Borough of Carnegie

BMP MANUAL
In undisturbed landscapes, rainwater runoff is part of the natural hydrologic cycle. Vegetation, soils, and a wide range of organisms filter, absorb, and use rainfall in their living processes. Evaporation and transpiration takes place. Excess precipitation infiltrates into groundwater and flows into surface waters, recharging aquifers and supporting aquatic life. The entire system is affected when the landscape is changed: impervious surfaces (paved streets, parking lots, rooftops, etc.) prevent runoff from percolating into the ground and cause it to pick up debris, sediments, chemicals, and other pollutants as it moves over the ground.

In order to assist municipalities, designers, developers, and engineers in understanding, developing, and implementing stormwater management programs, PA DEP published the PA Stormwater BMP Manual in 2006.

As noted in the Foreword:
"The Manual is intended to be a tool to achieve the regulatory standard to protect, maintain, and improve the Commonwealth's water resources through comprehensive site planning and BMP design guidance, while allowing for the continued growth and development of Pennsylvania."

As such, the Manual's main focus is on BMPs that help meet the requirements of MCMs 4 through 6. These BMPs are generally classified as "non-structural" (design and planning techniques that preserve natural systems and hydrologic functions on a site) and "structural" (more conventional 'brick and mortar' techniques, which may include vegetation and soil-based mechanisms.) The key distinction between Non-Structural and Structural BMPs is that the former are intended to prevent stormwater generation, while the focus of the latter is on mitigating unavoidable stormwater-related impacts.

When it comes to development, the Stormwater Management Program described in the Manual emphasizes a comprehensive total site design approach. Whether referred to as "Low Impact Development", "Conservation Design", or "Green Design", this environmentally sensitive approach to site development and stormwater management aims to minimize the effect of development on water, land, and air resources.

The use of best management practices, or BMPs, is the primary method of controlling stormwater discharges and preventing runoff from adversely affecting water quality. BMPs are a suite of planning processes, development methodologies, and specific techniques designed to be used in combination to:

• Prevent disturbances to natural systems and hydrologic functions; and
• Minimize the impacts of disturbances where they do occur.

This guidance Manual identifies information on resource planning, techniques for land development, and concepts and procedures for designing and implementing performance-based BMPs for managing stormwater for quantity and quality. The comprehensive planning approach to stormwater management emphasizes reducing the impacts of development activities through the mitigation process by employing practices that avoid and minimize potential impacts to watershed resources. The BMPs are designed to minimize or mitigate for the unavoidable impacts that development has on watershed resources.

This manual lists various BMPs and design standards, which are acceptable in Pennsylvania to manage stormwater and prevent pollution of waters of the Commonwealth. Stormwater management plans and the BMPs designed according to these standards when properly applied, implemented and maintained, will achieve the Department's water quality mandates established in 25 Pa. Code Chapter 93 and required by the federal Clean Water Act, federal NPDES regulations, the Pennsylvania Storm Water Management Act and the Pennsylvania Clean Streams Law. The proper utilization and implementation of these BMPs will protect and maintain existing uses and the level of water quality necessary to protect the existing uses, and will protect and maintain the water quality of high quality and exceptional value waters of the Commonwealth.

This Manual Provides:
- A standardized process for evaluating site design and BMP selection to minimize or eliminate the net change in post construction volume, rate and quality.
- A standardized approach to measuring pre-construction volume, rate and quality and post construction site runoff volume, rate and quality.
- Standardized specifications for BMPs to manage stormwater to minimize the net change in post construction runoff volume, rate and quality.

Alternate BMPs, not listed in this manual, that provide the same or greater level of protection, may also be used to attain the regulatory standard. It is incumbent on the person proposing the use of alternative BMPs to demonstrate their effectiveness with appropriate supporting analysis, calculations, test results or other documentation.