Schematic illustrating five different 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.

A review of key stormwater sizing terminology.

Term Definition
Better Site Design (BSD) Better site design refers to the application of non-structural practices at new development sites to reduce site impervious cover, conserve natural areas, and use pervious areas to more effectively treat stormwater runoff. Also know as low impact development.
Channel Protection (VcpVolume) Refers to the recommended runoff storage volume needed to control post-development bankfull flow velocities so they do not increase erosion in downstream channels. typically, detention and/or extended detention of intermediate sized storms (0.5 to 2.0 inches of rainfall) are used for this purpose. The channel protection volume is denoted as Vcp.
Design Storm An engineering term for a single rainfall event with a defined intensity, duration and statistical recurrence interval commonly ranging from 0.5 to 100 years. These single event storms are based on long-term rainfall data and are used in hydrologic models to predict the peak discharges and runoff volumes associated with each type of storm. Unless otherwise indicated, all design storms discussed in the Manual has a 24-hour duration and a Type II distribution.
Detention Time Various definitions for detention time exist in hydraulic manuals and in help screens on computer models. For this Manual, a simple method of computing detention time is recommended. Detention time is equal to the length of time starting at basin full (for a specific design storm) and ending either when the basin is dry (filtration or infiltration) or the basin has attained normal water level (stormwater ponds or constructed wetlands).
Extreme Storm Volume (Vp100 The greatest runoff storage volume is used to the peak discharges of infrequent but very large storm events to pre-development levels. The 100-year design storm, which has a statistical recurrence interval of occurring once in one hundred years, is used by most communities. Extreme floods can cause catastrophic damagae and even loss of life. The storage volume needed to store and detain them is denoted as Vp100. Note that stormes more "extreme" than the 100-year event do occur in Minnesota. The extreme term is used relative to other volume terms for perspective.
Hydrologic Soil Group (HSG) HSG is an USDA-NRCS designation given to different soil types to reflect their relative surface permeability and infiltrative capability. Group A soils have low runoff potential and high infiltration rates. Group B soils have moderate infiltration rates. Group C soils have low infiltration rates. Group D soils have high runoff potential with very low infiltration rates and consist chiefly of clay soils (TR-55, 1986).
Other Sensitive Receiving Waters In addition to special waters defined in the CGP, there are other receiving waters that merit additional management attention because of their sensitivity, as defined by various state and local entities. Recommended stormwater criteria exist for
• Lakes (most sensitive and sensitive)
• Wetlands (susceptible and non-susceptible)
• Drinking water source areas (ground and surface water)
• Impaired waters (computable and non-computable.
Overbank Flood Volume (Vp10) Refers to the runoff storage volume needed to prevent an increased frequency of floods that spill out of the channel and onto the floodplain where they may cause damage to the conveyance systems, property and infrastructure. Overbank flooding is normally controlled by detention of post-development 10-year storm so that pre-development peak discharge rates (as defined by state or local agencies) are maintained, and is denoted by Vp10 (assuming the local review authority requires control of the 10-year storm event).
Permanent Pool Volume (Vpp) The CGP requires that all wet sedimentation basins contain a permananet pool with a volume of 1,800 cubic feet of storage for each acre that drains to the basin. This equates to 1/2 inch of runoff per acre. The permanent pool must reach a minimum depth of three feet, stay below 10 feet and be configured to minimize scour and resuspension of solids.
Pre-Development Conditions The term pre-development conditions can be interpreted in many different ways. The MPCA uses land cover conditions immediately preceding the current development project as the CGP pre-development condition, whereas many other local and watershed managers use a more natural definition, such as meadow or woodlands in good shape (stated in a manner to help in CN selection). Obviously the CGP version will usually result in a smaller net runoff increase for most land that has anything but a natural cover. It is recommended that the CGP definition be used for its intended purpose as part of state permit issuance, but that the more natural pre-development condition be used if a stormwater manager wants to assume a more conservative condition.
Recharge Volume (Vre) Refers to the recommended volume of runoff which should be spread over pervious areas and otherwise infiltrated into the soil to promote groundwater recharge. The recharge volume is denoted as Vre and is normally included as part of the water quality volume.
Special Waters A list of eight categories of receiving waters are specifically designated as "special waters" in the CGP. Additional BMPs and enhanced runoff controls are required for discharges to the following special waters:
• Wilderness areas
• Mississippi River (Lake Itasca through Morrison County)
• Scenic or recreation river segments
• lake Superior
• Lake trout lakes
• Trout lakes
• Scientific and natural areas
• Trout streams
Total Storage Volume (Vts) For ponds built under the requirements of the CGP, the total storage volume required is the sum of the permanent pool volume (Vpp) plus the water quality volume (Vwq).
Water Quality Volume (Vwq) Generic term for the storage volume used to capture, treat and remove pollutants in stormwater runoff. It is normally expressed as a volume (watershed-inches or acre-feet) and is denoted by Vwq. For ponds and wetlands, the MPCA CGP defines it as the volume of live storage above the permanent pool (above the dead storage) used for water quality. For non-pond BMPs, MPCA defines the water quality volume in the same manner as the general definition above.

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 found at this link
Schematic plan drawing illustrating pond storage allocations. Allocations include the permanent pool (Vpp), water quality volume (Vwq), channel protection volume (Vcp), overbank flood volume (Vp10), and extreme flood volume(Vp100). A typical pond plan and profile CADD file exists for this layout.

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 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.

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.

Overview of unified stormwater sizing criteria

Target Current State or Local Sizing Criteria
Recharge (Vre) or Infiltration

Regular Waters: None required, although recharge and infiltration are strongly encouraged through better site design and stormwater credits.

CGP Special Waters: Full infiltration of the excess runoff volume from the two year-24 hour storm event is one option contained in the CGP to meet the requirement for temperature control for trout streams.

Other Sensitive Receiving Waters: Recharge recommended as a local option.

Recharge may be restricted or prohibited in certain settings, as outlined in the Recharge and Infiltration Criterion below.
Water Quality (Vwq)

Regular Waters: Apply MPCA Sizing Rule 1 or 3 in 2003 GCP, depending on BMP used.

CGP Special Waters: Apply MPCA Sizing Rule 2 or 4 in 2003 GCP, depending on BMP used. Additional BMPs required under CGP Appendix A (B) for special waters.

Other Sensitive Receiving Waters: Recommend expanded sizing criteria for most-sensitive lakes, trout streams (in CGP), susceptible wetlands and impaired waters, as described later in this section.
Channel Protection (Vcp)

Regular Waters: No current state requirement. It is recommended that communities adopt a criterion for either 24-hour extended detention of the 1-year, 24-hour design storm or one-half of the 2-yr, 24-hr pre-development peak flow when revising or adopting local stormwater ordinances for peak flow control (and eliminate two-year peak discharge requirements).

CGP Special Waters: One- and two-year design storm peak discharge and volume control required in four special water categories (wilderness, trout lakes, lake trout lakes, and scientific and natural areas).

Other Sensitive Receiving Waters: 12-hour detention of water is recommended as the most for discharge to trout streams (to prevent heating), while other sensitive receiving waters should maintain the 24-hour minimum.
Overbank Flood (Vp10)

Current Local Requirement: Varies, but typically involves either 10-year design storm peak discharge control, or a combination of 10- and 25-year design storm peak discharge control.

CGP Special Waters and Other Sensitive Receiving Waters: No additional requirements.
Extreme Storm (Vp100)

Current Local Requirement: Varies, but frequently involves 100-year design storm peak discharge control. In other cases, peak discharge control is waived if development is excluded from ultimate 100-year floodplain, or an acceptable downstream hydrologic analysis indicates it is not needed. Stormwater BMPs must be designed to provide safe overflow of the 100 year-peak discharge even if extreme storm control is not required at the site.

CGP Special Waters and Other Sensitive Receiving Waters: No additional requirements.

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

Assuming an orifice is also used to release Vcp, the invert of the Vcp orifice may be placed at the Vwq pool elevation (Ewq).

The average release rate for Vcp is computed as

$Q_{cp_avg} = (V_{cp} - V_{wq} / t_{cp}) - Q_{wq}$

where:

tcp = the intended channel protection volume detention time
Qwq= computed (using the above equation for Qwq), 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 Vcp. 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 hcp 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 Vcp pool elevation, designed to release Qp10 at pre-development rates (recommended).
• Extreme storm control outlet: an outlet with its invert at or slightly above the Vp10 pool elevation, designed to release Qp100 at pre-development rates (recommended), or at minimum to safely pass the Qp100 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.

The intent of this sizing criterion is to maintain groundwater recharge rates at development sites to preserve existing water table elevations and support natural flows in streams and wetlands. Under natural conditions, the amount of recharge that occurs at a site is a function of slope, soil type, vegetative cover, precipitation and evapotranspiration. Sites with natural ground cover, such as forest and meadow, typically exhibit higher recharge rates, less runoff and greater transpiration losses than sites dominated by impervious cover. Since development increases impervious cover, a net decrease in recharge rates is inevitable.

Recharge and/or infiltration criteria offer additional benefits, as they promote more on-site infiltration/filtration of stormwater runoff, and enable communities to offer stormwater credits that reduce the water quality storage volume. Recharge credits provide real incentives to apply better site design techniques at development sites that can reduce the size and cost of stormwater BMPs needed at some sites. To maximize recharge, designers should explore how to use pervious areas for infiltration early in the site layout process.

The recharge volume is considered to be part of the total water quality volume provided at a site and is not an additional CGP requirement (e.g., Vre is contained within Vwq). Recharge can be achieved either by a structural BMP (e.g., infiltration, bioretention, and filter), better site design techniques, or a combination of both.

There are currently no statewide recharge sizing requirements for regular waters in the state of Minnesota, although previous stormwater guidance has strongly promoted recharge and infiltration (MPCA, 2000 and MC, 2001). Also, infiltration can be used as one way to meet the state CGP requirement for permanent stormwater management. Recharge and infiltration are strongly encouraged through better site design and stormwater credits. There are three readily available options for how a community or stormwater manager could determine the amount of runoff to include as the Rev factor.

• Regular Waters: None required, although recharge and infiltration are strongly encouraged through better site design and stormwater credits.
• CGP Special Waters: Full infiltration of the excess runoff volume from the two year-24 hour storm event is one option contained in the CGP to meet the requirement for temperature control for trout streams.
• Other Sensitive Receiving Waters: Recharge recommended as a local option.Note that recharge may be restricted or prohibited in certain settings

Since there are no current required state infiltration requirements, any of the three approaches could be used. Stormwater managers are cautioned to review local conditions and select a method according to what can logically be expected, keeping in mind that the goal is to match pre-development volumes of infiltration as closely as possible in most cases. However, if the potential for using site development to enhance or increase local infiltration exists, for example through an infiltration basin, managers might choose another method to increase infiltration expectations.

### Modifications to recharge criteria

Infiltration and recharge of polluted stormwater runoff is not always desirable or even possible at some development sites. Therefore, most communities qualify their recharge and/or infiltration requirements to reflect special site conditions, protect groundwater quality, and avoid common nuisance issues. For example, the local review authority may require

• some form of special pre-treatment of stormwater runoff prior to infiltration for some land cover types and pollution source areas (e.g. parking lots);
• recharge be restricted or prohibited at specific industrial, commercial and transport-related operations designated as potential stormwater hotspots (PSH);
• recharge be prohibited or otherwise restricted within the vicinity of ground water aquifers, wellhead protection areas, individual wells, structures and basins;
• recharge be discouraged within certain geological zones such as active karst and bedrock, and adjacent to steep or unstable slopes; and/or
• recharge be reduced or waived for minor redevelopment projects that have previously compacted soil structures.

Treatment of stormwater runoff is needed to meet in-stream water quality standards and protect aquatic life and water resources. Extensive monitoring has revealed high concentrations of sediments, nutrients, bacteria, metals, oxygen-demanding substances, hydrocarbons and other pollutants in untreated stormwater runoff (Pitt et al., 2004) and demonstrated their impact on stream and lake quality (CWP, 1999 and CWP, 2003). A range of BMPs can provide a high degree of removal for stormwater pollutants (ASCE, 2004 and Winer, 2001). The 2000 state manual (MPCA, 2000) established a performance goal that BMPs provide a minimum degree of pollutant removal for a defined fraction of stormwater runoff events, which has been operationally defined as 90 percent sediment removal. A 50 percent total phosphorus removal can be assured to accompany this removal. Parts of the state CGP reference the 80 percent Total Suspended Solids (TSS) standard.

## Computing water quality volume

Designers in the state have traditionally relied on ponds for water quality treatment. The water quality volume is given by

$V_{wq} = 3630 IC$

where

Vwq = required water quality volume live storage in acre feet;
IC = new site impervious cover, in acres; and
3630 = conversion factor (to cubic feet).

The minimum pre-treatment volume recommended (not required in CGP) to protect non-pond BMPs from clogging and increase their longevity is 0.20 watershed inches.

## Modifications to water quality criteria

Most communities do not allow many exemptions to their basic water quality sizing criteria, although they may choose to reduce or exempt certain redevelopment and infill projects.

Water quality sizing criteria can be modified upward or downward. The first occurs when stormwater credits are offered to reduce water quality sizing when acceptable better site design techniques are applied on the site. The second occurs when sizing criteria are increased to provide an enhanced level of treatment to protect special waters, such as a nutrient sensitive lake or when local criteria exceed the state minimum.

Example of channel erosion along the Vermillion River.

The purpose of channel protection criteria is to prevent habitat degradation and erosion in urban streams caused by an increased frequency of bankfull and sub-bankfull stormwater flows. Channel protection criteria seek to minimize downstream channel enlargement and incision that is a common consequence of urbanization (Schueler and Brown, 2004). As fields and forests are converted to impervious surfaces, the volume and frequency of runoff is increased significantly. Research indicates that urbanization causes channels to expand two- to ten-times their original size to adjust to the increased volume and frequency of runoff caused by impervious cover, as well as the increased conveyance efficiency of curbs, gutters and storm drains (CWP, 2003 and 2004).

Urban stream channel enlargement significantly impacts stream habitat, water quality and public infrastructure. Bank erosion sharply increases total annual sediment yield and nutrient loads as nutrient-rich floodplain soils are eroded and transported downstream. In addition, channel erosion degrades and simplifies stream habitat structure, which diminishes aquatic biodiversity. Lastly, channel erosion can cause severe damage to bridge, culvert and sewer infrastructure and loss of private property.

Historically, two-year peak discharge control has been the most widely applied local criteria to control channel erosion in Minnesota, and many communities continue to use it today. More specifically, two-year peak control seeks to keep the post-development peak discharge rate for the 2-year, 24-hour design storm at pre-development rates. The reasoning behind this criterion is that the bankfull discharge for most streams has a recurrence interval of between 1 and 2 years, with approximately 1.5 years as the most prevalent (Leopold, 1964 and 1994), and maintaining this discharge rate should act to prevent downstream erosion.

Recent research, however, indicates that two-year peak discharge control does not protect channels from downstream erosion and may actually contribute to erosion since banks are exposed to a longer duration of erosive bankfull and sub-bankful events (MacRae, 1993 and 1996, McCuen and Moglen, 1988). Consequently, two-year peak discharge control may have some value for overbank flood control, but is not effective as a channel protection criterion, since it may actually reach peak flow that is too high and extend the duration of erosive velocities in the stream and increase downstream channel erosion. Communities may wish to drop two-year peak discharge control if they also need 10-year peak discharge control for overbank flooding when they adopt or revise stormwater ordinances as part of NPDES Phase II MS4 permits.

There are currently no state requirements to provide channel protection for regular waters, although a few local reviewing authorities have recently adopted channel protection criteria. Communities are encouraged to adopt new channel protection criteria (and eliminate two-year peak discharge control requirements) when they revise or adopt local stormwater ordinances to comply with municipal NPDES permits.

The state construction General Permit (CGP) references an “enhanced runoff control” criterion wherein the post-development runoff rate and volume need to be maintained for both the one-year 24-hour and two-year 24-hour storm. This criterion could have some channel protection applicability. The criterion only applies to a very restricted subset of special waters designated in the CGP permit, including

• wilderness areas;
• trout lakes;
• lake trout lakes; and
• scientific and natural areas.

The options suggested in the CGP for controlling temperature could also help reduce channel volume by infiltrating or otherwise controlling discharges to trout streams.

The recommended channel protection criteria described next should satisfy the CGP channel protection criterion. Channel protection is also highly recommended for trout streams and certain discharge situations to lakes and wetlands.

The recommended channel protection criterion is to provide 24 hours of extended detention for the runoff generated from the 1-year 24-hour design storm. This runoff volume generated is stored and gradually released over a 24-hour period so that critical erosive velocities in downstream channels are not exceeded over the entire storm hydrograph. As a very rough rule of thumb, the storage capacity needed to provide channel protection is about 60 to 65 percent of the one-year storm runoff volume. The rainfall depth for the one-year, 24-hour storm varies across the State, but can be inferred from intensity-duration-frequency [IDF] curves. The one-year, 24-hour rainfalls range between 1.8 and 2.5 inches across the State of Minnesota. Maximum extended detention time should be limited to 12 hours in trout streams to minimize stream warming, provided erosive velocities can be avoided. Channel protection has recently been adopted by the States of Maryland, New York, Vermont, and Georgia, and is relatively easy to compute at most development sites using existing hydrologic design models. The recommended criterion reduces the magnitude and duration of highly erosive stream channel velocities which should help to reduce downstream channel erosion.

For those wishing more in-depth analysis of channel protection flows, the following option could be used. The State of Washington (2004) has adopted an extremely stringent channel protection criterion that requires the duration of post-development peak stormwater discharges match pre-development durations for the entire range of storms between 50 percent of the one-year storm and the 50-year event. Designers must use a continuous hydrologic simulation model (HSPF, e.g.) to demonstrate compliance. As of 2004, hydrologic models such as TR-55 or P-8 that employ single event design storms are no longer allowed in Washington for design purposes. The goal of the peak duration matching criteria is to exactly replicate the pre-development frequency of peak discharge rates for all storm events that should provide a high level of channel protection. This approach was considered by the Manual Sub-Committee, but recommended only as an option because of the high cost of implementation.

Finally, a special study was undertaken in the Manual Version 2.0 revisions to look at the potential for cumulative peak flows released from detention BMPs to occur in an urban watershed. The parameters of the study focused on a 450 acre developing watershed within an urban area and the placement of detention BMPs in five sub-watersheds scattered within the larger basin. The results showed that when the design specifics contained in the Minnesota Stormwater Manual are used, there will not be an elevated cumulative flow that results. Further analysis showed that for this condition to result in the test watershed, releases from BMPs would have to occur before the passage of the overall watershed peak flow, a situation that would not occur under the Manual design. Some caution should be exercised when considering the likelihood of cumulative flows for large watersheds, where there might be a primary peak flow from near-source areas, followed by a secondary peak as the rest of the watershed contributes. In larger watershed situations, similar modeling for near, medium and far sub-watershed inflows should be undertaken.

As part of Version 2.0 revisions to the Manual, a special study was undertaken to also evaluate a rate control recommendation from the MPCA’s Protecting Water Quality in Urban Areas (2000). This document recommmneds using ½ of the peak runoff rate from the 2-year, 24-hour pre-development event. An analysis of this showed that it does, in fact, result in slightly less erosion potential than the 1-year, 24-hour extended detention noted above under worst case wet years. Use of either method should result in adequate channel protection.

This review also resulted in a recommendation that the method of considering instantaneous inflow of the entire runoff volume into the BMP facility not be used for purposes of routing the channel protection volume. Routing or modeling of inflow and outflow gives a more accurate representation of actual flow. The instantaneous volume method will, however, continue to be used by the state for implementation of water quality storage requirements under the construction General Permit. The instantaneous volume method can also be used as a first-cut estimation of the maximum amount of channel protection storage volume that will be needed since it will give a conservatively high estimate of this volume.

## Estimating channel protection storage volume

This discussion provides an easy method of determining the maximum amount of storage needed for the channel protection volume. This approach should only be used to determine that maximum volume since it results in a conservative volume estimate.

### Channel protection method for ponds

Step 1: Compute the runoff volume produced from the post-development 1-year, 24-hour design storm event, using TR-55, or equivalent.
Step 2: Assume the pond volume above the permanent pool instantaneously fills up. Note again that this will result in an estimate of the maximum volume needed and should only be used as a conservative preliminary estimate. Determine storage volume and Vcp maximum invert elevation using the short-cut method (described below).
Step 3: :Set the Vcp orifice invert at the permanent pool elevation and size initial diameter to drain the entire Vcp volume in 24 hours
Step 4: Compute the average peak discharge rate for the Vcp event (i.e., Vcp/24)
Step 5: Using TR-20, or equivalent, route the runoff through the pond and check to make sure that peak discharge for Vcp does not exceed twice the average discharge.
Step 6: Evaluate whether Vcp meets water quality (live storage) requirements.
• 6a. Compute the runoff volume for live storage using appropriate MPCA pond sizing equation.
• 6b. Assume live storage volume instantaneously fills above the permanent pool. The approach of instantaneously filling storage results in an estimate of the maximum storage volume; that is, no outflow is assumed during the filling process.
• 6c. Using TR-20, or equivalent, route the live storage volume through the channel protection orifice to ensure it satisfies the 5.66 cubic feet per second per surface-acre-of-the-full-water-quality-pool release.
• 6d. If the answer is YES (as it will be in most cases), the Vcp will be considered to meet the live storage requirement for water quality (Vcp can be substituted for live storage volume).
• 6e. If the answer is NO (which may rarely happen for very low density development sites in sensitive waters), then designers gradually reduce the Vcp orifice to meet minimum Vwq release requirements, and repeat steps 3 to 6. If new answer is YES, then Vcp will again be considered to meet live-storage water quality requirement.
• 6f. If answer is NO, then the designer must account for Vwq and Vcp separately, and designed using two separate orifices.
Step 7 (optional): For areas with 1-year and 2-year peak controls use TR-20 or equivalent to route the 2-year storm through the Vcp orifice. Check 2-year peak rate of discharge. If discharge is equal to or less than pre-development discharge rates, then outlet sizing is complete. If not, then raise the elevation of the Vp10 to an elevation that fully contains the volume of the 2-year storm below the Vp10 orifice and re-route the flows as a check. (This is rarely needed since two-year peak discharge control is usually waived when Vcp is provided).
Unit peak discharge Type 2 rainfall distribution
Detention time vs discharge ratios
Approximate detention basin routing

### Channel protection method for non-pond BMPs

Note: No permanent pool is involved in design (although a micropool is recommended to keep orifice from clogging).

In most cases, an extended detention basin is incorporated into the treatment train after the non-pond BMP to provide storage for channel protection and flood control design storms. In some cases, temporary channel protection storage can be incorporated into the non-pond BMP if adequate pretreatment is provided, and if an outlet structure is sized to ensure that the infiltration or filtration basins are empty 48 hours after the storm (e.g., infiltration basin or bioretention in the pond floor).

Channel protection within the extended detention pond is designed following Steps 1 to 5, as outlined above.

• It is strongly recommended that two-year peak discharge control be waived when channel protection is provided.
• The effect of channel protection on total required volume of stormwater storage is modest when the local reviewing authority already requires 50- or 100-year peak discharge control.

### Shortcut sizing for channel protection

This method presents the TR-55 “short-cut” sizing technique, used to size practices designed for extended detention, slightly modified to incorporate the flows necessary to provide channel protection.

#### Storage volume estimation

This section presents a modified version of the TR-55 (NRCS, 1986) shortcut sizing approach. The method was modified by Harrington (1987) for applications where the peak discharge is very small compared with the uncontrolled discharge. This often occurs in the 1-year, 24-hour detention sizing.

Using TR-55 guidance, the unit peak discharge (qu) can be determined based on the curve number and time of concentration. Knowing qu and T (extended detention time), qo/ qi (peak outflow discharge/peak inflow discharge) can be estimated from a plot of detention time versus discharge ratios (see figures on right).

Then using q0/qi, we can estimate Vs/Vr (see figures on right). For a Type II or Type III rainfall distribution, Vs/Vr can also be calculated using the following equation

$V_s/V_r = 0.682 – (1.43 (q_o/q_i)) + (1.64 (q_o/q_i)^2) – (0.804 (q_o/q_i)^3)$

where

Vs = required storage volume (acre-feet);
Vr = runoff volume (acre-feet);
qo = peak outflow discharge (cfs); and
qi = peak inflow discharge (cfs).

The required storage volume can then be calculated by

$V_s = (V_s/V_r)(Q_d)(A) / 12$

where:

VSubscript text and Vr are defined above;
Qd = the developed runoff for the design storm (inches); and
A = total drainage area (acres).

## Channel protection criteria evaluation

The Channel Protection method currently recommended in version 1.1 of the Minnesota Stormwater Manual (MN Manual) for the design of wet detention ponds is to detain the post-development runoff from a 1-year, 24-hour storm event to be released over a 24-hour period. Another channel protection method recommended in MPCA’s (2000) Protection of Water Quality In Urban Areas and applied in Washington state suggests wet-detention pond discharge at ½ of the peak discharge from the pre-development 2-year, 24-hour storm event.

The two design methods are compared to evaluate their respective effectiveness. If warranted, updates to the Minnesota Stormwater Manual will be recommended based on the results of the analysis.

### Methodology

A wet-detention pond for analysis of the two channel protection criteria was sized for a generic but typical development, consistent with the requirements of the Minnesota Construction General Permit (CGP). The generic subwatershed characteristics typical of an urban area development and used in the modeling exercise are defined in the table below.

Subwatershed and channel characteristics

Status Total area (ac) Sub-area (ac) Land Use Channel Width (feet) Slope % Capillary Suction (inch) Initial Soil Deficit Saturated Hydraulic Conductivity (Inch/hr) Depression Storage (Inch)
Pre-development 50 - Agriculture - undeveloped mix 400 0.01 8 0.3 0.15 0.05
Post-development 50 - Low to medium density residential 700 0.01 8 0.3 0.15 -
Pervious 50 30 Low to medium density residential 700 0.01 8 0.3 0.15 0.05
Impervious (-40%) 50 20 Low to medium density residential 700 0.01 8 0.3 0.15 0.3

Wet detention pond sizing was performed in accordance with the methodology outlined in the MN Manual (version 1.1, revised in 2006). A total of four wet-detention ponds were designed using the following design events:

1. 1-yr 24-hr Instantaneous volume1 design (K124)
2. 1-yr 24-hr routed2 design (R124)
3. ½ the pre-development 2-yr 24-hr runoff peak Instantaneous volume design (K224)
4. ½ the pre-development 2-yr 24-hr runoff peak routed design (R224)

1This refers to the simultaneous introduction of the entire runoff volume into a pool at once.
2”Routed” through the use of a detailed hydrologic model, such as XP-SWMM which was used in this exercise

Applying the sizing criteria outlined in the manual, the water quality, channel protection, over bank flood protection, and the 100-year extreme flood protection volumes were calculated. The tables below summarize the watershed runoff response and the resulting uniform sizing volumes for pre and post-development conditions.

Watershed runoff characteristics

Given:
Area = 50 ac. total watershed area in acres draining to pond
IC = 20 ac. new impervious area in acres (40%)
Watershed peak discharge and runoff volume:
Event Pre-development Post-development Unit
Q1-yr 24-hr 6.7 53.5 cfs
Q2-yr 24-hr 11.6 69.0 cfs
Q10-yr 24-hr 36.7 131.9 cfs
Q100-yr 24-hr 81.8 222.0 cfs
V1-yr 24-hr 0.3 3.9 ac-ft
V2-yr 24-hr 0.6 4.9 ac-ft
V10-yr 24-hr 3.1 8.9 ac-ft
V100-yr 24-hr 7.5 14.7 ac-ft

Pond design sizing criteria

Event Pre-development Post-development Unit
Vpp 2.1 ac-ft Permanent pool volume (0.5" X 50 acres)
Vwq 0.8 ac-ft Water quality volume (0.5" X 50 acres)
Vts 2.9 ac-ft Total storage volume (Vpp + Vwq
Vcp 3.9 ac-ft Channel protection volume (includes water quality volume)
V10 8.9 ac-ft Over bank flood protection volume
V100 14.7 ac-ft Extreme flood volume

The pond sizing methodology applied for evaluation of ½ the pre-development 2-year 24-hour event was similar (applied the same water quality, over bank protection, and extreme event volume criteria) with the exception of the channel protection outlet. The channel protection outlet was sized to restrict release of the 2-year 24-hour event to ½ the pre-development peak discharge from the 2-year 24-hour storm event.

The design calculations resulted in the pond stage and storage shown in the table below.

Pond stage / storage

Elevation Side slope height per 1" vertical Area Incremental volume Cumulative volume Volume above permanent pool Voume requirement / treatment level
Feet feet acres ac-ft ac-ft ac-ft
100 3 0.333 N/A 0 N/A
101 3 0.363 0.3 0.3 N/A
102 3 0.394 0.4 0.7 N/A
103 3 0.427 0.4 1.1 N/A
104 10 0.461 0.4 1.6 N/A
105 10 0.583 0.5 2.1 N/A Vpp 2.1 ac-ft
106.3 3 0.764 0.9 3.0 0.9 Vwq 2.1 ac-ft
107 3 0.796 0.5 3.5 1.4 Vts 2.9 ac-ft
109.9 3 0.933 2.5 6.0 3.9 Vcp 3.9 ac-ft
112 3 1.039 2.1 8.1 6.0
114.7 10 1.184 3.0 11.1 9.0 V10 8.9 ac-ft
116 10 1.437 1.7 12.8 10.7
118.4 1.967 4.1 16.9 14.8 V100 14.7 ac-ft

Test channel grain size distribution.
Downstream channel cross section schematic.

The scenarios presented for the Channel Protection Volume exceed the base CGP requirement for Water Quality Volume. The discharges from the four scenarios were checked to verify that they all met the CGP requirement of not more than 5.66 cubic feet per second (cfs) per surface acre of the full Water Quality pool. This criterion applies to the drainage into the Water Quality pool, which equates to the runoff from ½ inches over the entire drainage basin, plus ½ inches from the new 20 acre impervious area for non-Special Waters of the State. The table below reflects the runoff from the 1-year (2.4 inches) and 2-year (2.8 inches) rainfall events, which are substantially more than the Water Quality pool sizing criteria, hence the values exceeding the base requirement (5.66 cfs x 0.9 acres from the bottom of the 10-yr pool in Wet detention pond - Over bank protection (10-year 24-hour) outlet configuration table = 5.1 cfs).

The relative channel erosive potential of the four wet detention ponds resulting from the 1-year and ½ the 2-year design criteria were evaluated for a wet season. The year 2002 hourly precipitation from Flying Cloud Airport, CITY, was simulated in XP-SWMM to represent the pond performance in a wet year.

Discharge from the XP-SWMM 2002 continuous simulation became input into the erosion model CONCEPTS (Conservation Channel Evolution and Pollutant Transport System, U.S. Department of Agriculture, Agricultural Research Service, Research Report No. 16, E.J. Langendoen, 2000). The CONCEPTS model requires hydraulic, soils, and channel data for a simulation. The output from the XP-SWMM model was converted to cubic meters per second and put into the proper format for CONCEPTS. The bed and bank material was assumed to be: sand with gravel, medium- to fine-grained (SP). The hypothetical channel was designed as a two-stage channel, whereby approximately the 2-year flow was handled by the main channel and the 100-year flow was allowed to flood a secondary bench and channel conveyance area.

Considering the idealized channel cross-section (see figure to right), note that artificial sides from horizontal station 0 to 2 meters and from 8.5 to 10.5 meters were added in the unlikely event the modeled stage exceeded the flood bench elevations. Channel bank side slopes were 1H:1V while the floodplain side slopes were 2H:1V. Three cross sections were modeled at 0, 0.5, and 1 kilometer longitudinal stations at a 0.5 percent slope. Because of the fairly short duration modeling time (May to October, 2002), the channel was modeled to be fairly erodible so that differences in erosion caused by the different flow regimes would be apparent. The only change between the different scenarios modeled in CONCEPTS was the flow input file. The total change in cross-section at the mid-point and the total sediment load over the course of the simulation were examined and are reported in a subsequent section. The model is capable of simulating bank failure, but this feature was not selected so that only bed erosion and sediment transport were simulated to accentuate the downcutting versus side-slope failure aspect of the erosion.

Note that according to the RFP, only the results for the wet year 2002 are reported. This will have some bearing on the conclusions that can be drawn relative to normal and dry years. The wet year scenario is considered a worst case situation.

### Results/Discussion

Overall, differences in the four pond designs are noticeable between the “routed” design and the Instantaneous volume design methods. In all cases, the Instantaneous volume design volumes will be greater in size because they involve input of the entire runoff volume at once without outflow during accumulation. The routed option includes continually outflowing water concurrent with the runoff volume filling the pond. This difference contributes, for example, to the 1-year Instantaneous volume having a higher discharge than the routed based on a less constricted orifice to pass the stored volume. For the ½ 2-yr scenario, the outflow orifice is adjusted to meet the peak matching requirement, so the orifice sizes are different but the peak discharge values are the same (5.8 cfs or ½ of 11.6 cfs pre-development 2-year peak).

The Wet detention pond - Over bank and extreme flood protection (10-year 24-hour) outlet configuration tables (shown below) are included to show the progression of design steps as the complete pond is modeled. The most critical finding of this exercise is that the Instantaneous volume method for both the 1-year and ½ 2-year scenarios results in twice the size of pond than the routed method at more than 6 feet greater HWL. This is simply because of the more realistic approach that the routed method uses to release water as the pond is filling rather than assuming that all of the water collects before any discharge occurs. Since most designs done today use sophisticated computer programs, such as XP-SWMM, language in the updated Manual should recommend that routed methods be used to more realistically portray pond design and behavior. If a conservative answer is desired, the Instantaneous volume method could be selected

Wet detention pond - Channel protection / water quality outlet configuration

Design methodology
1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) 1/2 Pre-development 2-yr 24-hr peak Instantaneous volume 1/2 Pre-development 2-yr 24-hr peak routed (R224)
Orifice diameter (inches) 10.5 6.9 10.0 11.6
Oriface elevation (ft) 105.0 105.0 105.0 105.0
Surface area at orifice elevation (ac) 0.6 0.6 0.6 0.6
Peak discharge per design event (cfs) 6.4 2.1 5.8 5.8
Peak water elevation per design event (ft) 107.9 108.2 108.7 108.5

Note that the routed method for the 1-year design (R124) is much less than the Instantaneous volume due to the more constricted orifice.

Wet detention pond - Over bank protection (10-yr 24-hr) outlet configuration

Design methodology
1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) 1/2 Pre-development 2-yr 24-hr peak Instantaneous volume 1/2 Pre-development 2-yr 24-hr peak routed (R224)
Weir length (ft) 0.8 3.7 0.8 4.8
Weir elevation (ft) 109.2 108.2 109.9 108.6
Surface area at weir elevation (ac) 0.9 0.9 0.9 0.9
Peak discharge per design event (cfs)1 36.7 36.0 36.7 36.4
Peak water elevation per design event (ft) 114.7 110.2 114.77 110.1

1Because the 10-year flow rate is designed to match the pre-development peak, the values in this row should be all less than or equal to 36.7 cfs.

Wet detention pond - Extreme flood (10-yr 24-hr) outlet configuration

Design methodology
1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) 1/2 Pre-development 2-yr 24-hr peak Instantaneous volume) 1/2 Pre-development 2-yr 24-hr peak routed (R224)
Weir length (ft) 3.0 3.9 3.1 4.6
Weir elevation (ft) 114.72 110.2 114.7 110.1
Surface area at weir elevation (ac) 1.2 0.9 1.2 0.9
Peak discharge per design event (cfs)1 81.8 81.5 81.8 81.4
Peak water elevation per design event (ft) 118.4 112.0 118.4 111.8
Designed surface area at HWL (ac) 2.01 1.0 2.0 1.0

1Note that the Instantaneous volume method requires twice the surface area (4 percent vs 2 percent of drainage area) and about 6 more feet of storage depth than the routed method for this design event. This is the result of introducing the entire event volume at once versus the more realistic routing of inflow and outflow.

Graph 1: 1 year 24 hour storm event (2.4 inches)
Graph 2: 2 year 24 hour storm event (2.8 inches)

Graphs 1-4 illustrate the hydrographs for the 1-year, 2-year, 10-year and 100-year events for the routed designs. The Instantaneous volume scenarios are not included because they are not routed. Attention for this task is placed on the smaller events wherein flow generally stays below or close to bank-full. The 10-year and 100-year events will exceed bank-full.

In Graph 1, a horizontal line portrays the pre-development discharge level of 6.7 cubic feet per second (cfs). Both peaks are less than the pre-development peak. The 1-year design scenario begins discharging water as soon as it starts to flow into the pond, peaks at 2.1 cfs and then de-waters in 24 hours after the last inflow occurs. Although the duration of flow extends well beyond that of the ½ 2-yr scenario design, the peak flow is well below half.

Graph 2 illustrates a 2-year event for the routed scenarios and shows how the ½ 2-year requirement is met (1/2 of the pre-development peak of 11.6 cfs = 5.8 cfs). The 1-year design scenario has a higher peak (in this case about 6.5 cfs) because more water flows over the weir due to the constricted orifice; that is, the 1-year design configuration is exceeded. Again for both design scenarios, less than pre-development peak flow is maintained.

Graph 3: 10 year 24 hour storm event (4.2 inches)
Graph 4: 100 year 24 hour storm event (6.0 inches)

Graph 3 shows the behavior from a 10-year event where flow entering the pond greatly exceeds the two design scenario assumptions. In this case, flow is well over the orifices and weirs installed for the 1- and 2-year events. In spite of this, both designs keep peak flow close to pre-development levels. Graph 4 shows similar behavior for the 100-year event.

The tables below present the results of running the four design scenarios as presented in previous tables for the 1-, 2-, 10- and 100-year events, ranging from 2.4 to 6.0 inches. The first table below shows the High Water Level (HWL) that would be reached for each of the design rainfall events. Note that the Instantaneous volume approach would result in a depth of about 2 feet greater than the routed methods. The second table displays the peak discharge rates that result from the four design scenarios. The higher rates for the R124 design for the events greater than 1-year are again reflecting the constricted orifice and the movement of more water over the weir as a result. In all cases in this table, flow rates are less than or nearly equal to pre-development conditions.

The current Manual recommendation for the 1-year 24-hour channel protection criteria resulted in the most restrictive outlet for the “routed” option. This results in the lowest peak discharge for the 1-year 24-hour design event (2.1 cfs), but in a slight exceedance for the 2-year event. Peak discharges for the larger design events (2-year 24-hour and greater) increase for the same pond. The tail of the hydrograph for this design was longer than the ½ 2-year design for both the 1-year and 2-year events (Graphs 1 and 2). The implication of this behavior for downstream channel erosion is discussed in a later section.

Generally, the benefit of the reducing peak flow below the pre-development level is that it can bring flow below the shear stress level of some streams, and in these modeled cases, brings flow below pre-development conditions, which was the goal expressed in the current Manual. However, the actual benefit needs to be determined on a case-by-case basis depending upon factors such as soil type, channel slope and flow velocity. That is, either of the control options (1-year or ½ 2-year) produces the desired result of lower peak, but exposes the base channel to higher flow than pre-development because of the slower release of stored water.

High water level (feet)

Elevation (ft) in pond based on different outlet configurations
Design rainfall event 1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) Development 2-yr 24-hr peak Instantaneous volume Development 2-yr 24-hr peak routed (R224)
1-yr 24-hr (2.4 inches) 107.9 108.2 107.9 107.8
2-yr 24-hr (2.8 inches) 108.7 108.7 108.7 108.5
10-yr 24-hr (4.2 inches) 111.3 110.2 111.3 110.1
100-yr 24-hr (6.0 inches) 113.8 112.0 113.9 111.7

Peak discharge (cfs)

Outflow rate (cfs) based on different outlet configurations
Design rainfall event Pre-development 1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) Development 2-yr 24-hr peak Instantaneous volume Development 2-yr 24-hr peak routed (R224)
1-yr 24-hr (2.4 inches) 6.7 4.22 2.1 3.9 4.9
2-yr 24-hr (2.8 inches) 11.6 4.9 6.7 4.5 5.7
10-yr 24-hr (4.2 inches) 36.7 11.2 36.8 11.0 35.5
100-yr 24-hr (6.0 inches) 81.8 29.1 81.5 29.1 81.4

Peak discharge (cfs) during a wet year (2002)

Design rainfall event Pre-development 1-yr 24-hr Instantaneous volume 1-yr 24-hr routed (R124) Development 2-yr 24-hr peak Instantaneous volume Development 2-yr 24-hr peak routed (R224)
2002 14.4 6.72 19.8 6.5 16.3

Graph 5: Peak discharge during a wet year

The peak discharge from the ponds simulated during the (wet) year 2002 are summarized in the previous table and illustrated in Graph 5. Graph 5 shows that the R124 design consistently exceeds all other design peak discharges for large events (yielding over 5 cfs). In most cases, the R224 design follows at about 70 to 80 percent of R124. This implies that designing for the 1-year option currently contained in the Manual will yield higher peak flow during a wet year than using ½ of the 2-year design. A similar statement cannot be made for a normal or dry year because this analysis was only run for 2002, a wet year.

The Instantaneous volume design scenarios yield markedly lower peak rates, but this approach is overly conservative, not reflective of today’s design practices and results in a much over-sized pond, and is therefore not recommended

It is important to note that the highest pre-development peak (14.4 cfs) is exceeded throughout 2002 by the routed events. The R124 and R224 designs are geared to the smaller (1- to 2-year events) and cannot be expected to keep peak flows at pre-development rates for larger events, particularly during a wet year when available storage might not be available.

### CONCEPTS erosion modeling results

The CONCEPTS erosion prediction model was run based on the output of the wet year 2002 (about 36 inches from March-October at Flying Cloud Airport) runoff modeling. The figures below shows the change in cross-section at the modeled channel midpoint (0.5 kilometer) in response to annual flows during 2002. Note that the channel is small because it is sized to handle only the theoretical modeled watershed for purposes of analyzing the impact of various runoff scenarios on the channel.

Tabular results from three events during the 2002 modeled period are presented in the table below. This table displays three “snapshot” events (2.85 inches, 2.76 inches and 3.63 inches, respectively) and tabulates the entire 2002 period modeled in the total column. Note that the units of flow differ from XP-SWMM based on the CONCEPTS format. Information is presented in the table for peak flow rate, change in the width of channel bottom, change in the depth of the thalweg (stream centerline), sediment yield and water depth.

Tabular results from three events during the 2002 modeled period

Results at 0.5 KM
6/21/2002 8/16/2002 9/5/2002 Total
Pre-Dev Peak flow rate (cms) 0.164 0.197 0.376 -
K124 Peak flow rate (cms) 0.134 0.152 0.262 -
R124 Peak flow rate (cms) 0.422 0.362 0.550 -
K224 Peak flow rate (cms) 0.125 0.140 0.182 -
R224 Peak flow rate (cms) 0.428 0.256 0.662 -
Pre-Dev Change in width (m) 0.000 0.000 0.000 0.000
K124 Change in width (m)) 0.000 0.000 0.000 0.000
R124 Change in width (m) 0.000 0.000 0.000 0.000
K224 Change in width (m) 0.000 0.000 0.000 0.000
R224 Change in width (m) 0.000 0.000 0.000 0.000
Pre-Dev Change in Thalweg depth (m) (net dewnward erosion) -0.002 -0.013 -0.006 -0.022
K124 Change in Thalweg depth (m) (net dewnward erosion) -0.001 -0.002 0.001 -0.039
R124 Change in Thalweg depth (m) (net dewnward erosion) 0.008 -0.006 -0.009 -0.056
K224 Change in Thalweg depth (m) (net dewnward erosion) -0.011 -0.003 -0.006 -0.026
R224 Change in Thalweg depth (m) (net dewnward erosion) -0.003 -0.003 -0.005 -0.037
Pre-Dev Sediment yield (1000 kg) 3.120 4.798 6.163 19.66
K124 Sediment yield (1000 kg) 3.737 1.589 7.473 43.42
R124 Sediment yield (1000 kg) 5.819 5.907 11.260 51.13
K224 Sediment yield (1000 kg) 1.341 1.545 4.331 35.77
R224 Sediment yield (1000 kg) 7.846 5.540 6.959 37.62
Pre-Dev Change in water depth (m) 0.259 0.285 0.368 -
K124 Change in water depth (m) 0.241 0.261 0.287 -
R124 Change in water depth (m) 0.267 0.369 0.430 -
K224 Change in water depth (m) 0.232 0.247 0.282 -
R224 Change in water depth (m) 0.268 0.331 0.411 -

K124 = Instantaneous volume 1 yr, 24 hr
K224 = Instantaneous volume 1/2 2 yr, 24 hr
R124 = Routed 1 yr, 24 hr
R224 = Routed 1/2 2 yr, 24 hr

The “initial” condition (see figure to right) represents the starting point for the modeling year with no erosion. The “pre-development” line shows that some erosion occurs (about 0.02 meters or 0.8 inches) even under pre-development conditions with no development. The routed 1 year, 24 hour (R124) design results in the most channel incision (about 0.06 meters or 2.2 inches) for the scenario modeled, consistent with the highest peak flow noted in the table above and the series of modeled events portrayed in Graph 5. The width of the bottom “cut” is also larger than the other design scenarios by about 0.09 meters (3.5 inches). Also note particularly in Graphs 1 through 4 that R124 extends the duration of flow for those particular precipitation events. The erosion is likely caused by a combination of relatively higher peak flows and extended duration of runoff, resulting in the highest sediment yield of the design options. The table above shows an increase over pre-development sediment yield conditions of 31,470 kilograms (about 31 tons) for the wet year 2002 for the entire modeled 1 kilometer reach.

Changes in cross-section at channel mid-point during 2002
Water depths during June 21, 2002 event of 2.85 inches over 13 hours
Water depths during August 16, 2002 event of 2.76 inches over 7 hours
Water depths during September 5-6, 2002 event of 3.63 inches over 15 hours

The routed ½ 2-yr design (R224) is only slightly less erosive (0.02m or 0.8 inches) than the R124 scenario. The R224 design also results in a narrower bottom cut than the R124 design. The R224 design had the second greatest peak runoff rates for 2002 and the tail of its hydrograph for individual events (Graphs 1 through 4) is generally of shorter duration than the R124 approach. The R224 design results in an increase of 17,960 kilograms (17.7 tons) over pre-development sediment yield.

The Instantaneous volume design options were modeled even though they are not recommended. There is essentially no difference in the Instantaneous volume options over the entire modeled year.

The figures to the right illustrate the water depth changes for the three highlighted events during 2002. Again note that the channels are purposely small to reflect only the results of the modeled watershed and that the horizontal scale changed from the previous figure. All water depths exceeded bank-full flow onto the flood bench.

The 6/21/2002 event portrayed shows essentially the same water depth for the R124 and R224 designs, exceeding bank-full and only slightly higher than pre-development conditions. This event totaled 2.85 inches over a period of 13 hours.

Figures to the right show some differences between the R124 and R224 levels. In both cases, the R124 design results in greater water depth beyond bank-full than the R224 design. The 8/16/2002 event totaled 2.76 inches over 7 hours. This event resulted in the greatest difference (0.04 meters or 1.5 inches) in water depth between R124 and R224. The 9/5-6/2002 event was a 3.63 inches event in two episodes over 15 hours. This event resulted in only a slight difference (0.019 meters or 0.75 inches) between the two routed designs.

The water depth results are consistent with previous findings favoring the ½ 2-year design over the 1-year, 24 hour extended detention design, but not by a large margin. The combination of deeper downward cutting, higher sediment yield and greater water depth for the R124 option tends to favor use of the ½ 2-year pre-development peak matching approach. Consistent with previous findings, the Instantaneous volume designs generally result in reduced erosion because they involve conservative over-design based on the instantaneous need to store the entire event volume. If a conservative approach is desired, the Instantaneous volume approach could be used. However, it is recommended that a routed design approach be used as a better depiction of reality and because techniques available to designers can easily accommodate the added complexity.

### Conclusions

1. The “Instantaneous volume” method of design assumes that the entire runoff event volume is instantaneously present in the pond at the conclusion of the runoff event. This results in a higher than realistic (conservative) volume because no outflow is assumed while the volume collects in the pond. The “routed” method is more reflective of current practice in that it uses simple to sophisticated routing methods to put water into and out of the pond during the event. This approach yields a more realistic pond behavior and should be recommended for the Manual revision. The Instantaneous volume approach is an option when a conservative over-design is desired.
2. The magnitude of peak flow and duration of elevated flow varies among the four design approaches (K124, R124, K224, R224) examined by design event. In general, the differences are most apparent during the 1-year and 2-year storm events and reflect the differences noted above.This is pertinent to the channel protection design which focuses on this range of events. Even though there are differences, all four design scenarios show that peaks will be reduced below pre-development conditions.
3. The current channel protection designs recommended in the Manual (1-year extended detention) and in the MPCA Protecting Water Quality in Urban Areas (1/2 2-year pre-development peak matching) both appear from this analysis to work well at reducing peak flows for the 1- and 2-year rainfall events. The methods also appear to be able to contribute to matching pre-development peaks for the 10- and 100-year events under the modeled conditions. The 1-year extended detention method does appear to result in higher peaks and potentially higher erosion during a modeled wet year, which argues in favor of the ½ 2-year pre-development peak matching approach.
4. The largest downward erosion and greatest water depth (above bankfull) of the modeled channel during the wet modeled year of 2002 occurred for the routed 1-year, 24-hour (R124) design configuration. The routed ½ 2-year, 24-hour configuration (R224) showed less erosion in the channel and lower water depths.
5. Applying either of the two design methodologies using a routing approach should work to keep erosive flows at a minimum during small events (less than 2 inches). When events exceed 2 inches, the hydraulic outlet controls designed for a detention pond become a major factor as water levels rise above the channel protection criteria for design. If control of events more than 2 inches is needed to achieve a goal, adaptations in the outlet to allow more water to leave the facility at lower flows might be needed.
6. The findings of the modeling in this exercise and the literature review at the end confirm that there is no simple or uniform answer when it comes to erosion control on a particular reach of stream. For small events (less than 2 inches), the two approaches evaluated herein are acceptable. The safest approach when erosion is a major concern is to analyze the soil and channel conditions (soil type, shear stress, velocity, water depth, channel slope, sediment load, etc.) and perform a total erosion potential analysis that determines the actual conditions under which erosion will begin, and then design to not exceed those conditions. Several authors (below) seem to validate the theme of the Minnesota Stormwater Manual that runoff volume reduction will result in less erosion downstream of urban areas.

## Modifications to channel protection criteria

There are some practical limitations in applying channel protection criteria to small development sites because orifice diameters or weir sizes become extremely small and are prone to clogging. As a result, it is recommended that localities waive the channel protection requirements at small sites that have less than three acres of impervious cover. Channel protection need not be applied at sites that have a discharge condition that will not likely cause a local channel erosion problem, such as sites that directly discharge to

• fourth order or higher streams and rivers (data on stream order is available from DNR or the USGS from the National Hydrography Dataset.); or
• lakes and reservoirs where the development area is less than 5 percent of the watershed area upstream of the development site.

## Overbank flood protection criteria (Vp10)

The goal of this criterion is to prevent flood damage to conveyance systems and infrastructure and reduce minor flooding caused by overbank floods. Overbank floods are defined as floods which exceed the bankful capacity of the channel and spill over to the floodplain where they can damage property and structures. The key management objective is to protect downstream structures (houses, businesses, culverts, bridge abutments, etc.) from increased flows and velocities from upstream development.

Most local reviewing authorities establish an overbank design storm that is matched with the same design storm used to design open channels, culverts, bridges, and storm drain systems. Most localities in Minnesota require that post-development peak discharge rates from the 10-year and/or 25-year, 24-hour design storm event be controlled to pre-development rates.

In general, the storage volume needed to manage the 25-year return design storm is much greater than the 10-year design storm. Modeling has shown that control of the 10-year storm coupled with control of the 100-year storm effectively attenuates storm frequencies between these two events (e.g., the 25-year storm). Even without attenuation of the 100-year event, 10-year control provides a significant control for the 25-year storm (approximately 70 to 80 percent).

Consequently, most communities across the state have adopted the 10-year design storm control for overbank protection, since it requires less storage volume and provides some de-facto control for the 25-year storm. The choice of what design storm(s) to target for overbank control is always a local decision, and normally depends on whether the 10- or 25-year design storm has historically or currently been used as the basis for the design of conveyance systems and culverts.

## Extreme flood control criteria (Vp100)

The goal of extreme flood criteria is to maintain the boundaries of the pre-development 100-year floodplain, reduce risk to life and property from infrequent but very large floods and protect the physical integrity of a stormwater BMPs and downstream infrastructure.

The accepted design storm to manage extreme storms in most communities in Minnesota is the 100-year, 24-hour event. Designers are required to control the post-development 100-year, 24‑hour peak discharge rate to locally defined pre-development levels. Communities should carefully reassess extreme flood criteria since it requires the largest storage volume and greatest cost of any stormwater sizing criteria.

Communities may elect to waive 100-year peak discharge criteria in certain situations. The most common situation is when they have a buffer or floodplain ordinance that effectively excludes development from ultimate 100-year floodplain. Designers may also need to demonstrate that no downstream structures exist within the 100-year floodplain and that bridges and other infrastructure can safely pass the storm using an acceptable downstream analysis. This approach accomplishes the goal of extreme flood control by protecting the downstream ultimate 100-year floodplain rather than providing expensive upstream storage.

Hydrologists have often noted that extreme flood criteria may not always provide full downstream control from the out-of-bank events, due to differences in timing of individual peak discharges in the downstream portion of the watershed. Depending on the shape and land use of a watershed, it is possible that upstream peak discharge may arrive at the same time a downstream structure is releasing its peak discharge, thus increasing the total discharge. As a result of this “coincident peaks” problem, it is often necessary to evaluate conditions downstream from a site to ensure that effective out-of-bank control is being provided. Hydrologic and hydraulic models that can be used for analysis of downstream effects are provided in Appendix B.

Debo and Reese (1992) proposed the concept of the “10 percent rule” as the point to which a downstream analysis should extend. This is operationally defined as the downstream point where the development site represents 10 percent of the total contributing drainage area of a watershed. They contend that the hydrologic effects of flooding stabilize and remain constant further downstream. A typical downstream analysis will need a hydrologic investigation of the site area draining to a proposed detention facility and of the contributory watershed to the location of the 10 percent rule for the 10- and 100-year storms. As a minimum, the analysis should include the hydrologic and hydraulic effects of all culverts and/or obstructions within the downstream channel and assess whether an increase in water surface elevations will impact existing buildings or other structures. The analysis should compute flow rates and velocities for pre-developed conditions and proposed conditions both with and without the detention facility.

While the 10 percent rule is useful in establishing a limit for assessment, stormwater program managers still have some basic issues that need to be addressed. For example:

• Is a downstream analysis always required?
• Should a downstream analysis be required on a case-by-case basis?
• Is a certain site size threshold required to trigger the analysis?
• What should the analysis include (culverts, channel erosion, flooding, etc.)?
• What data requirements are necessary for the analysis and what methods should be employed?

The following recommendations are provided to help answer these questions.

• A downstream analysis is probably warranted for projects over 50 acres that posses more than 25 percent impervious cover or when deemed appropriate by the reviewing authority when existing conditions are already causing a problem (e.g., known drainage or flooding conditions documented in a regional plan or existing channel erosion is evident).
• A typical downstream analysis will require a hydrologic investigation of the site area draining to a proposed detention facility and of the contributory watershed to the location of the 10 percent rule for the 10- and 100-year storms. A hydraulic analysis of the stream channel below the facility to the location of the 10 percent rule will also be necessary (a HEC-RAS water surface profile analysis, e.g.). Depending on the magnitude of the impact and the specific conditions of the analysis, additional information and data may be necessary such as collecting field run topography, establishing building elevations and culvert sizes or investigating specific drainage concerns or complaints.
• If the hydrologic investigation shows that flow rates and velocities (for Vp10 and Vp100) with the proposed detention facility increase by less than 5 percent from the pre-developed condition, and no existing structures are impacted, then no additional analysis is necessary. If the flow rates and velocities increase by more than 5 percent, then the designer should either redesign the detention structure, evaluate the effects of no detention structure, or propose corrective actions to the impacted downstream areas. Additional investigations may be required by the approving authority on a case-by-case basis depending on the magnitude of the project, the sensitivity of the receiving water resource, or other issues such as past drainage or flooding complaints. Special caution should be employed where the analysis shows that no detention structure is required. Stormwater designers should be able to demonstrate that runoff will not cause downstream flooding within the stream reach to the location defined by the 10 percent rule.

A local community may elect to waive the Vp100 criteria when a development project

• directly discharges to a large reservoir or lake;
• directly discharges to a 4th order or larger stream or river;
• is smaller than five acres in area;
• is a redevelopment or infill project; or
• is within an area where a regional flood model indicates that 100-year control is not needed at a particular site.

Some Minnesota communities base their extreme storm design on a rain-on-snow scenario, rather than a specific design storm approach. Under this scenario, communities may define the effective 100-year event as having as much as 7.2 inches of equivalent rainfall that needs to be controlled to pre-development levels. There is little basis for this approach in Minnesota based on rainfall records or experience, and it clearly results in costly over-control, although some communities chose to continue its use for conservative design in land-locked basins. The State of Minnesota has many different kinds of mandated special watershed and water resource designations that directly influence how stormwater is managed at a site. When these are combined with the even more numerous water resource designations created by localities and watershed organizations (see for example, WCWC, 2003 and EOR, 2000), there is a great deal of potential for overlap and confusion. Indeed, in many regions of the state there can be more area designated and managed for specially protected waters than for regular ones.

This page presents a condensed framework for managing stormwater when these sensitive waters need additional protection. Please note there is a focus on additional stormwater management practices that can be used to supplement protection of sensitive receiving waters. Some of these waters currently have limited protections under state or local programs, while others do not. The material in this section is offered as guidance when further stormwater management is deemed necessary or desirable by state or local decision makers.

In addition to the eight specific “special waters” mentioned in the state CGP, there are several specifically protected waters that may warrant supplemental protection relative to stormwater. These include calcareous fens, all DNR designated Public Waters, many kinds of wetlands, shoreland/floodplain areas, areas with active karst, drinking water source areas, impaired waters and the Mississippi River Critical Area. On-line maps and lists exist to help designers and reviewers determine if their development project is located in a special water of the state.

The many different local and state receiving waters noted above that could be addressed by supplemental stormwater management fall into five basic groups:

• lakes;
• trout resources;
• drinking water source areas;
• wetlands; and
• impaired waters.

Some of these groups may be further divided into management subcategories, as shown in the right column of the table below. These subcategories will each be discussed in greater detail.

Classification of receiving waters used in this manual

Group Name Stormwater Management Subcategory
Lakes Most-Sensitive
Sensitive
Trout Resources Trout Protection
Drinking Water Ground Water
Surface Water
Wetlands Susceptible
Non-Susceptible
Impaired Waters Computable Pollutants
Non-Computable

The table below compares the main stormwater management criteria and considerations for all five groups listed in the above table. The text following the table discusses the details of application for each of the receiving water classes.

Summary of suggested stormwater criteria for MN receiving waters

Group Name Suggested Stormwater Sizing Criteria1
Most-Sensitive Lakes

Recharge (Vre): HIGHLY RECOMMENDED if lake is fully or partially ground water dependent Water Quality (Vwq): Site-Based Phosphorus Load Reduction: To apply this HIGHLY RECOMMENDED criteria for the most sensitive lakes, a manager would compute pre- and post-development phosphorus load at the development site using Simple Method, P8, SLAMM or an equivalent water quality model. A TP load reduction is then calculated for the site based on a range of no change to a 25% reduction in post-development load, if additional load reduction is warranted. Designers document load reduction achieved using a list of BMP removal efficiencies. If on-site compliance is not possible, an offset fee can be charged which is equivalent to cost of removing similar mass of phosphorus elsewhere in the watershed. The offset may be applied at time of permit application in the form of bond held by the issuing agency

Channel Protection (Vcp): HIGHLY RECOMMENDED if site drains to tributary stream to a lake
Sensitive Lakes

Recharge (Vre): HIGHLY RECOMMENDED if lake is fully or partially ground water dependent

Water Quality (Vwq): Increased Water Quality Sizing: RECOMMEND MPCA pond rule 2 or 4 for “special waters”, depending on the BMP used. For sites with more than 30% Site IC, the Walker Rule (Section IX) presents another option that will result in similar TP removal

Shorter BMP List: Only BMP designs or combination with a TP removal capability exceeding 50% should be used

Channel Protection (Vcp): RECOMMENDED if site drains to tributary stream to a lake
Trout Streams

Recharge (Vre): RECOMMENDED in certain situations

Water Quality (Vwq): RECOMMEND apply MPCA Sizing Rules 2 and 4, depending on BMP used. Comply with SWPPP requirement in CGP, as well as prohibited industrial discharges into infiltration systems. No infiltration of stormwater without pre-treatment. If site is designated as a potential stormwater hotspot, no infiltration of runoff should be allowed. Special precautions are advised for karst areas.
Surface Water Drinking Supplies

Recharge (Vre): RECOMMENDED for watersheds, with caution for ground water functioning as a surface water source

Water Quality (Vwq): RECOMMEND apply the same sizing criteria as Sensitive Lakes

Comply with SWPPP requirement in CGP
Susceptible Wetlands

Maintain Wetland Hydroperiods: For wetlands identified as HIGHLY or moderately susceptible in Table 10.12, meet the storm bounce and inundation duration limits set forth in Table 10.13. This may be done through infiltration, extended detention, or diversion

Recharge (Vre): HIGHLY RECOMMENDED

Water Quality (Vwq): RECOMMEND site-based phosphorus load reduction as described for Most Sensitive Lakes to control nutrients.

Channel Protection (Vcp): RECOMMENDED if channel is direct tributary to the wetland

No use of natural wetlands for stormwater treatment (REQUIRED)

No constrictions at wetland outlets (RECOMMENDED)
Non-Susceptible Wetlands

Recharge (Vre): RECOMMENDED

Water Quality (Vwq): No untreated stormwater discharges to wetlands (REQUIRED) which is operationally defined as providing Vwq via MPCA Rules 1 and 3

Channel Protection (Vcp): RECOMMENDED only if channel is direct tributary to the wetland
Impaired Waters for Computable Pollutants

Water Quality (Vwq): If new development site is located in a watershed subject to a TMDL that has no remaining stormwater allocation, designer may need to document no net increase in pollutant load; RECOMMEND using the general method proposed for the Most Sensitive Lakes but using the appropriate pollutant. Currently, sufficient data are only available to perform this calculation for sediment, phosphorus, nitrogen, ammonia, and bacteria

Channel Protection (Vcp): RECOMMENDED when water body impaired for sediment or sediment related pollutant
Impaired Waters for Non-Computable Pollutants

Water Quality (Vwq): For the remaining ten pollutants which may be subject to a TMDL but have no remaining stormwater allocation, designers should satisfy appropriate MPCA sizing rules.

Channel Protection (Vcp): RECOMMENDED when water body impaired for sediment related pollutant

1REQUIRED- CGP requirement
HIGHLY RECOMMENDED- Essential to provide adequate management and good engineering
RECOMMENDED- Suggested for good management and engineering

## Lakes

Research has shown that development can increase eutrophication, bacteria and turbidity levels in lakes. According to a national survey of 3,700 urban lakes, more than 80 percent were found to be either eutrophic or hyper-eutrophic (U.S. EPA, 1980). Urban and urbanizing lakes receive higher phosphorus loads than non-urban lakes because urban watersheds, particularly those under construction, produce higher unit area phosphorus loads from stormwater runoff, compared to other watersheds (Caraco and Brown, 2001).

Impacts of eutrophication on lake and reservoir quality include the following (Brown and Simpson, 2001):

• nuisance algal blooms in the summer;
• reduced dissolved oxygen in the bottom of the lake;
• fish kills due to low dissolved oxygen;
• taste and odor problems with drinking water;
• formation of THMs (trihalomethanes)and other disinfection byproducts in water supplies;
• increased cost to treat drinking water;
• reduced water clarity;
• decline in fish community (more rough fish,fewer game fish);
• blockage of intake screens by algal mats;
• reduced quality of boating, fishing and swimming experience;
• decline in lakefront property values;
• floating algal mats and/or decaying algal clumps; and
• increased density of aquatic weeds in shallow areas.

From a stormwater management standpoint, lakes can be divided into three management categories based on their current trophic status and sensitivity to additional phosphorus loads.

• Most-Sensitive Lakes are normally defined as being oligotrophic.
• Sensitive Lakes are considered to be mesotrophic or slightly eutrophic.
• A third category of all other lakes would include eutrophic and highly eutrophic lakes that are not generally categorized as sensitive because of their relatively poor quality. These lakes should be treated under regular stormwater programs.

The lake designation is normally made by the local or regional lake management authority, although the state may do so for certain special waters such as trout lakes or lake trout lakes. Often, the lake management designation has already been made by the local, watershed, regional or state agencies, or perhaps even by a university or local educational institution. If no designation has been made, the local review authority should consult available data on water clarity, phosphorus content and algal abundance (using Chlorophyll-a as a surrogate measure). If none of these data exist, the local review authority may want to collect lake monitoring data to make a designation. Future phosphorus loadings should also be considered when making a stormwater management designation for a lake.

As a general rule, all surface water drinking supplies, such as water supply reservoirs and river intakes should be managed using the same stormwater sizing criteria as Sensitive Lakes, given the importance of controlling bacteria, toxic pollutants and turbidity that can threaten drinking water quality. The ensuing section presents stormwater guidance for most-sensitive and sensitive lakes, including enhanced sizing criteria and recommendations for BMP design and selection.

### Most-sensitive lakes

The following adjustments to the basic sizing criteria are recommended for lakes designated as most-sensitive:

• Recharge: Highly recommended
• Water Quality: Highly recommend a site-based phosphorus load reduction if the lake in question is a state-designated “special water” under the CGP or falls under a similar local regulatory designation; also highly recommended for non-designated lakes with documented oligotrophic quality.

Under this criterion, designers would demonstrate that no increase in total phosphorus (TP) loads will occur at a site from pre-development to post-development conditions using a site-based TP load calculation. The designer could use the Simple Method (Schueler, 1987) or equivalent to compute pre-development and post-development TP loads at the site and determine the pollutant removal requirement (in pounds). The designer would then propose a series of BMPs that maximize the amount of phosphorus removal at the site to reach the desired condition. This criterion provides a major incentive to design for maximum phosphorus removal which is essential for managing most-sensitive lakes. Site-based phosphorus reductions have been adopted by several communities in Minnesota, which vary between no change in phosphorus load to as much as a 25 percent reduction from pre-development conditions. The step-wise computational approach is outlined below.

• Calculate site imperviousness before and after development.
• Calculate the pre-development phosphorus load.
• Calculate the expected post-development pollutant load.
• Calculate the pollutant removal requirement.
• Identify feasible BMPs and calculate load removed (using standard BMP removal efficiencies).
• Select off-site mitigation option, if needed.

If a designer cannot meet the total removal requirement, they could be allowed to pay an offset fee that is equivalent to the cost of removing an equivalent amount of phosphorus elsewhere in the watershed.

Channel Protection: Highly recommended if the site drains to a direct tributary stream to a lake.

BMP Selection: The following BMP design and selection guidance is recommended for lakes designated as most-sensitive.

The foremost concern is to choose BMPs with a proven ability to reliably remove high levels of phosphorus. Soluble phosphorus is of particular interest since it is most readily available for algal uptake. Therefore, any BMP employed to protect most-sensitive lakes protection should have a moderate to high capability to remove total and soluble phosphorus.

Infiltration practices tend to have the highest phosphorus removal, but are not always be feasible due to soil constraints or lack of the 3 foot separation distance between the bottom of the infiltration device and the seasonally saturated water table. Pond systems are generally a reliable removal option for both soluble and total phosphorus. Filters are fairly effective at removing total phosphorus, but exhibit little or no capability to remove soluble phosphorus. This can be explained by the fact that most sand filters have no biological or chemical processes to bind soluble phosphorus. The addition of organic matter or binding agents to sand filters may show promise in boosting removal, but early monitoring of experimental filters have yet to demonstrate this result conclusively (Schueler, 2000a).

Wetlands have a highly variable capability to remove both soluble and particulate forms of phosphorus. The variability can be explained in part by internal phosphorus cycling within the wetland, sediment release, and vegetative dieback during the non-growing season (Schueler, 1992). Factors such as soil pH, oxygen conditions, nutrient saturation and presence of calcium, magnesium or iron in the soil can also make a big difference in whether phosphorus is removed or released. The best design variation for phosphorus removal in the stormwater wetland group is the pond-wetland system (e.g., wetland with a relatively large portion of its storage devoted to a deep pool).

### Sensitive lakes

The following recommends that stormwater ponds and constructed wetlands discharging to sensitive lakes be sized larger to increase the retention time for additional phosphorus removal. These designs are more conservative than the MPCA sizing rule and could be considered by local authorities interested in greater protection for sensitive lakes. Recommended adjustments to the standard stormwater sizing criteria for Sensitive Lakes are:

Recharge: Highly recommended.

The MPCA water quality sizing Rule 2 should be applied to size stormwater ponds (e.g., ponds located within special waters). If the site has more than 30 percent impervious cover, the Walker Rule (see below) presents a size option that should result in similar TP load reductions in ponds (see File:Issue paper D - unified sizing criteria for Minnesota.pdf for more discussion). The Walker Rule was developed in the upper Midwest to maximize retention time needed within a pond to promote maximum algal uptake of phosphorus and subsequent settling between storm events. The Walker Rule seeks to attain an average pond retention time of about two weeks. Based on the distribution of storm events in the upper Midwest, Walker (1987) recommended all storage via a permanent pool storage volume equivalent to 2.5 inches multiplied by the site runoff coefficient. Based on the Minnesota rainfall frequency spectrum, the Walker Rule would capture about 98 percent of all runoff producing events each year, resulting in very little bypass of untreated runoff. In addition, runoff from many storm events is retained within the pond over several storm cycles to help improve phosphorus uptake. The pond designer should allocate total storage to the permanent pool under the Walker Rule. The total storage in acre-feet needed under the Walker Sizing Rule is provided using the following equation.

I. Walker Rule

$V_{wq} = 3630 RA$

where

Vwq = required water volume in cubic feet;
R = runoff for design storm (inches) and is given by

$2.5 FI +((2.5-0.2S)^2/(2.5+0.8S)) (1.0 - FI)$;

A = area, in acres;
FI = Fraction of total impervious cover of site;
S = (1000/CN) - 10;
CN = curve number; and
3630 = conversion factor (to cubic feet).

The MPCA water quality sizing Rule 3 should be applied if the designer is using a BMP other than a pond (e.g., non-BMPs draining to special waters), keeping in mind that a minimum water quality storage volume of 0.2 watershed inches is recommended for pre-treatment, regardless of site impervious cover.

Channel Protection: Limited application.
Channel protection is recommended if a development site drains to a tributary stream to a lake.
Channel protection is not needed if the site discharges directly to the lake.
BMP Selection: The following guidance on BMP design and selection is offered for Sensitive Lakes:
Designers should only apply BMPs that have a total phosphorus removal rate exceeding 50 percent.
Four kinds of BMPS are not recommended in sensitive lakes: media filters, wet vegetated swales, micropool extended detention ponds, and extended detention wetlands. If these BMPS are used, they need to be combined with more effective BMPs in a treatment train. By contrast, infiltration, wet ponds, and bioretention have high phosphorus removal rates, and are strongly encouraged in Sensitive Lakes.

Recommended BMPs for Sensitive Lakes.

BMP Group BMP Design Variation Recommended for Lake Watersheds
Bioretention Bioinfiltration Yes
Biofiltration If appropriate filter media is used.
Filtration Media No
Vegetative Filter (dry) Yes
Vegetative Filter (wet) No
Infiltration Infiltration Trench Yes
Infiltration Basin Yes
Stormwater Ponds Flow-Through (Wet) Pond Yes
Wet ED Pond Yes
Micropool ED Pond No
Constructed Stormwater Wetlands Shallow Wetland Yes
Pond/Wetland Yes
ED Shallow Wetland No

In addition, designers and plan reviewers should evaluate every BMP to look for ways to maximize phosphorus removal (see table below). For example, the use of multiple treatment pathways is encouraged (e.g., directing runoff to a filtering or infiltration BMP, and then routing it a wet pond).

Summary of stormwater design recommendations to enhance phosphorus removal.

BMP Design Design recommendations
Infiltration
Filtration (includes practices with an underdrain)
• Organic filters are a source of soluble phosphorus and should not be used
• Practices with engineered media are acceptable when the appropriate filter media is used
• Employ finer-grained media in the filter bed with a small diameter (15 microns), or provide a finer-grained layer at mid-depth in the filter profile
• The process for pretreatment and/or filtration should extend from 36 to 48 hours, where possible
• Filters should be oriented to provide maximum solar exposure
• Wet swales are not recommended because they may export phosphorus
• Open channels should be designed to promote maximum sediment retention
Stormwater ponds1
• Pond aeration is encouraged
• Design wet ponds with a depth no greater than 10 feet to prevent stratification and potential release of phosphorus from bottom sediments
• Avoid the use of dry or dry extended detention ponds
• Designers should consider the snowmelt runoff volume and design ponds for seasonal operation
• Use a surface or mid-depth release from the pond
• Landscape pond to discourage geese
• Add shallow benches and wetland areas to enhance the plankton community
• Pond sediments may be amended to retain phosphorus (e.g. iron, aluminum)
Constructed Stormwater wetlands
• Pond/ wetland system is the preferred wetland design
• Use a surface or mid-depth release from the wetland
• Maximize surface micro-topography
• Landscape wetland to discourage geese

1The recommendations for constructed ponds are from the original Minnesota Stormwater Manual. MPCA anticipates updating this information in the near future.

Also, as a general rule, no BMPs should be located inside the shoreline buffer, as defined by the local reviewing authority.

## Trout streams

Trout populations are threatened by stream habitat degradation, stream warming, possible chloride toxicity, and other impacts associated with upland development. Trout are very sensitive to increases in water temperature. The optimal temperature range for adult trout is from about 57°F to 65°F. Generally, adult trout can survive warmer temperatures if cool water refuge is present in the form of ground water upwelling or springs. Juvenile trout, fry and eggs are much more susceptible to warm water temperatures and are not able to tolerate temperatures much above 68°F (Emmons & Olivier Resources, 2000). Stream warming also harms trout by reducing dissolved oxygen available for fish and aquatic life. Increased temperatures can also increase the metabolic rates of aquatic organisms and increase their sensitivity to other pollutants, parasites, and diseases (SSL SWCD, 2001).

The reduction in streamside forest cover removes much of the mechanisms that keep a stream cool. The heating of impervious surfaces by solar radiation also warms precipitation that runs over them and potentially into a stream. A series of monitoring studies have documented the stream warming effect in urban trout streams (Roa-Espinosa et al. 2003; SSL SWCD, 2001; Johnson, 1995; Galli, 1990).

Sedimentation is also a major concern for trout. Construction runoff, channel erosion and road sand all increase sediment loads which can impair streambed habitat in trout streams. Excess sediment can affect the productivity of a trout stream in several ways. Sediment can impede trout respiration by clogging gill plates. In addition, sediment deposition can destroy spawning habitat and harm the benthic organisms upon which the trout feed.

Road salt may also significantly impact trout habitat. Chloride is one of the main components of road salt, and is extremely soluble in water. As a result, there is virtually no way to remove chloride once it gets into either surface or groundwater. Chloride levels are the highest in late winter as initial melting occurs from snow containing significant amounts of road salt and stream flows are lowest. The chloride from the salt can be toxic in trout streams during some meltwater events. For more detained discussion see Cold climate impact on runoff management.

The following adjustments to the basic sizing criteria are recommended to protect trout streams.

Recharge: Highly recommend infiltration as part of stormwater control.
It is highly recommended that all excess runoff volume above that produced from the pre-development 2- year, 24-hour storm event should be infiltrated for designated trout streams, where soils conditions permit. The state CGP contains this method as one option to meet mandated temperature control for designated trout streams.
Water Quality: Discourage use of ponds/wetlands
Use MPCA water quality Sizing Rule 3 for non-pond BMPs volume determination for special waters, and infiltrate and/or filter this volume at the site regardless of soil conditions (e.g., bioretention, dry swales, infiltration, and better site design practices). Discharge from ponds or wetlands with standing water to trout streams is discouraged. If they are used, they should be sized according to MPCA Sizing Rule 2, incorporate temperature controls, and have an extended detention time no longer than 12 hours.
Channel Protection: Highly recommended
Given the importance of trout habitat, it is highly recommended that channel protection criteria be applied to all trout streams. If soils do not permit infiltration of the channel protection volume, then designers should provide 12 hour extended detention of 1-year, 24-hour runoff volume in a thermally acceptable pond option. Note that CGP allows up to 24 hours, but 12 is recommended in the Manual. Release of the 1-yr, 24-hour volume in 12 hours should be compared with the ½ 2-yr pre-development peak matching method described previously to determine which approach would result in less heating of the stored water.
BMP Selection: There are quite a few do’s and don’ts when it comes to BMP design for trout streams.

Designers should look for ways to incorporate the following design features into their BMPs:

• consider using forested wetland design (see Capiella 2005b);
• maximize use of better site design techniques;
• use infiltration and bioretention to the maximum extent possible;
• construct BMPs “off-line”;
• shade pilot and outflow channels and micropools by planting trees and shrubs;
• plant trees to the maximum extent possible in the stormwater practices and buffers;
• manage buffers to maximize forest cover and shading in riparian areas;
• pre-treat roadway runoff to reduce sediment and road sand discharges to streams; and
• develop and test roadway spill management plans.

Designers should ensure that each BMP does not have

• a large, unshaded permanent pool or shallow wetland;
• extensive and unshaded pilot and outflow channels within the BMP;
• an extended detention time longer than 12 hours;
• extensive exposed riprap or concrete channel;
• an on-line or in-stream location;
• a location within the forested buffer; and
• infiltration practices that are undersized or lack pre-treatment.

## Drinking water source areas

### Groundwater

This group includes any groundwater recharge areas that supply water used for drinking water supply. The management goal is to maintain groundwater recharge while preventing the possibility of groundwater contamination. Groundwater is a critical water resource, as many residents depend on groundwater for their drinking water, and the health of many aquatic systems depends on steady recharge to maintain surface water bodies throughout the year. For example, during periods of dry weather, groundwater sustains flows in streams and helps to maintain the hydrology of wetlands. Because development creates impervious surfaces that prevent natural recharge, a net decrease in groundwater recharge rates can be expected in urban watersheds. Thus, during prolonged periods of dry weather, stream flow sharply diminishes. In smaller headwater streams, the decline in stream flow can cause a perennial stream to become seasonally dry.

Urban land uses and activities can also degrade groundwater quality if stormwater runoff is directed into the soil without adequate treatment. Certain land uses and activities are known to produce higher loads of metals and toxic chemicals and are designated as potential stormwater hotspots or “PSHs”. Soluble pollutants, such as chloride, nitrate, copper, dissolved solids and some hydrocarbons can migrate into ground water and potentially contaminate wells. Stormwater runoff should never be infiltrated into the soil from sites designated as a PSH. Below is a list of business operations at potential stormwater hotspots (adapted from MDE, 2000). Note that road surfaces are not always considered PSHs unless a history of contaminated water has occurred.

• vehicle salvage yards and recycling facilities;
• outdoor liquid container storage;
• vehicle service and maintenance facilities;
• vehicle and equipment cleaning facilities;
• public works storage areas;
• fleet storage areas (bus, truck, etc.);
• facilities that generate or store hazardous materials;
• industrial sites;
• commercial container nursery;
• marinas (service and maintenance);
• large parking lots;
• transportation routes and fueling areas;
• large chemically managed turf areas.

Stormwater hotspots commonly occur as commercial, industrial, institutional, municipal, or transportation-related operations that produce higher levels of stormwater pollutants, and/or present a higher potential risk for spills, leaks or illicit discharges. Runoff from these operations may contain soluble pollutants which cannot be effectively removed by current BMPs and can contaminate ground water quality.

Typical sources of nutrients, metals, hydrocarbons, toxins and other pollutants that can be generated from PSH are summarized in the table below. It should be noted that not all of these operations or activities will actually generate pollution at an individual stormwater hotspot. In fact, many industrial operations are highly regulated under state and federal programs. There are, however, many small or unregulated facilities (such as gas stations or auto salvage yards) that are of concern because of the potential for release of toxic material to stormwater.

Stormwater Pollutants Associated With Common Operations at Potential Stormwater Hotspots

Operation or Activity Nutrients Metals Oil / Hydrocarbons Toxics Others
Vehicle Repair Minor Major Major Major
Vehicle Fueling Minor Major Major Major (MTBE not used in MN)
Vehicle Washing Major Moderate Moderate Major Water Volume
Vehicle Storage Not Pollutant Moderate Major Minor Trash
Outdoor Storage Moderate Moderate Moderate Moderate
Liquid Spills Moderate Moderate Major Major
Dumpsters Moderate Moderate Moderate Major Trash
Building Repair Minor Moderate Moderate Moderate
Building Maintenance Not Pollutant Major Minor Moderate
Parking Lot Maintenance Minor Moderate Major Moderate Chloride
Turf Management Major Not Pollutant Not Pollutant Major Pesticides
Landscaping Major Not Pollutant Not Pollutant Major Pesticides
Swimming Pool Discharges Not Pollutant Not Pollutant Not Pollutant Not Pollutant Chlorine
Golf Courses Major Minor Not Pollutant Major Pesticides
Hobby Farms/Race Tracks Moderate Not Pollutant Not Pollutant Not Pollutant Bacteria
Construction Moderate Minor Minor Moderate Trash, Sanitary Waste, Sediment
Marinas Moderate Moderate Moderate Major Bacteria
Restaurants Moderate Not Pollutant Major Not Pollutant Grease

The management goal in groundwater drinking water source areas is to prevent possible groundwater contamination by preventing infiltration of untreated hotspot runoff. At the same time, recharge of unpolluted stormwater is needed to maintain flow in streams and wells during dry weather. As such, structural BMPs alone should not be relied upon as a sole stormwater management strategy at a PSH. A stormwater pollution prevention plan for a PSH should also incorporate a combination of

• good housekeeping;
• preventive maintenance;
• spill prevention and clean-up;
• employee training;
• inspections;
• record-keeping; and
• chemical use restrictions.

The following adjustments to the standard stormwater sizing criteria are recommended to protect the quality of groundwater drinking water source areas.

Water Quality: Enhanced sizing and pre-treatment. MPCA water quality sizing Rules 2 or 4 should be applied to development sites within groundwater drinking water source areas, depending on whether a pond or non-pond BMP option is being considered. A minimum of 0.2 watershed-inches of effective pre-treatment is recommended for non-pond BMPs to remove pollutants prior to any infiltration or soil filtration.

Recharge: Encouraged in limited situations. Infiltration is encouraged at residential subdivisions to increase groundwater recharge through rooftop disconnections and other better site techniques. Commercial and institutional rooftops can also be disconnected as long as they are not a potential stormwater hotspot. No infiltration or recharge of runoff from potential stormwater hotspot operations should be allowed to reduce the risk of groundwater contamination. Caution on the source of infiltrating water should be exercised in all cases.

BMP Selection: The following guidance on BMP design and selection is offered to protect groundwater drinking water source areas.

• In general, infiltration of clean runoff from residential and non-residential rooftops is encouraged with acceptable pre-treatment.
• Stormwater ponds, wetlands, bioretention, and filters are effective surface treatment
• No infiltration from PSHs, especially those with potentially high chloride levels and/or vulnerable ground water resources.
• Minimum setbacks from wells, septic systems, sinkholes and wellhead protection zones in conformance with state and local regulations (contact Minnesota Department of Health) and plans.
• Avoid pooling or infiltrating stormwater in active karst areas.

Additional BMP design criteria for groundwater protection are presented in the following table.

BMP design considerations for groundwater aquifer protection

BMP Group Design Consideration
Bioretention
• OK with proper caution for PSH
Filtration
• OK with proper caution for PSH
• Open channels are OK, but polluted runoff must be adequately pretreated
Infiltration
• Provide a 100-foot horizontal separation distance from wells and three-foot vertical distance from the water table
• No PSH runoff, unless treated by another practice, such as a filtering system
• Needed pre-treatment of all runoff except rooftop
Stormwater Ponds
• May need liner if A soils or active karst are present. The CSW permit requires a liner in areas of active karst.
• Pre-treat PSH runoff
• Provide a separation distance from well or water table to BMP
Constructed Stormwater Wetlands
• May need liner if A soils or active karst are present. The CSW permit requires a liner in areas of active karst.
• Pre-treat PSH runoff
• Provide a separation distance from well or water table to BMP

## Surface water

There is a large portion of Minnesota residents served by drinking water obtained from a surface water source. The supplies for the St. Cloud, Minneapolis and St. Paul metropolitan areas are obtained mostly from the Mississippi River; St. Paul’s supply is supplemented by both small stream flow and ground water. Several other cities throughout the state are also supplied by smaller rivers such as the Minnesota/Blue Earth, Red Lake and Red Rivers, by Lake Superior or by large abandoned quarries in the Iron Range. In each of the river source areas, protection of the surface water source reaches far beyond the local border to the entire watershed feeding the supply intake. For the quarries, inflow occurs primarily from groundwater sources that must be protected as noted in the previous section. Lake Superior itself requires attention, as do the tributary streams that feed it.

Each of the surface water sources is preparing or has prepared a source water protection plan in which they identify potential pollutants of interest and the likely source of those pollutants. They also must put together a plan to protect the source of water. This plan, as is the case for the Mississippi River communities, can stretch far upstream (or up-gradient for groundwater) to areas not under the control of the served communities. This severely limits the direct control that the supplied communities have over pollution generating activities. Fortunately, a willingness to help protect these drinking water source areas has led to multi-community cooperative protection efforts.

The pollutants mentioned in the previous groundwater section certainly all apply to surface water sources. In addition, surface water suppliers have to be concerned about such things as sediment, phosphorus, nuclear waste (Mississippi River suppliers), any cargo hauled through the watersheds on rail or roads, or on the water in barges, PSHs, fire-fighting runoff and a myriad of other potential surface water contaminants. All of the precautions mentioned in the previous section for groundwater source areas should also be applied to surface waters that provide drinking water.

The management goal in surface water drinking water source areas is to prevent possible source contamination by preventing any potential contaminant from reaching either the stream or river providing the water or any groundwater inflow that will eventually feed a surface water source. Pollution prevention and emergency response become primary BMP approaches for source waters. The list of focal BMPs remains similar to the groundwater list noted previously with the addition of good watershed management to control pollutants associated with nonpoint sources.

The following adjustments to the standard stormwater sizing criteria are recommended to protect the quality of surface water drinking water source areas.

Water Quality: Enhanced sizing and pre-treatment. MPCA water quality sizing Rules 2 or 4 should be applied to development sites within surface water drinking water source areas that are determined in a source water protection plan to be critical to maintaining the quality of the source water. A minimum of 0.2 watershed-inches of effective pre-treatment is recommended for non-pond BMPs to remove pollutants prior to any infiltration or soil filtration.

Recharge: Encouraged for watersheds, with caution for groundwater feeding a surface water source.

Infiltration is encouraged within watersheds upstream of drinking water intakes from surface water. Protective measures consistent with the previous groundwater supply section are encouraged for groundwater feeding surface water sources.

BMP Selection: Supplemental BMPs should follow those suggested for Sensitive Lakes. The following guidance on BMP design and selection is offered to protect surface water source areas.

• A pollution prevention plan is essential for the entire area draining to the surface water intake.
• Stormwater ponds, wetlands, bioretention, and filters are effective surface treatment.
• No infiltration or direct runoff in the vicinity of the intake from PSHs, especially those with potentially high chloride levels and/or vulnerable ground water resources.
• An emergency response plan should be prepared for spill response in areas critical to supply protection.

## Wetlands

For a long time, wetlands were viewed as wastelands that were better drained or filled. It is estimated Minnesota has lost nearly 42 percent of its original wetland acreage (MN SWAG, 1997). Wetlands are now recognized as performing many important watershed functions and services, and their direct disturbance is closely regulated. there are several state, local and federal aspects of wetland regulation and management.

Naturally occurring quantities of runoff with seasonal fluctuations are essential for the maintenance of a wetland, and moderate amounts of nutrients and sediment in the runoff can increase a wetland’s productivity. However, excessive stormwater runoff has the potential to alter the hydrology, topography, and the vegetative composition of a wetland (U.S. EPA, 1993). For example, an increased frequency and duration of inundation can degrade native wetland plant communities or deprive them of their water supply.

Stormwater inputs can also cause changes in water or soil chemistry that can degrade wetlands. This is a particular concern for wetlands with a narrow pH range such as acidic sphagnum bogs and alkaline calcareous fens (MN SWAG, 1997). Calcareous fens are the rarest wetland plant community in Minnesota, and as such are specially protected. These fens are peat-accumulating wetlands dominated by distinct groundwater inflows having specific calcium carbonate chemical characteristics. Flows are circum-neutral to alkaline, with high concentrations of calcium and low dissolved oxygen content. The water chemistry creates a unique environment for a disproportionately large number of rare, threatened, and endangered wetland plant species compared to other plant communities in the Great Lakes region (MN SWAG, 1997). Changes in wetland water quality can alter the nature of the plant community, encouraging invasive species, and reducing sensitive species that are preferred by fish, mammals, birds, and amphibians for food and shelter (U.S. EPA, 1993).

Stormwater runoff inputs can exceed the water depths and frequency/duration of inundation prevalent in natural wetlands. Deposition of sediment carried by urban stormwater can have the same effect, causing replacement of diverse species with monotypes of reed-canary grass or cattails, which are much more tolerant of sedimentation and fluctuating water levels. Schueler (2000b) reported that invasive or aggressive plant species are favored when water level fluctuation (WLF) is high (e.g., reed-canary grass). The result is low vegetative diversity and lower quality wildlife habitat values (MN SWAG, 1997). A modest change in WLF sharply decreases plant species richness, and amphibian species richness a study in the Pacific Northwest (Horner, et al., 1996). Some communities have used existing wetlands for stormwater treatment by increasing the depth of ponding on a permanent or temporary basis. The end result is the transformation of a natural wetland into a stormwater wetland, with the attendant loss of diversity and functional values.

Not all wetlands respond in the same way to the impact of stormwater runoff. In the context of this Manual, wetlands can be defined as Susceptible or Non-Susceptible to stormwater runoff, based on the MN SWAG (1997) wetland classification scheme. This classification provides a useful framework for managing stormwater inputs to different types of wetlands.

Highly susceptible wetland communities can be composed of dozens of plant species. The table below presents the MN SWAG classification of wetland types according to their presumed susceptibility to degradation by stormwater. Given this diversity of wetland types, it is not surprising that wetlands have a broad range of tolerance to stormwater runoff. Some wetlands (e.g. calcareous fens) are sensitive to any disturbance and will show signs of degradation with even low-level inputs of urban stormwater. Note that Susceptible Wetlands are defined as highly and moderately susceptible and Non-Susceptible Wetlands are defined as slightly and least in the table.

Susceptibility of wetland types to degradation by stormwater input

Susceptible Non-Susceptible
Highly Susceptible Wetland Types 1 Moderately Susceptible Wetland Types 2 Slightly Susceptible Wetland Types 3 Least Susceptible Wetland Types 4
• Open Bogs
• Coniferous Bogs
• Calcareous Fens
• Low Prairies
• Coniferous Swamps
• Lowland Hardwood Swamps
• Seasonally Flooded Basins
• Shrub-carrsa
• Alder Thicketsb
• Shallow Marshesd,e
• Deep Marshesd,e
• Floodplain Forestsa
• Shallow Marshesc
• Deep Marshesc
• Gravel Pits
• Cultivated Hydric Soils
• Dredged Material / Fill Material Disposal Sites
1. Special consideration must be given to avoid altering these wetland types. Inundation must be avoided. Water chemistry changes due to alteration by stormwater impacts can also cause adverse impacts. Note: All scientific and natural areas and pristine wetlands should be considered in this category regardless of wetland type.
2a, 2b, 2c. Can tolerate inundation from 6 inches to 12 inches for short periods of time. May be completely dry in drought or late summer conditions.
2d. Can tolerate +12” inundation, but adversely impacted by sediment and/or nutrient loading and pro- longed high water levels.
2e. Some exceptions.
3a. Can tolerate annual inundation of 1 to 6 feet or more, possibly more than once/year.
3b. Fresh meadows which are dominated by reed-canary grass.
3c. Shallow marshes dominated by reed-canary grass, cattail, giant reed or purple loosestrife.
4. These wetlands are usually so degraded that input of urban stormwater may not have adverse impacts.

Notes: There will always be exceptions to the general categories listed above. Use best professional judgment. Pristine wetlands are those that show little disturbance from human activity.

The following adjustments to the standard stormwater sizing criteria are recommended to protect wetlands from the indirect impact of stormwater runoff. Note that wetlands are highly regulated within the state and that all federal, state and local/watershed authorities should be consulted before any activity is initiated on any parcel of land that appears to be a wetland.

Recharge: Highly recommended for Susceptible Wetlands. Many Susceptible Wetlands are dependent on groundwater to maintain their natural hydrology so it is important to maintain recharge at a consistent rates in the contributing source area to the wetland. Recharge is also recommended for Non-Susceptible Wetlands that are dependent on groundwater.

Water Quality: Recommend site based phosphorus load reduction. Site-based phosphorus load reduction for Susceptible Wetlands using the method described for Most-Sensitive Lakes are recommended to control nutrients. Site-based nutrient load reduction should be used for nutrient sensitive bogs and calcareous fens. No untreated stormwater discharges should be allowed to Non-Susceptible Wetlands, which are operationally defined as providing water quality volume according to MPCA sizing Rules 2 and 4 (depending on the type of BMP chosen). Currently, the MPCA interprets the CGP as requiring a permanent pool in constructed stormwater wetland systems. While this seems appropriate for a [Stormwater ponds|pond]]/wetland system, it does detract from the bioretention character of other wetland BMPs. The application of a permanent pool to constructed stormwater wetland systems that behave as bioretention systems should be considered for change in the next CGP update.

In addition, Susceptible Wetlands should not be used for stormwater treatment. A Non-Susceptible Wetland should only be used for stormwater treatment if designers can demonstrate that it will restore wetland functional value, and only when approved by the local government unit acting as approving agency under the Minnesota Wetland Conservation Act.

Channel Protection: Limited. Channel Protection is recommended only when a channel is a direct tributary to a wetland.

Other:Maintain wetland hydroperiod. Designers should maintain the hydroperiod of Susceptible Wetlands following development to prevent detrimental impacts. Any wetlands present at the site should be investigated in the field to determine their wetland type and contributing hydrologic source area, and then determine if any additional runoff will be delivered to the wetland as a result of the proposed project. Based on this determination, a wetland will be classified as either Susceptible or Non-Susceptible.

The following table presents hydroperiod guidelines for wetlands, developed by MN SWAG (1997) for use unless better site-specific data are available. The term “existing” means the existing hydrologic conditions. If there have been recent significant changes in conditions, it means the conditions that established the current wetland. Designers then model the effect of runoff discharge from the site on the wetland to ensure they conform to the storm bounce and inundation duration guidelines standards using infiltration, extended detention, diversion or other methods.

Recommended hydroperiod standards for wetlands (Source: State of Minnesota Storm-Water Advisory group, 1997

Hydroperiod standard
Susceptible
Non-susceptible
Highly susceptible wetlands Moderately susceptible wetlands Slightly susceptible wetlands Least susceptible wetlands
Storm bounce Existing Existing plus 0.5 feet Existing plus 1.0 feet No limit
Discharge rate from wetland Existing Existing Existing or less Existing or less
Inundation period for 1 and 2 year precipitation event Existing Existing plus 1 day Existing plus 2 days Existing plus 7 days
Inundation period for 10 year precipitation event and greater Existing Existing plus 7 days Existing plus 14 days Existing plus 21 days
Run-out control elevation (free flowing) No change No change 0 to 1.0 feet above existing run out 0 to 4.0 feet above existing run-out
Run-out control elevation Above delineated wetland Above delineated wetland Above delineated wetland Above delineated wetland

BMP Selection: Additional guidance on BMP design to protect wetlands is offered below.

• BMPs such as stormwater wetlands, infiltration systems, and bioretention are encouraged to treat runoff prior to discharge to a wetland.
• Direct pipe outfalls to wetlands should be restricted (e.g., not allowed, allowed if energy dissipated, or routed through a pre-treatment system).
• Stormwater should be routed around sensitive wetlands using a diversion or bypass system.
• Constrictions at wetland outlets should be avoided.
• Natural wetlands should not be used for stormwater treatment, unless they are severely impaired and construction would enhance or restore wetland functions; if natural wetlands are used in this manner, MN Rules 7050 establishes the sequence of avoid, minimize and compensatory replacement.
• The discharge of untreated stormwater to a wetland is prohibited.

## Impaired waters

Under the Clean Water Act, Minnesota administers water quality standards which consist of numeric and narrative criteria that protect the physical, chemical and biological integrity of surface waters in the state. These criteria are set to maintain seven designated or beneficial uses of water in the state. The state routinely monitors the quality of its waters to determine if they are meeting their designated uses. If monitoring indicates that water quality standards are not being met and/or designated uses are not being achieved, the state lists the water as being “impaired”. This, in turn, triggers the Total Maximum Daily Load (TMDL) provisions of the Clean Water Act.

A TMDL study consists of an analysis to determine what pollutant reduction is needed to achieve water quality standards, and is normally conducted at the watershed scale. A TMDL determines the amount of pollutants that a waterbody can receive from both point and nonpoint sources and still meet water quality standards (e.g., no impairment). Water quality sampling and computer modeling determine how much each pollutant source needs to be reduced to assure the water quality standard is met.

Impaired waters include streams and lakes that do not meet their designated uses because of excess pollutants or identified stressors. As of 2004, 916 lakes and 199 river and stream segments were listed as impaired waters for Minnesota (MPCA, 2004). Each listed water will ultimately require a TMDL based on the assessment. Currently, there are 14 pollutants causing water quality standard violations in some part of the state.

• sediment
• phosphorus
• nitrogen
• ammonia
• fecal coliform
• oxygen demand
• turbidity
• chloride
• DDT
• dieldrin
• mercury
• PCBs
• toxaphene
• dioxin

To date (fall 2005), only four final TMDLs and their corresponding implementation plans have been completed in Minnesota, so many listed waters currently lack a TMDL or are in the process of developing one.

While none of the completed TMDL implementation plans currently contain stormwater requirements, they may eventually be included if stormwater pollution is determined to be a significant source of the listed pollutant. Therefore, development projects that occur in a listed watershed may require a higher level of stormwater treatment, regardless of whether a TMDL has been completed or not. The main reason is that both municipal NPDES Phase I or II stormwater permits and individual construction general permits must be consistent with the load allocations and pollutant reductions contained in an approved TMDL. If stormwater runoff is likely to be a significant pollutant source within a listed watershed, the local review authority may elect to require higher levels of stormwater treatment to restore the impaired water.

Some general guidance on how to deal with stormwater pollutant loads at development sites located within listed waters is provided below.

First, the local review authority should check with MPCA to determine:

• whether any local waters as listed as impaired;
• which pollutant(s) is causing the impairment to be listed;
• the estimated watershed area to the receiving water;
• the timeframe under which the water will fall under the TMDL program; and
• whether stormwater is expected to be a significant source of the impairment for the indicated pollutant.

Second, if an impairment exists, the local reviewing authority should determine whether the indicated pollutant is considered computable or non-computable. In the context of stormwater, “computable” is defined as a pollutant for which enough data exist to perform a site-based pollutant load calculation that documents no increase or even a reduction in pollutant loading. By contrast, “non-computable” pollutants lack enough data to perform a reliable site based pollutant reduction calculation. See File:Issue paper E - receiving water based stormwater criteria final.pdf for determining pollutant computability. Computable pollutant must pass the following four tests:

• enough stormwater EMC data is available to characterize its average level in stormwater;
• stormwater concentrations are high enough to constitute a major source in the stormwater load allocation;
• sufficient BMP performance data are available to estimate expected removal for a range of stormwater practices; and
• stormwater removal rates are high enough to warrant performing the calculation.

Currently, only five pollutants meet all four criteria -- sediment, phosphorus, nitrogen, ammonia, and fecal coliform bacteria). A stormwater strategy to deal with computable and non-computable pollutants within listed is offered below.

Water Quality: Computable pollutants If a new development site is located in a watershed subject to a TMDL that has no remaining stormwater allocation, the local review authority may wish to adopt a “no net increase” policy for the listed computable pollutant (e.g., sediment, phosphorus, nitrogen, ammonia or fecal coliform). Pollutant removal calculations should be conducted on a site-by-site basis, using the general method proposed for the Most-Sensitive Lakes, adapted for the listed pollutant.

Water Quality: Non-computable pollutants Since non-computable pollutants lack enough data to perform a site-based load reduction calculation, they can only be managed by increasing the Vwq assuming that a higher level of pollutant reduction will occur within the BMP. In these situations, the local review authority may wish to require that development sites satisfy MPCA water quality volume sizing Rules 2 or 4, depending on the type of BMP employed.

Channel Protection: Recommend for waters listed for sediment or sediment related pollutant. Given the importance of channel erosion in the sediment budget of urban streams, it is advisable to require channel protection criteria in watersheds that are listed for sediment. In all cases, the local review authority should check with MPCA to determine what, if any, water quality or channel protection requirements need to be addressed as part of TMDL implementation.

BMP Selection: The selection and design of specific BMPs to address impaired water pollutant reductions will be determined through the TMDL process.

## 6. Literature review summary

Channel enlargement/erosion is the result of changes in runoff distribution and lower watershed sediment yield. This paper compares peak rate control to sediment yield control methods and evaluates erosion potential using five sediment transport equations. The major finding is that both control methods increase the duration of lower flows and cause changes that would result in increased channel erosion. Additionally, the use of detention ponds reduces the sediment load to the stream and can cause further degradation of the stream bed. The author’s recommendation is to evaluate the impact of reductions in sediment load and to match the “shape and magnitude of the predevelopment hydrograph over a range of geomorphically important flows”.
This paper summarizes modeling indicating that similar erosion protection for streams is provided by extended detention of the 1 year 24 hour storm and the “distributed runoff control” method (the method recommended by MacRae, 1993). The methods provide protection for storms of less than 2 inches; for larger rainfall, the two methods are the same as peak rate control. The paper also summarized a study of specific stream reaches and found that site specific morphologic studies are necessary to fully evaluate stormwater management needs They recommend a three step process of assessing geomorphic conditions, determining stability thresholds, and determining allowable stormwater release.
The authors found that recurrence interval for bankfull flow (effective discharge) for Illinois streams was less than 1.1 years for each of the about 20 streams analyzed.
• Ferguson, B.K. and T. Deak, 1994. Role of Urban Storm-Flow Volume in Local Drainage Problems. Jour. of Water Resources Planning and Management (ASCE), 120(4): 523-530.
This study shows that the increase in runoff volume alone can cause increases in flooding where restrictions in flow occur. The recommended solution is the use of infiltration instead of detention as a management practice.
The authors found that erosion potential can be predicted with probability density functions (PDFs) of channel velocity and duration of flows.
The authors report that streams in wetland environments show different bankfull characteristics than non-wetland alluvial streams. Bankfull cannot be defined the same way for wetland streams as for non-wetland streams.
This paper uses two-dimensional stream scour analysis to evaluate effect of peak rate control and “over-control” (detention that allows sediment transport rate to remain at predevelopment levels). Neither successfully maintained pre-development sediment transport to a level that would not cause scour or aggradation of the channel. The author’s recommendation is to use a third method called “distributed runoff control” that follows the over-control method but also allows larger flows through to make use of the floodplain and limit erosion within the channel.
The author reports that peak rate control to the 2-year event increases mid-bankfull events that are most important for erosion control of streams in urbanized areas. The author’s model indicated that the duration of mid-bankfull flow increased by 4 to 10 times as the level of development increased even though rate control facilities were in place.
The authors found that stormwater management using detention only causes changes in the timing of storage and increases in volume. These changes cause increases in the duration of bankfull flows which lead to stream erosion. Detention basins do not adequately mimic natural storage, do not result in similar stream sediment transport patterns or bankfull flow durations, and can have a regional impact downstream of the facility. The authors’ recommendation is to use storage that is distributed spatially throughout a site and releases water distributed over time while also reducing the runoff volumes.
• McCuen, R.H. and G.E. Moglen, 1988. Multicriterion Stormwater Management Methods. Jour. of Water Resources Planning and Management (ASCE), 114(4): 414-431.
This paper provides design methods for channel and erosion based criteria for stormwater management. It emphasizes that multiple criteria are needed to ensure that the problem to be addressed by stormwater management is adequately solved. The paper demonstrates that peak rate control alone does not address the issue of channel erosion.
The authors report that infiltration basins distributed over a watershed provide peak rate control up to 30% (based in this case on a subwatershed equivalent CN reduction of 5 for each subwatershed with an infiltration basin). The incremental inclusion of new BMPs can result in an equally good solution to an initial determination of all optimal locations for BMPs, as long as the basins are installed in highly developed areas.
The authors found that the assumption of 1.5- to 2-year recurrence interval for bankfull flow is inappropriate to Midwestern streams and rivers. The paper evaluates bankfull flow in Ohio streams and finds a range between 0.3 and 1.5-year as the recurrence intervals corresponding to bankfull flow. Assuming a 1.5 to 2-year recurrence interval for bankfull flow will result in incised channels if it is inappropriate to the stream in question. The authors’ recommendation is to evaluate the specific stream to determine the bankfull flow and recurrence interval.
• Rohrer, C.A and L.A. Rosner, 2006. Matching the Critical Portion of the Flow Duration Curve to Minimize Changes in Modelled Excess Shear. Water Science and Technology, 54(6-7): 347- 354.
The authors propose to manage stream erosion by determining the critical flow causing streambed erosion and control flows only above that level to match critical portions of the flow duration curve. Urbanization increases runoff volume unless volume reductions are implemented. Without volume reductions, the duration of flows will increase regardless of the type of detention used. This study indicates that if the duration of flows above the critical threshold for erosion of the bed and bank remains the same, erosion of the channel will not increase. Changes in sediment supply from the watershed, however, could alter the threshold for erosion in the stream. For highly erodible channels the flow duration curve needs to be matched entirely from pre- to post-development to prevent channel erosion. The study suggests that reducing the total volume of runoff “may provide a better method for stormwater management for watersheds that drain to fine grained non-cohesive or erodible clay streams.”
• Shields, F.D., R.R. Copeland, P.C. Klingeman, M.W. Doyle and A. Simon, 2003. Design for Stream Restoration. Jour. of Hydraulic Engineering (ASCE), 129(8): 575-584.
The authors define channel-forming discharge, bankfull discharge and return-interval discharge and describe how to use these concepts in stream restoration.
This paper provides an overview of basic streamflow concepts.

Small redevelopment sites can pose special challenges for stormwater design, given their small size, intensive use, and compacted soils. Redevelopment projects are also not covered under the CGP unless they created more than one acre of new impervious surface or are part of a larger related planned development. Communities may wish to develop special sizing criteria for smaller redevelopment so that the cost to comply with stormwater requirements does not become a barrier to smart growth.

The following guidance is offered for handling redevelopment projects. It has been adapted from several recent manuals that represent a balanced approach to stormwater management for these sites.

The first issue is how to define what is meant by infill and redevelopment, which may be different in each locality. One accepted definition is that redevelopment is “any construction, alteration, or improvement that disturbs greater than or equal to 5,000 square feet of existing impervious cover performed on sites where the existing land use is commercial, industrial, institutional, or residential.” Note that this definition does not fall under the purview of the CGP.

The second issue is to provide some greater flexibility in how redevelopment projects can comply with basic stormwater sizing criteria. This is done by proposing stormwater management guidance that a redevelopment will

• provide a reduction in impervious area; or
• implement stormwater management practices; or
• a combination of both to result in an improvement to water quality.

More specifically, redevelopment projects introduce a chance to reduce existing site impervious area. Where site conditions prevent the reduction of impervious area, stormwater management practices could be implemented to provide water quality control for at least 20 percent of the site’s impervious area as a general guideline.

When a combination of impervious area reduction and stormwater management practice implementation is used for redevelopment projects, the combination of impervious area reduction and the area controlled by a stormwater management practice should equal or exceed 20 percent coverage of the project size.

The MPCA may allow practical alternatives where conditions prevent impervious area reduction or on-site stormwater management. Practical alternatives include, but are not limited to

• fees paid in an amount specified by the approving agency and then dedicated to stormwater management;
• off-site stormwater treatment practice implementation for a drainage area comparable in size and impervious cover to that of the project;
• watershed or stream restoration; or
• stormwater retrofitting.

The recharge, channel protection storage volume, overbank, and extreme flood protection volume requirements specified in the Manual do not apply to redevelopment projects unless specified in an approved and adopted basin plan.