sustaining the hydrologic balance of surface and ground water;
5. Disconnecting, decentralizing and distributing sources and discharges;
6. Slowing runoff down, and not speeding it up;
7. Preventing potential water quality and quantity problems;
8. Minimizing problems that cannot be avoided;
9. Integrating stormwater management into the initial site design process; and
10. Inspecting and maintaining all BMPs.

Chapter 3 of the Manual provides an in-depth discussion of these comprehensive principles and the control guidelines required to meet the standards. Read More >

To assist engineers in developing stormwater management plans that meet the recommended control guidelines, Chapter 8 of the Manual offers a look at widely used methodologies for estimating the total runoff volume, the peak rate of runoff, and the runoff hydrograph from land surfaces under a variety of conditions. More importantly, it provides a series of flowcharts and worksheets to assist in the preparation, presentation, and review of project data needed to formulate an acceptable stormwater management plan. Read More >