Retrofitting: Infiltration, Filtration & Bioretention

Integrating stormwater management practices into developed sites

Retrofitting can be used to achieve highly effective stormwater management that reduces runoff volume, increases ground water recharge, improves surface water quality, provides thermal benefits and helps mimic pre-development hydrology. Retrofits such as rain gardens and swales are versatile because they can be constructed in small areas and easily integrated into existing residential and commercial sites. This fact sheet provides a list of practices and applications for small-scale sites and includes discussion on the associated benefits and costs. Links to example municipal cost-share and incentive programs for stormwater retrofitting are provided in the Additional Resources section.

This is a picture of Bioretention facility in St Paul MN
Bioretention facility in St Paul MN Image Courtesy of Emmons & Olivier Resources, Inc.

Benefits / Pollution Reduction

Retrofitting is a way to rehabilitate watersheds that have a significant amount of imperviousness and little stormwater treatment. When properly designed, constructed, and maintained, BMP retrofits, as discussed here, increase the aesthetics of an area by providing green space and/or stormwater educational opportunities. Retrofitting has the potential to help achieve nondegradation requirements and TMDL allocations for impaired waters as well as protect resources that may be experiencing increased pressure from other areas of the watershed. Stormwater retrofits are generally employed to:

  • Fix past mistakes and maintenance problems
  • Solve chronic flooding problems
  • Serve as stormwater demonstration and education opportunities
  • Trap trash and floatables
  • Reduce runoff volumes to combined sewers
  • Restore stream corridors
  • Reduce specific pollutants
  • Reduce downstream channel erosion
  • Support stream restoration efforts
  • Recharge shallow ground water

The preceding list was adapted from the manual: Urban Stormwater Retrofit Practices created by the Center for Watershed Protection (see Additional Resources) which has tables listing retrofit objectives and implementation options, including pollutant removal capabilities of retrofit options.

Program Development & Implementation

Retrofit programs can be effectively implemented on both public and private properties. Municipally driven projects can be used as examples to encourage private landowner participation. MS4s can also encourage landowners to install retrofit practices using a variety of methods such as those identified below. Using a MS4 directed approach along with a program that encourages landowner retrofits will generally realize the greatest benefits.

Retrofit Designs

Through appropriate planning and BMP selection, almost any type of BMP can be used in retrofitting existing infrastructure. BMP selection depends on site location and characteristics. The existing land use and site objectives can be indicators of which BMPs are more likely to be successful in a retrofit situation.

The following list of retrofit BMPs are examples categorized based on the likely application of each. These lists are not meant to exclude BMPs from particular land uses but rather provide a guide to identifying the best BMPs for your MS4. Individual project constraints will dictate which BMP is the most appropriate in a given situation. Descriptions of these practices and design recommendations can be found in the Minnesota Stormwater Manual or the Center for Watershed Protection Manual 3: Urban Stormwater Retrofit Practices Version 1.0.

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