New England Stormwater Retrofit Manual

The Stormwater Retrofit Guidance Manual is a key tool for improving New England’s water resources.  The guidance is based on how stormwater treatment occurs within structural controls with the understanding that achieving any water quality improvement is beneficial.

In retrofit scenarios, it is often challenging to determine the stormwater control measures (SCM) best suited for the site. The manual is based on the concept that incorporating some stormwater treatment for developed sites is better than omitting all together because prescribed design standards cannot be fully met. The manual guides users to develop SCMs based on their core functional and treatment components and encourages the user to piece components together in configurations that best fit project and site-specific needs. The range of sites and scales where this guidance can be applied varies from watershed scale planning to the design of small-scale measures inserted into reconstruction projects.

The manual presents the US EPA (Environmental Protection Agency) SCM Performance Curves as a tool to quantify water quality benefit (i.e. pollutant removal credit) for a range of sizes and types of SCMs to aid in the selection process and justify the retrofit.

The manual takes practitioners through the fundamental approach for retrofit, sizing, design and performance characterization, an introduction of performance curves and stormwater control measure design criteria including detailed sizing requirements and guidance. The manual offers an alternative approach that is practical, achievable, and can offer solutions to communities’ size restrictions, while achieving measured and verified infiltration and pollution goals. This manual does NOT intend to replace existing federal, state or local requirements for stormwater management. Instead it should be used as supplemental guidance and a resource to support smart choices in site design as it relates to the mitigation of stormwater impacts from existing developed areas.

For the purposes of the manual, a retrofit is defined as the addition of stormwater controls on a currently developed site. Retrofits stormwater control measures (SCMs) can be incorporated into planned construction work or constructed as stand-alone stormwater improvements.

This manual…

  1. Provides research-based guidance on planning, siting, and designing retrofit (SCMs) to manage stormwater in existing or reconstructed development situations where regulatory requirements do not dictate prescribed specifications.
  2. Discusses both the Opportunistic Approach (incorporation of SCMs into already planned and needed construction projects) and Planning Approach (proactively planning retrofits and prioritizing sites where retrofits are possible and beneficial) to identifying and installing stormwater retrofits.
  3. Promotes the use of SCM Performance Curves to optimize SCM selection and sizing based on treatment performance.
  4. Reviews SCM unit operations and processes for treatment so that designers are empowered to be flexible and creative with their designs.
  5. Provides unique design guidance custom for the retrofit scenario when prescriptive regulatory guidance may be too restrictive.


Situations where retrofits are encouraged, and with which this manual will assist, include:

  1. Implementing stormwater controls as part of maintenance and planned construction.
  2. Demonstrating stormwater treatment to the maximum extent practicable (MEP).
  3. Implementing stormwater controls to meet total maximum daily load (TMDL) targets.
  4. Identifying and prioritizing stormwater management interventions to protect or improve a local water resource.
  5. Enhancing existing structural SCMs for improved performance controls (e.g., increased pollutant load and runoff volume reductions).
  6. Understanding cost/benefit for various control measures and options

The manual was developed by the University of New Hampshire Stormwater Center and VHB in close collaboration with the Environmental Protection Agency Region 1, the Rhode Island Department of Environmental Management, Department of Transportation representatives from Massachusetts and Rhode Island, and representatives from Vermont and Connecticut. This manual was developed with financial support from the SNEP Network, a project of the New England Environmental Finance Center. 

2022 SNEP Symposium presentation: Introducing the Stormwater Retrofit Manual
Link to Recording.


Webinar: The New England Stormwater Retrofit Manual: Upgrading the performance of your stormwater management system for better watershed health. July 26, 2022. EPA’s Soak Up the Rain Series. 


New England Stormwater Retrofit Manual Decisions Basis Memo: This memorandum serves as documentation received by the Technical Advisory Committee and provides rationale on why the comments were unable to be addressed and/or how they were incorporated into other pieces of the manual. 


Performance Curves Overview: The Stormwater Control Measure (SCM) performance curves were developed by EPA and serve as a tool that helps quantify water quality benefits of various structural stormwater controls. The curves relate to the depth of runoff treated of various controls to pollutant reduction.