A unified framework is presented for addressing stormwater sizing criteria in the context of the 2003 MPCA Construction General Permit (CGP) and local stormwater management requirements, if chosen by the local community. The unified approach addresses five different sizing criteria, as shown below:

  • Recharge
  • Water quality
  • Channel protection
  • Over bank flooding
  • Extreme storms

Once the basic stormwater sizing criteria are defined for regular waters, the chapter then describes how they can be adapted to provide greater protection for special and other sensitive waters of the state. The goal of the unified framework is to develop a consistent approach for sizing stormwater practices that can:

  • Perform effectively: Manage enough runoff volume to actually solve the stormwater problem it is intended to address.
  • Perform efficiently: Manage just enough runoff volume to address the problem but not over-control it. More storage is not always better, and can greatly increase construction costs.
  • Be simple to administer: Be understandable, relatively easy to calculate with current hydrologic models, and workable over a range of development conditions and intensities. In addition, criteria should be clear and straightforward to avoid needless disputes between design engineers and plan reviewers when they are applied to development sites, while also eliminating any competitive disadvantages that are created when uniform regulations do not exist.
  • Promote better site design: Be structured in a manner so that property owners have real incentives to reduce storage volumes (and costs) by applying better site design techniques (Chapter 4) to receive stormwater credits (Chapter 11).
  • Be flexible to respond to special site and receiving water conditions: Be expanded to adequately protect unique receiving waters, and to be reduced or eliminated in certain development situations where they are inappropriate or infeasible.

A unified framework for sizing stormwater practices provides greater consistency and integration among the many city, county, watershed organization, regional and statewide stormwater requirements and ordinances adopted over the years. It also establishes a common framework to address all stormwater problems caused by development sites over the entire spectrum of rainfall events. The unified approach still provides communities with flexibility to develop stormwater criteria adapted for local conditions, within overall context of the 2003 MPCA CGP. In addition, Chapter 8 presents more detailed guidance on the appropriate design assumptions for accepted hydrologic models used in design.