Unified sizing criteria

A unified framework is presented for addressing stormwater sizing criteria in the context of the 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

In addition to these sizing criteria for regular waters, this Manual describes how sizing criteria 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 to receive stormwater credits; and
• 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. The Manual provides more detailed guidance on the appropriate design assumptions for accepted hydrologic models used in design.

Overview of the unified stormwater sizing framework

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. This terminology is described in the following table.

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. Rainfall frequency maps have been developed for Minnesota for several return frequencies. 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 bankfull and sub-bankfull 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.

The unified approach proposes to standardize the basic approach to stormwater design for regular waters of the state, while also defining certain site conditions or development scenarios where individual stormwater sizing criteria may be relaxed or waived. The unified framework also clearly indicates when sizing criteria need to be enhanced to provide a higher degree of water resource protection for special or other sensitive waters. The table below profiles the recommended or required sizing criteria for each of the five stormwater management zones for both regular and special waters. Each of these management zones is discussed in a separate page in this Manual. Each page describes how the sizing criteria are calculated and the conditions where they may be relaxed or waived. This information is a general guide and designers should always check with their local review authority and the MPCA to determine the appropriate sizing criteria used in their community.

Channel protection outlet: an outlet designed to release V_cp over a period of 24 hours (minimum V_cp detention time is recommended to be 12 hours). The V_cp pool elevation can be read from the pond stage-storage relationship.

Assuming an orifice is also used to release V_cp, the invert of the V_cp orifice may be placed at the V_wq pool elevation (E_wq).

The average release rate for V_cp is computed as

$^Q_{cp,avg} = (V_{cp} - V_{wq} / t_{cp}) - Q_{wq}^$

Where:

t_cp = the intended channel protection volume detention time
Q_wq= computed (using the above equation for Q_wq), with the head value calculated as $^h_{wq} = (E_{wq} + E_{cp} / 2) - E_{PermPool}^$

From the stage-storage table, find the elevation associated with V_cp. The average head on the channel protection outlet can then be calculated as

$^h_{cp,avg} = (E_{cp} - E_{wq}) / 2^$

Again, the required orifice cross sectional area can then be indirectly computed using the orifice equation

$^Q_{cp,avg} = CA_{cp} \sqrt{2gh_{cp,avg}}^$

The diameter of the orifice is then

$^D_{cp} = 2\sqrt{A_{cp} / \pi}^$

The rate of discharge from the channel protection orifice for any head value h_cp on the channel protection outlet can then be computed as

$^Q_{cp} = CA_{cp} \sqrt{2gh_{cp}}^$

The combined flow out of the water quality orifice and channel protection orifice at a given water surface elevation can be computed by adding together the discharges from the two structures, for the head values corresponding to the specified water surface elevation.

• Overbank flood control outlet: an outlet (typically a weir) with its invert at the V_cp pool elevation, designed to release Q_p10 at pre-development rates (recommended).
• Extreme storm control outlet: an outlet with its invert at or slightly above the V_p10 pool elevation, designed to release Q_p100 at pre-development rates (recommended), or at minimum to safely pass the Q_p100 with 1 to 2 feet of freeboard below the top of the embankment. Check with local officials to determine whether a principal spillway can be used to manage extreme storm flows, or if an emergency spillway (broad-crested weir or earthen embankment, not susceptible to obstruction) is necessary.