Chapter 9: Stormwater Retrofits

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This chapter provides guidance for retrofitting sites that are already developed to reduce the adverse impacts of existing stormwater runoff. A “retrofit” is a project that modifies an existing developed site for the primary purpose of improving the quality of and reducing the quantity of stormwater discharge. This is primarily achieved through disconnecting, and therefore reducing, Directly Connected Impervious Area (DCIA), as defined in Chapter 2 - Stormwater Impacts. Stormwater retrofits can be used to disconnect DCIA by converting impervious surfaces to pervious surfaces, redirecting runoff from impervious surfaces to adjacent pervious areas, and adding new or modifying existing structural stormwater Best Management Practices (BMPs) to infiltrate or reuse stormwater runoff from impervious areas.

This chapter describes the reasons for and benefits of stormwater retrofits, various retrofit approaches and types, identification and design of stormwater retrofits, quantifying retrofit benefits (i.e., crediting), and common retrofit applications. Additional guidance on stormwater retrofits can be found in the information resources at the end of this chapter.

Why Retrofit? - Objectives and Benefits of Stormwater Retrofits

Overview of how retrofitting can provide many water quality and quantity benefits and achieve stormwater objectives highlighted throughout the Manual such as reducing pollutant loads and meeting pollution load reduction targets.

Retrofit Approaches

This section highlights two approaches to implementing retrofits: the Opportunistic Approach and the Retrofit Planning Approach, which consists of 8 steps. Both approaches can be used together.

Retrofit Types

Information about each of the different types of retrofits:

  • Impervious Area Conversion
  • Impervious Area (Simple) Disconnection
  • Modifying Existing Structural Stormwater BMPs
  • New Structural Stormwater BMPs

Retrofit Applications

Information about incorporation of retrofits applications into various land use settings and sites, including roads and right-of way retrofits and parcel based retrofits.

Retrofit Selection

Table containing retrofit factors and which considerations should be addressed when selecting the right retrofit for the site.

Retrofit Sizing and Crediting

Information on sizing retrofits for the site using stormwater BMP curves and sizing guidance, in addition to credit which can be received for DCIA disconnection.

Additional Information and Resources and Stormwater Retrofits

Further links / resources on retrofits as well as a table breakdown of when DCIA is considered disconnected and the reduction credit which can be received.