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

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.

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