Unified sizing criteria

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.

Overview of the Unified Stormwater Sizing Framework

This section reviews the key stormwater sizing concepts and terminology used in the chapter and presents an overview of the unified framework for managing stormwater in Minnesota. The terminology and abbreviations associated with various stormwater sizing criteria can be confusing at times, as the state and local reviewing authorities often define or interpret them in a slightly different manner. The specific meanings of important terms used in this chapter are described in Table 10.1.

In the course of a year, anywhere from 35 to 50 precipitation events fall on Minnesota. Most events are quite small but a few can be several inches in depth. A rainfall frequency spectrum describes the average frequency of the depth of precipitation events (adjusted for snowfall) that occur during a normal year. Figure 10.1 provides an example of a typical rainfall frequency spectrum for Minnesota (MSP airport) which shows the percent of rainfall events that are equal to or less than an indicated rainfall depth. Similar graphs for other locations are contained in Appendix B. As can be seen, the majority of storms are relatively small but a sharp upward inflection point occurs at about one-inch of rainfall.

The unified sizing approach seeks to manage the entire frequency of rainfall events that are anticipated at development sites. The runoff frequency spectrum is divided into five management zones, based on their relative frequency, as follows:

• Recharge: targets rainfall events that create little or no runoff but produce much of the annual ground water recharge at the site.
• Water Quality: targets rainfall events that deliver the majority of the stormwater pollutants at the site.
• Channel Protection: targets storms that generate bankful and sub-bankful floods in the stream that cause channel enlargement.
• Overbank Floods: targets large and infrequent storm events that spill over to floodplain and cause damage to infrastructure.
• Extreme Storms: controls the largest, most infrequent and most catastrophic floods that threaten property and public safety (e.g., commonly known as the 100-year storm.

The goal of stormwater management is to provide effective control over each management zone in order to produce post-development hydrology that most closely resembles state or locally defined pre-development conditions at the development site. Each criterion defines a unique storage volume that should be managed at the site. They are best understood visually as a layer cake that has progressively larger layers from bottom to top, with recharge volume being the narrowest layer at the bottom and extreme storm control comprising the thickest layer at the top. Figure 10.2 shows how the five storage volumes interact in a stormwater BMP.