Stormwater wetlands articles

# Overview

Example of a stormwater wetland in a suburban area.

This section provides an overview of stormwater wetlands. It includes a discussion of permit applicability, function within the treatment train, cold climate and retrofit suitability, and role in water quality and quantity treatment.

## Function within stormwater treatment train

Stormwater wetlands are typically installed at the downstream end of the treatment train (they are considered an end-of-pipe BMP). Stormwater wetland size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, when a stormwater wetland is constructed, it is likely to be the only management practice employed at a site, and therefore must be designed to provide adequate water quality and water quantity treatment for all regulated storms.

## MPCA permit applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least one acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, stormwater wetlands currently fall under the “Wet Sedimentation Basin” category described in Section 18 of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the wetland will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice. Of note, the MPCA will evaluate the need to keep stormwater wetlands under the “wet sedimentation basin” category in future CGP revisions and consider it as a bioretention system instead.

The following terms are used throughout this Manual to distinguish various levels of stormwater wetland design guidance:

Required:Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).

Highly recommended:Indicates design guidance that is extremely beneficial or necessary for proper functioning of the wetland, but not specifically required by the MPCA permit.

Recommended:Indicates design guidance that is helpful for stormwater wetland performance but not critical to the design.

Of course, there are situations, particularly retrofit projects, in which a stormwater pond is constructed without being subject to the conditions of the permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular stormwater wetland, depending on where it is situated both jurisdictionally and within the surrounding landscape.

## Retrofit suitability

As a retrofit, stormwater wetlands have the advantage of providing both educational and habitat value. One disadvantage of wetlands, however, is the difficulty in storing large amounts of runoff without consuming a large amount of land. Therefore, the most common type of wetland retrofit involves the modification of an existing dry or wet pond.

## Special receiving waters suitability

The following table provides guidance regarding the use of wetlands in areas upstream of special receiving waters.

Design restrictions for special water or other sensitive receiving watersheds.

BMP
Watershed Management Category
A
Lakes
B
Trout Waters
C
Drinking Water*
D
Wetlands
E
Impaired Waters
Wetlands Some variations NOT RECOMMENDED due to poor P removal, combined with other treatments. NOT RECOMMENDED
except for wooded wetlands
RECOMMENDED RECOMMENDED
but no use of natural wetlands
RECOMMENDED

*Applies to groundwater drinking source areas only; use the sensitive lakes category to define BMP Design restrictions for surface water drinking supplies

This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.

## Cold climate suitability

Wetland performance can be diminished in spring months when large volumes of runoff occur in a relatively short time and carries the accumulated pollutant load from the winter months. Because stormwater wetlands are relatively shallow, freezing of the shallow pool can occur. Also, freezing of inlet and outlet structures can occur, which will reduce performance of the stormwater wetland. To avoid these problems, the Center for Watershed Protection (Caraco and Claytor, 1997) made some general design suggestions, which are adapted as follows:

• Inlet pipes should not be submerged, since this can result in freezing and upstream damage or flooding.
• Burying all pipes below the frost line can prevent frost heave and pipe freezing. Wind protection can also be an important consideration for pipes above the frost line. In these cases, designs modifications that have pipes “turn the corner” are helpful.
• Increase the slope of inlet pipes to a minimum of 1 percent to prevent standing water in the pipe, reducing the potential for ice formation. This design may be difficult to achieve at sites with flat local slopes.
• If perforated riser pipes are used at the outlet, the minimum opening diameter should be ½ inch. In addition, the pipe should have a minimum 6 inch diameter.
• When a standard weir is used, the minimum slot width should be 3 inches, especially when the slot is tall.
• Baffle weirs can prevent ice reformation during the spring melt near the outlet by preventing surface ice from blocking the outlet structure.
• Alternative outlet designs that have been successful include using a pipe encased in a gravel jacket set at the elevation of the aquatic bench as the control for water quality events. This practice was both avoids stream warming and is also a non-freezing outlet.
• Trash racks should be installed at a shallow angle to prevent ice formation.

## Water quantity treatment

Stormwater wetlands are well-suited to provide channel protection and overbank flood protection. As in ponds, this is accomplished with live storage (extended detention) above the permanent pool.

Information: It is highly recommended that when providing water quantity control in stormwater wetlands, the smallest possible bounce (vertical water level fluctuation) be designed for in order to limit the amount of stress on the vegetation.

## Water quality treatment

Pollutants are removed from stormwater runoff in a wetland through uptake by wetland vegetation and biota (algae, bacterial), vegetative filtering, soil adsorption, and gravitational settling in the slow moving marsh flow. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons.

Pollutant removal efficiencies and optimum effluent concentrations for selected parameters are provided in following two tables.

Percent removal of key pollutants for stormwater wetlands. Removals represent median values from Winer (2000) and are rounded.

Practice TSS
(%)
Total
Phosphorus(%)
Total
Nitrogen(%)
Metals1
(%)
Bacteria
(%)
Hydro-
carbons(%)
Stormwater Wetlands 75 40 30 40 802 852

1. Average of zinc and copper.
2. Based on fewer than five data points (i.e., independent monitoring studies)

Typical BMP best attainable effluent concentrations. Values from ASCE BMP database and Winer 2000

Practice TSS (mg/l) TP (mg/l) TN (mg/l) Cu (ug/l) Zn (ug/l)
Wetlands 6 0.2 1.7 3.0 50

## Limitations

The following general limitations should be recognized when considering installation of stormwater wetlands:

• They require more land than other practices;
• They requires careful design and planning to ensure wetland hydrology is maintained; and
• Water quality behavior can change seasonally

# Types of stormwater wetlands

The three major stormwater wetland types include shallow wetlands, a pond/wetland system, and an extended detention wetland

Stormwater wetlands are constructed with varying amounts of the following three components:

• Shallow marsh area
• Permanent micropool area
• Storage volume above the normal water level

The amount of each of the components named above depends on the desired type of stormwater wetland (e.g., shallow wetland). The figure at right shows the three major types of stormwater wetlands presented in this Manual.

Stormwater wetland design must be tailored to site characteristics; however, some general RECOMMENDED design criteria for shallow wetland, ED shallow wetland, and pond/wetland design are presented below:

Summary of wetland design criteria

Design Criteria Shallow Wetland Pond/Wetland ED Shallow Wetland*
Wetland/Watershed Ratio (Ac/Ac) 0.2 0.1 0.1
Minimum Drainage Area (Ac) 25 10 25
Length to Width Ratio (minimum)(ft/ft) 2:1 2:1 2:1
Extended Detention (ED) No Optional Yes
Allocation of Vwq (pool/marsh/ED) in % 25/75/0 70/30/0(includes pond volume) 25/25/50
Allocation of Surface Area (deepwater/low marsh/high marsh/semi-wet) in % 20/35/40/5 45/25/25/5 (includes pond surface area) 10/35/45/10
Forebay Required Required Required
Micropool Required Required Required
Outlet Configuration Reverse-slope pipe or hooded broad-crested weir Reverse-slope pipe or hooded broad-crested weir Reverse-slope pipe or hooded broad-crested weir

*Note: The ED Shallow Wetland design guidance does not meet the MPCA requirements for permanent volume. The guidance may be applied in a stormwater retrofit situation where area requirements preclude the use of a Shallow Wetland

The proportions of deep water, low marsh, high marsh, and semi-wet marsh are presented above and are defined as:

• Deepwater zone – From 1.5 to 6 feet deep. Includes the outlet micropool and the deepwater channels through the wetland facility. This zone supports little emergent wetland vegetation but may support submerged or floating vegetation. It is consistent with the Cowardin Wetland Classification of palustrine aquatic beds.
• Low marsh zone – From 6 to 18 inches below the normal permanent pool or water surface elevation. This zone is suitable for the growing of several emergent wetland plant species.
• High marsh zone – From 6 inches below the pool to the normal pool elevation. This zone will support a greater density and diversity of wetland species than the low marsh zone. The high marsh zone should have a higher surface area to volume ratio than the low marsh zone.
• Semi-wet zone – Those areas above the permanent pool that are inundated during larger storm events. This zone supports a number of species that can survive flooding.

# Design criteria for stormwater wetlands

The following terms are used in the text to distinguish various levels of stormwater wetland design guidance:

Warning: Required:Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).

Highly recommended: Indicates design guidance that is extremely beneficial or necessary for proper functioning of the wetland, but not specifically required by the MPCA permit.

Recommended: Indicates design guidance that is helpful for stormwater wetland performance but not critical to the design.

## Physical feasibility initial check

Before deciding to construct a wetland for stormwater management, it is helpful to consider several items that bear on the feasibility of using a wetland at a given location. The following list of considerations will help in making an initial judgment as to whether or not a wetland is the appropriate BMP for the site. Note that none of these guidelines are strictly required by the MPCA Permit, and it may be possible to overcome site deficiencies with additional engineering or the use of other wetlands.

Warning: Groundwater Protection – It is Required that stormwater wetlands treating runoff from Potential Stormwater Hotspots (PSHs) provide excellent treatment capabilities. In some cases (depending on the land use and associated activities), lining the stormwater wetland may be necessary to protect groundwater, particularly when the seasonally high groundwater elevation is within three feet of the practice bottom.
• Drainage area – 25 acres minimum Highly Recommended, ensuring hydrologic input sufficient to maintain permanent pool; 10 acres (or less) may be acceptable, particularly if the groundwater table is intercepted and a water balance indicates that a permanent pool can be sustained.
• Space required – Approximately 2 to 4 percent of the tributary drainage area is Recommended for the wetland footprint.
• Minimum head – The elevation difference Recommended at a site from the inflow to the outflow is a minimum of 2 feet. The relatively small head requirement makes stormwater wetlands a feasible practice in areas with shallow soils.
• Minimum depth to water table – In general, there is no minimum separation distance required with stormwater wetlands. In fact, intercepting the groundwater table is common and helps sustain a permanent pool. However, some source water protection requirements may dictate a separation distance if there is a sensitive underlying aquifer, which means that a liner might be required for portions of the wetland with standing water. A Level 2 liner is recommended.
• Soils – Underlying soils of hydrologic group “C” or “D” should be adequate to maintain a wetland. Most group “A” soils and some group “B” soils may require a liner. A Level 2 liner is recommended. A site specific geotechnical investigation should be performed. Also, if earthen embankments are to be constructed,it will be necessary to use suitable soils.
• Karst – Stormwater wetlands are a preferred management technique over stormwater ponds in karst areas, but it is Recommended that maximum pool depths be 3 to 5 feet. If stormwater wetlands are used in areas, impermeable liners may be needed.
Warning: The CSW permit requires liners for stormwater ponds constructed in areas of active karst
• Cold water fisheries – Stormwater wetlands may not be appropriate practices where receiving waters are sensitive cold water fisheries due to the potential for stream warming from wetland outflows. Suitable vegetative canopy may lessen potential negative effects.

## Conveyance

Inflow Points

Warning: It is Required that inlet areas be stabilized to ensure that non-erosive conditions exist during events up to the overbank flood event (i.e., Qp10)
• It is Highly Recommended that inlet pipe inverts be located at the permanent pool elevation if the wetland contains a pool. Submerging the inlet pipe is can result in freezing and upstream damage during cold weather.
• It is Highly Recommended that inlet pipes have a slope of no flatter than 1 percent, to prevent standing water in the pipe and reduce the potential for ice formation.
• It is Highly Recommended that pipes be buried below the frost line to prevent frost heave and pipe freezing.
• It is Highly Recommended that trenches for pipes be over-excavated and backfilled with gravel or sand to prevent frost heave and pipe freezing.
• It is Highly Recommended that where open channels are used to convey runoff to the wetland, the channels be stabilized to reduce the sediment loads.

Stormwater wetland outfalls should be designed not to increase erosion or have undue influence on the downstream geomorphology of the stream.

• It is Highly Recommended that a stilling basin or outlet protection be used to reduce flow velocities from the principal spillway to non-erosive velocities (3.5 to 5.0 feet per second).
• Flared pipe sections that discharge at or near the stream invert or into a step-pool arrangement are Recommended over headwalls at the spillway outlet.
• It is Recommended that tree clearing be minimized along the downstream channel and that a forested riparian zone be reestablished in the shortest possible distance. It is also Recommended that excessive use of riprap be avoided, to minimize stream warming in channels with dry weather flow.
• Local agencies (Watershed Districts, Watershed Management Organizations (WMOs), municipalities, etc.) may have additional outlet control requirements.

## Pre-treatment

Sediment forebays are the commonly used pre-treatment method for stormwater wetlands, although other features, such as grassed swales, could be used to remove sediment from runoff before it enters the wetland system. A forebay or equivalent pre-treatment should be in place at each inlet to ease the maintenance burden and preserve the longevity of the stormwater wetland. See the section on Stormwater ponds for design guidance.

## Treatment

Permanent Pool (Vpp) and Water Quality Volume (Vwq) Stormwater wetlands follow similar sizing criteria as stormwater ponds. See the Stormwater ponds section for guidance on sizing the permanent pool volumes, water quality volume, and depth.

Information: A water balance is recommended to ensure sufficient inflows to maintain a constant wetland pool and sustain wetland vegetation during prolonged dry weather conditions. This is of particular importance in stormwater wetlands.

The basic approach to performing a water balance is as follows:

• Check maximum drawdown during periods of high evaporation and during an extended period of no appreciable rainfall to ensure that wetland vegetation will survive.
• The change in storage within a wetland = inflows – outflows.
• Potential inflows: runoff, baseflow and rainfall.
• Potential outflows: Infiltration, surface overflow and evapotranspiration.
• Assume no inflow from baseflow, no outflow losses for Infiltration or for surface overflows. The validity of these assumptions need to be verified for each design.
• Therefore, change in storage = runoff - evapotranspiration.

If a liner is required for the stormwater wetland, it should be designed following the same guidance as for stormwater ponds.

Grading and site layout Site layout and grading affect the pollutant removal capability of the stormwater wetlands as well as the ease of maintenance. Performance is enhanced when multiple cells, longer flowpaths, high surface area to volume ratios, and complex microtopography are used. Specific design considerations for site layout include:

• It is Recommended that, to the greatest extent possible, stormwater wetlands be irregularly shaped and long flow paths be maintained.
• Microtopography (small, irregular 6 to 24 inch variations in bottom topography) is Recommended to enhance wetland diversity.
• It is Highly Recommended that at least 25 percent of the wetland pool volume of a stormwater wetland be in deepwater zones with a depth greater than four feet.
• It is Highly Recommended that a minimum of 35 percent of the total surface area of stormwater wetlands should have a depth of 6 inches or less, and at least 65 percent of the total surface area shall be shallower than 18 inches (see the section on Mosquito control and stormwater management).
• It is Highly Recommended that a micropool be excavated at the wetland outlet to prevent resuspension of sediments.
• It is Highly Recommended that the extended detention associated with the Vwq and Vcp not extend more than three feet above the permanent pool at its maximum water surface elevation.
• It is Highly Recommended that berms be used to separate wetland cells. This reduces the incidence of freezing and requires less maintenance than pipes or concrete weirs.
• Structures such as fascines, coconut rolls, straw bales, or carefully designed stone weirs can be used to create shallow marsh cells in high-energy areas of the stormwater wetland.
• It is Highly Recommended that the perimeter of all deep pool areas (4 feet or greater in depth) be surrounded by an access bench and aquatic bench, as described in the Stormwater ponds section. The aquatic benches can be incorporated into the pond microtopography.

## Landscaping plan

It is Highly Recommended that a qualified landscape professional prepare a Landscaping Plan that includes both plant materials, bedding materials and maintenance schedules. There are many references describing suitable native species of plants for Minnesota. The reader is referred to the section on Minnesota plant lists as well as to Shaw and Schmidt, 2003.

The following guidelines are Recommended for landscaping of stormwater wetland facilities.

• A landscaping plan shall be provided that indicates the methods used to establish and maintain wetland coverage. Minimum elements of a plan include: delineation of pondscaping zones, selection of corresponding plant species, planting plan, sequence for preparing wetland bed (including soil amendments, if needed) and sources of plant material.
• Vegetation selection should be based on the anticipated hydrologic function of the stormwater wetland (e.g. water level fluctuation).
• Design should consider control – predation by carp, geese, deer, etc.
• Donor soils for stormwater wetland mulch should not be removed from natural wetlands.
• Wetland soils mixes often contain wetland plant propagules that help to establish the plant community.
• The landscaping plan should provide elements that promote greater wildlife and waterfowl use within the stormwater wetland and buffers.
• The planting schedule should reflect the short growing season. Designers should consider incorporating relatively mature plants, or planting dormant rhizomes during the winter.
• It is Recommended that a landscape architect or another landscape professional be consulted in selection of wetland plants.
Warning: If a minimum coverage of 50 percent is not achieved in the planted wetland zones after the second growing season, a reinforcement planting is REQUIRED.

## Constructed wetlands buffers and setbacks

Warning: It is Required that a 50 foot setback between high water levels of stormwater ponds and public water supply wells be provided. It is assumed that constructed wetlands fall under the definition of stormwater ponds in MDH Rule 4725.4350.
• It is Highly Recommended that a buffer extending 25 feet outward from the maximum water surface elevation be provided. Permanent structures (e.g., buildings) should not be constructed within the buffer. This distance may be greater under local regulations.
• The buffer should be contiguous with other buffer areas that are required by existing regulations (e.g., stream buffers).
• It is Highly Recommended that existing trees should be preserved in the buffer area during construction. It is desirable to locate forest conservation areas adjacent to ponds. To help discourage resident geese populations, the buffer can be planted with trees, shrubs and native ground covers.

## Safety

Warning: It is Required that public safety be considered in every aspect of stormwater wetland design.
• The principal spillway opening should not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter should be fenced to prevent a hazard.
• The access and aquatic benches should be landscaped to prevent access to the wetland.
• Warning signs prohibiting swimming, skating, and fishing should be posted.
• Wetland fencing is generally not encouraged, but may be required by some municipalities. A preferred method is to grade to eliminate steep drop-offs or other safety hazards.
• Dam safety regulations should be strictly followed with stormwater wetland design to ensure that downstream property and structures are adequately protected.

## Design procedure

As previously indicated, if the stormwater wetland is being designed to meet requirements for permanent stormwater management in the MPCA CGP, the design criteria of the permit for wet sedimentation basins apply. The following procedure is based on those criteria. If the stormwater wetland is being designed as a retrofit or is not subject to the criteria listed in the MPCA permit, then the criteria listed in the permit are not required to be followed but may be used for general guidance.

### Step 1: Make a preliminary judgment as to whether site conditions are appropriate

Make a preliminary judgment as to whether site conditions are appropriate for the use of a stormwater wetland, and identify the function of the wetland in the overall treatment system.

A. Consider basic issues for initial suitability screening, including:

• Site drainage area
• Soils
• Slopes
• Space required for wetland
• Depth to water table
• Receiving waters

B. Determine how the wetland will fit into the overall stormwater treatment system

• Are other BMPs to be used in concert with the constructed wetland?
• Will a pond be part of the wetland design and if so, where?

### Step 2: Confirm local design criteria and applicability

A. Determine whether the wetland must comply with the MPCA Permit.

B. Check with local officials and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

### Step 3: Confirm site suitability

A. Perform field verification of site suitability. If the initial evaluation indicates that a wetland would be a good BMP for the site, it is Recommended that a sufficient number of soil borings be taken to ensure wetland that conditions (hydrologic and vegetative) can be maintained after construction. The number of borings will vary depending on size of the site, parent material and design complexity. For example, a design that requires compacted earth material to form a dike will likely require more borings than one without this feature.

• It is Recommended that the soil borings or pits be five feet below the bottom elevation of the proposed stormwater wetland.
• It is Highly Recommended that the field verification be conducted by a qualified geotechnical professional.

B. Perform water balance calculations if needed.

### Step 4: Compute runoff control volumes and permanent pool volume

Calculate the Permanent Wetland Pool Volume Vpp, if needed, Water Quality Volume Vwq, Channel Protection Volume Vcp, Overbank Flood Protection Volume Vp10, and the Extreme Flood Volume Vp100.

Warning: If the wetland is being designed as a wet detention pond under the MPCA permit, then a Permanent Wetland Pool Volume, Vpp, of 1800 cubic feet of storage below the outlet pipe for each acre that drains to the wetland is Required.

This can be calculated by

$V_{pp} = 1800 A$

or

$V_{pp} = 0.0417 IC$

where:

A = total watershed area in acres draining to the pool.

In the case where the entire Vwq is to be treated with other BMPs and the wetland is being constructed only for rate control, a permanent pool may not be required, although it still may be desirable.

The water quality volume, Vwq, can be calculated as:

For normal waters:

$V_{wq} = 0.0417 IC$

For special waters (see Unified sizing criteria):

$V_{wq} = 0.0833 IC$

where:

Ai = the new impervious area in acres.

It is recommended that the Channel Protection Volume, Vcp, be based on the 1-yr, 24-hr rainfall event, though local ordinances may be more restrictive. It should be noted that the Vcp is inclusive of the Vwq. In other words, the Vwq is contained within the Vcp.

Schematic showing a shallow wetland profile

If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq to be treated by the stormwater wetland. If some portion of the other control volumes is treated by other BMPs, it can be subtracted from the overall Vcp, Vp10, and Vp100 to determine the volume to be treated by the wetland. The configuration of the various storage allocations is shown in the stormwater wetland profile in the schematic to the right.

Additional details can be found in Unified sizing criteria

### Step 5: Determine pre-treatment (sediment forebay) volume

In the absence of adequate upstream treatment by other BMPs, it is Highly Recommended that a sediment forebay or similarly effective pre-treatment system be provided at each inlet providing 10 percent or more of the total design inflow, with a Recommended volume equal to 10 percent of the computed wetland permanent pool volume (Vpp) in a pool 4 to 6 feet deep. The forebay storage volume counts toward the total Vpp requirement and may be subtracted from the Vpp for subsequent calculations. Similarly, the storage volume from other BMPs used upstream of the constructed wetland in the treatment train counts toward the total Vwq requirement and may be subtracted from it.

### Step 6: Allocate the remaining Vpp and Vwq volumes

Allocate the remaining Vpp and Vwq volumes among marsh, micropool, and ED volumes. Taking into consideration that 10 percent of the required permanent pool volume has already been allocated to the pre-treatment forebay, to meet the CGP or local requirements the remaining required volume may be allocated between marsh, micropool, and ED volumes using the recommendations presented in the table below.

Design restrictions for special water or other sensitive receiving watersheds.

BMP
Watershed Management Category
A
Lakes
B
Trout Waters
C
Drinking Water*
D
Wetlands
E
Impaired Waters
Wetlands Some variations NOT RECOMMENDED due to poor P removal, combined with other treatments. NOT RECOMMENDED
except for wooded wetlands
RECOMMENDED RECOMMENDED
but no use of natural wetlands
RECOMMENDED

*Applies to groundwater drinking source areas only; use the sensitive lakes category to define BMP Design restrictions for surface water drinking supplies

### Step 7. Determine wetland location and preliminary geometry

Determine wetland location and preliminary geometry, including distribution of wetland depth zones. This step involves initially laying out the wetland design and determining the distribution of wetland surface area among the various depth zones (high marsh, low marsh, and deep water). A stage-storage relationship should be developed to describe the storage requirements and to set the elevation of the wetland pool elevation, the water quality volume, the extended detention volume (if applicable), the channel protection volume, etc.

The proportion of surface area recommended to place in the various depth zones for each type of constructed wetland is shown in the table above. Other guidelines for constructed wetland layout are:

• Provide maintenance access (10 foot width for trucks/machinery)
• Length to width ratios as presented in the above table

### Step 8. Consider water quality treatment volume variations for frozen conditions (Highly Recommended)

When the pond and sediment forebay are frozen, much of the storage is rendered ineffective because stormwater runoff can flow over the ice and bypass the intended treatment. To alleviate this problem, additional extended detention storage (which is available even under frozen conditions) can be designed into the pond by increasing the extended detention storage volume designated for water quality control, or by adding a weir structure to the sediment forebay overflow area (see discussion in Cold climate impact on runoff management).

The average snowmelt volume can be computed from the following equation:

Average snowmelt volume (depth/unit area)= Average snowpack depth at the initiation of the snowmelt period x Typical snowpack water at time of melt – Estimated infiltration volume likely to occur during a 10-day melt period.</center>

A series of maps will allow the designer to determine the average depth of snowpack existing at the start of spring snowmelt, the water content of the snowpack during the month of March and the expected infiltration.

### Step 9. Compute extended detention outlet release rate(s), and establish Vcp elevation

Shallow Wetland: The Vcp elevation is determined from the stage-storage relationships and the outlet is then sized to release the channel protection storage volume over a 24-hour period (12-hour extended detention may be warranted in some cold water streams). The channel protection outlet should have a minimum diameter of 3 inches and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged 12 to 18 inches below the elevation of the wetland pool, or 6 inches below the normal ice depth, where outlet depths permit, is recommended. Adjustable gate valves can also be used to achieve these equivalent diameters.

1. The desired release rate may then be calculated by

$Q_{cp} = V_{cp} / t$

where:

t = the detention time in seconds determined above.

Check to determine if Qcp is less than or equal to 5.66 cubic feet per second per acre of surface area of the wetland. If Qcp meets the criterion, proceed to the next step in the process. If Qcp is greater than 5.66 cubic feet per second, the release time should be increased or a two-stage outlet should be used whereby the first outlet is able to discharge Vwq to meet the permit requirements. A two-stage outlet procedure is presented for the ED Shallow Wetland.

2. The average head is calculated as

$h_{avg} =(EL_{cp} - EL_{wp}) / 2$

where:

ELcp = the elevation of the channel protection volume; and
ELwp = the wetland pool elevation.

3. Given the design release rate an outlet may be sized using either the weir or orifice equations.

4. The discharge from the wetland can then be computed for any elevation between ELcp and the wetland pool elevation.

ED Shallow Wetland: Based on the elevations established in Step 6 for the extended detention portion of the water quality volume, the water quality outlet is sized to release this extended detention volume in 24 hours. If a water quality orifice is used, it should have a minimum diameter of 3 inches, and should be adequately protected from clogging by an acceptable external trash rack. A reverse slope pipe attached to the riser, with its inlet submerged one foot below the elevation of the permanent pool, is a recommended design. Adjustable gate valves can also be used to achieve this equivalent diameter. The Vcp elevation is then determined from the stage-storage relationship. The invert of the channel protection outlet is located at the water quality extended detention elevation, and the structure outlet is sized to release the channel protection storage volume over a 24-hour period (12-hour extended detention may be warranted in some cold water streams).

Steps to compute the ED outlet are similar to those presented above for the Shallow Wetland. In this procedure Vwq is equal to the extended detention volume.

1. The time period over which to release the Vwq volume is typically 24 hours, though this time may be reduced to 12 hours depending on thermal concerns of receiving bodies of water.

2. The release rate may then be calculated by

$Q_{wq} = V_{wq} / t$

where:

t = the detention time in seconds.

Check to determine if Qwq is less than or equal to 5.66 cubic feet per second per acre of surface area of the wetland. If Qwq meets the criterion, proceed to the next step in the process. If Qwq is greater than 5.66 cubic feet per second, the release time should be increased.

3. The average head is calculated as

$h_{avg} = (EL_{WQ} - EL_{WP}) / 2$

where:

ELwq = elevation of the water quality volume elevation in ft
ELwp = wetland pool elevation.

4. Depending upon the outlet configuration, use the weir or orifice equation to calculate the outlet size.

5. The discharge from the wetland through the primary outlet device can then be computed for any elevation between ELwq and the wetland pool elevation. The next step is to calculate the secondary outlet size to drain the channel protection volume.

6. The release rate may then be calculated by

$Q_{cp} = (E_{wq} + E_{cp} / 2) - E_{PermPond}$

where:

t = time in seconds. Check to determine if Qcp meets all design requirements.

7. The average head is calculated as

$h_{cp-avg} = (E{cp} - E{WQ}) / 2$

where:

Ecp is the elevation of the channel protection volume and Ewq is the water quality elevation.

8. The appropriate outlet equation can then be used to calculate the outlet’s opening size based on the Qcp computed above. For example, if an orifice is used for an outlet, its opening size, Acp, can be computed as

$A_{CP} = Q_{CP} / (C (2g / s^2) h_{avg}^{0.5})$

where:

g = gravitational constant equal to 32.2 [feet/s2]
C = discharge coefficient, which can be conservatively estimated to be 0.6.

The diameter of the opening can then be solved for

$d_{CP} = 2 (A_{CP} / π)^{0.5}$

9. The discharge from the wetland can then be computed for any elevation above the water quality elevation as

$Q_{cp} = Kh^{0.5}$

where:

K = CACP (2g0.5); and
h = ELWS - ELWP - dCP / 2

### Step 10. Calculate Qp10 (10-year storm) release rate and water surface elevation

Set up a stage-storage-discharge relationship for the control structure for the desired number of outlets and the 10-year storm. The procedure will be similar to that outlined above for the Shallow ED wetland.

### Step 11. Design embankment(s) and spillway(s)

Size emergency spillway, calculate 100-year water surface elevation, set top of embankment elevation, and analyze safe passage of the Extreme Flood Volume (Vp100).

At final design, provide safe passage for the 100-year event. Attenuation may not be required.

The following guidelines should also be followed (see NRCS Practice Standard 378 for further guidance):

• Embankments should be stabilized with vegetation (no trees) or riprap.
• Embankments may require a core-trench if geotechnical considerations warrant.
• Embankment side slopes should not be steeper than 1V:3H on the front, 1V:3H on the back (impounded side).
• Minimum embankment top width is 6 feet (8 feet if equipment access is necessary).
• Material consolidation and shrinkage needs to be factored into embankment design.
• Emergency overflows must be stabilized

To prevent freezing and associated blockage of the inflow, inlet pipes should not be completely submerged, and to the extent possible they should be buried below the frost line. It is also important to design the inlet to reduce or prevent scour, by including riprap or flow diffusion devices such as plunge pools or berms. To prevent standing water in the pipe, which reduces the potential for ice formation in the pipe, increase the slope to 1 percent if conditions permit.

### Step 13. Design sediment forebay

It is recommended that a sediment marker be included in the forebay to indicate the need for sediment removal in the future. Also, a hard bottom surface in the forebay will make sediment removal easier, but note that a hard bottom surface will likely result in reduced vegetative and biotic processes that remove pollutants.

### Step 14. Design outlet structures

Be aware of concerns associated with frozen conditions, particularly the risk of clogging or blockage of outlet structures with ice.

For weir structures, the minimum slot width should be 3 inches.

The minimum outlet pipe diameter should be 18 percent, with a minimum slope of 1 percent.

Outlet pipes should be buried below the frost line to the extent possible. Information on frost depths can be found from the Minnesota Department of Transportation

If a riser pipe with an orifice outlet is used, the orifice should be protected by a hood that draws water from 12 to 18 inches below the normal wetland pool elevation, or 6 inches below the normal ice layer if known, if outlet site conditions permit.

Trash racks should be installed at a shallow angle in order to discourage ice formation.

A baffle weir or skimmer can be used to keep organic floatables in the wetland and prevent ice or debris from blocking the outlet.

Also, outlet pipes through the embankment should be equipped with an anti-seepage collar to prevent failure.

### Step 15. Design maintenance access and safety features

Warning: Maintenance access to the pond, forebay, and inlet and outlet structures is Required. The access routes should be designed with a minimum 10 foot width and maximum 15 percent slope.

Safety features such as obstructive planting that make access difficult, signs warning against fishing and swimming, fencing, and grates over outlet structures should be included as appropriate.

Aesthetic enhancements such as trails or benches can also be included

Warning: If an outlet structure is greater than five feet deep, it is Required that OSHA health and safety guidelines be followed for safe construction and access practices.

Additional information can be obtained from OSHA.

### Step 16. Check expected pond performance against regulatory requirements.

Check that Vwq is detained for an average of 12 hours.

Check that the Vwq release rate does not exceed 5.66 cubic feet per second per acre of pond area.

Determine applicable requirements for Vcp volume and release rate, and verify that the constructed wetland performs adequately for the appropriate design event.

Determine applicable requirements for Qp10 and Qp100 release rates (e.g., pre-development rates), and check release rates (and freeboard) for the appropriate design events.

### Step 17. Prepare vegetation and landscaping plan.

A landscaping and planting plan by a qualified professional for the pond and surrounding area should be prepared, utilizing native vegetation wherever possible. See Major design elements section for guidance on preparing vegetation and landscaping management plan.

### Step 18. Prepare operation and maintenance (O&M) plan.

Warning: Preparation of a plan for operation and maintenance of the pond and associated structures and landscaping is Required.

See the Operation and Maintenance section for further details.

### Step 19. Prepare cost estimate.

Refer to the Cost Considerations section below for guidance on preparing a cost estimate for constructed wetlands.

Stormwater wetland cost estimate worksheet.

Description Units Quantity Unit Cost Total Estimated Price Project Title Owner Location Project Number Date Tree removal - up to 12" diameter each $350.00$0.00 Clear and grub brush square yard $1.50$0.00 Tree protection - temp. fence lineal foot $3.00$0.00 Topsoil - 6" depth, salvage on site square yard $4.50$0.00 Excavation - deepwater zone - 4' average depth square yard $5.00$0.00 Excavation - marsh zone - 1' average depth square yard $1.00$0.00 Grading square yard $1.50$0.00 Hauling off-site - 5' depth square yard $5.00$0.00 Inlet structure each $2,000.00$0.00 Outlet structure each $3,500.00$0.00 Sod - above vegetative bench square yard $4.50$0.00 Soil preparation square yard $25.00$0.00 Seeding - vegetative bench square yard $0.50$0.00 Planting square yard $30.00$0.00 Subtotal $0.00 10% Contingencies$0.00 Subtotal $0.00 Apply MN Location Factor$0.00 TAL CONSTRUCTION COST $0.00 Debris removal per visit$100.00 $0.00 Remove invasive plants per visit$500.00 $0.00 Replant wetland vegetation per plant$10.00 $0.00 Repair erosion square yard$75.00 $0.00 Sediment removal and disposal cubic yard$10.00 $0.00 Mow per visit$150.00 $0.00 Gate / valve operation per visit$125.00 $0.00 Inspection per visit$125.00 $0.00 Subtotal$0.00 Apply MN Location Factor $0.00 TOTAL ANNUAL O&M COST$0.00 Minnesota Location Factors Bemidji 0.963 Brainerd 1.003 Detroit Lakes 0.962 Duluth 0.991 Mankato 0.990 Bemidji 0.963 Minneapolis 1.035 Rochester 0.983 St. Paul 1.000 St. Cloud 1.002 Thief River Falls 1.042 Willmar 0.961 Windom 0.935

Note: Suggested unit costs are based on RS Means prices for Spring, 2005, then factored into an area basis based on typical design features for Constructed Wetlands BMPs. To be used for preliminary cost estimation

# Construction specifications for stormwater wetlands

CADD-based details for pond and wetland systems are contained in the Computer-aided design and drafting (CAD/CADD) drawings section. The following details, with specifications, have been created for stormwater ponds/wetlands:

• Typical Pond Plan and Profile
• Shallow Wetland Plan and Profile
• Extended Detention Shallow Wetland Plan and Profile
• Outlet Structure Details

# Operation and maintenance of stormwater wetlands

## Overview

Maintenance is necessary for a stormwater wetland to operate as designed on a long-term basis. The pollutant removal, channel protection, and flood control capabilities of stormwater wetlands will decrease if:

• wetland pool elevations fluctuate dramatically;
• debris blocks the outlet structure;
• pipes or the riser are damaged;
• invasive plants out-compete the wetland plants;
• sediment accumulates in the stormwater wetland, reducing the storage volume;
• slope stabilizing and desirable wetland vegetation is lost; or
• the structural integrity of the embankment, weir, or riser is compromised.

Stormwater wetland maintenance activities range in terms of the level of effort and expertise required to perform them. Routine stormwater wetland maintenance, such as mowing and removing debris or trash, is needed multiple times each year, but can be performed by citizen volunteers. More significant maintenance, such as removing accumulated sediment, is needed less frequently but requires more skilled labor and special equipment. Inspection and repair of critical structural features such as embankments and risers, needs to be performed by a qualified professional (e.g., structural engineer) that has experience in the construction, inspection, and repair of these features.

## Design phase maintenance considerations

The following references may be consulted for more information on stormwater wetland maintenance:

Implicit in the design guidance in the previous sections, many design elements of stormwater wetland systems can minimize the maintenance burden and maintain pollutant removal efficiency.
Warning: Primarily, providing easy access (typically 8 feet wide) to stormwater wetlands for routine maintenance is Required

Mosquito control is of particular concern in the case of stormwater wetlands. They can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations, but no design will eliminate them completely. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat.

## Construction phase maintenance

The construction phase is another critical step where O&M issues can be minimized or avoided.

Inspections during construction are needed to ensure that the stormwater wetland is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided at this link.

## Post construction operation and maintenance

Proper post-construction maintenance is important to the long-term performance of a stormwater wetland. Potential problems due to lack of maintenance include:

• A clogged outlet structure can increase water levels, killing vegetation and reducing the wetland’s ability to attenuate and store floods. Water quality can be compromised by not providing adequate storage time.
• Excess sediment can reduce storage volumes leading to many of the problems outlined above.
• Nuisance issues such as beaver and muskrat burrows/dens can threaten the integrity of embankments.

Some important post construction maintenance considerations are provided below. More detailed maintenance guidance can be found in the Stormwater Pond and Wetland Maintenance Guidebook (CWP, 2004).

Warning: It is Required that a legally binding and enforceable maintenance agreement be executed between the practice owner and the local review authority.
• Adequate access must be provided for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles. It is Recommended that a maintenance right of way or easement extend to ponds from a public or private road.
• It is Highly Recommended that stormwater wetlands be inspected annually during winter freeze periods to look for signs of improper operation.
• It is Highly Recommended that sediment removal in the forebay occur every 2 to 7 years or after 50 percent of total forebay capacity has been lost. In areas where road sand is used, an inspection of the forebay and stormwater wetland should be scheduled after the spring melt to determine if clean-out is necessary.
• Sediments excavated from stormwater wetlands that do not receive runoff from confirmed hotspots are generally not considered toxic or hazardous material, and can be safely disposed by either land application or land filling. Sediment testing may be necessary prior to sediment disposal when a confirmed hotspot land use is present.
• Periodic mowing of the stormwater wetland buffer is Highly Recommended along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year), prairie, or forest.

General maintenance activities and schedule are provided in the following table.

Typical inspection/maintenance frequencies for stormwater wetlands

Inspection Items Maintenance Items Frequency
• Ensure that at least 50% of wetland plants survive.
• Check for invasive wetland plants.
Replant wetland vegetation One time - After First Year
• Inspect low flow orifices and other pipes for clogging
• Check the permanent pool or dry pond area for floating debris, undesirable vegetation.
• Investigate the shoreline for erosion
• Monitor wetland plant composition and health.
• Look for broken signs, locks, and other dangerous items.
• Mowing – minimum Spring and Fall
• Remove debris
• Repair undercut, eroded, and bare soil areas.
Monthly to Quarterly or After Major Storms (>1”)
• Monitor wetland plant composition and health.
• Identify invasive plants
• Assure mechanical components are functional
• Trash and debris clean-up day
• Remove invasive plants
• Harvest wetland plants
• Replant wetland vegetation
• Repair broken mechanical components if needed
Semi-annual to annual
• All routine inspection items above Inspect riser, barrel, and embankment for damage
• Inspect all pipes
• Monitor sediment deposition in facility and forebay
• Pipe and Riser Repair
• Forebay maintenance and sediment removal when needed
Every 1 to 3 years
Monitor sediment deposition in facility and forebay Forebay maintenance and sediment removal when needed 2-7 years or 50% loss of sediment forebay storage
Remote television inspection of reverse slope pipes, underdrains, and other hard to access piping
• Sediment removal from main pond/wetland
• Pipe replacement if needed
5-25 years

# Calculating credits for stormwater wetlands

Warning: Models are often selected to calculate credits. The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.
 Recommended pollutant removal efficiencies, in percent, for constructed ponds. Sources. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen TSS TP PP DP TN Metals Bacteria Hydrocarbons 85 50 91 0 30 70 60 80
Information: The discussion of credits applies only to wet ponds. Dry ponds do not receive credit for volume or pollutant removal

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

 Recommended pollutant removal efficiencies, in percent, for constructed wetlands. Sources. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen TSS TP PP DP TN Metals Bacteria Hydrocarbons 73 38 69 0 30 70 60 80

This page provides a discussion of how constructed basins (constructed ponds and constructed wetlands) can achieve stormwater credits.

## Overview

Schematic showing characteristics of a constructed pond or constructed wetland.
Information: The discussion of credits applies only to wet ponds. Dry ponds do not receive credit for volume or pollutant removal

Stormwater ponds and stormwater wetlands are the most common types of constructed basins. Constructed basins have a permanent pool of water and are built for the purpose of capturing and storing stormwater runoff. These basins are constructed, either temporarily or in a permanent installation, to prevent or mitigate downstream water quantity and/or quality impacts. Several types of constructed basins and wetlands (stormwater basins, constructed stormwater ponds, wet ponds, forebays, wet sedimentation basins, wet detention ponds, constructed wetlands, stormwater wetlands, etc) are included in this general category. Generally stormwater ponds do not have a significant area of vegetation. Stormwater wetlands do have significant vegetation that enhances the nutrient removal of the basin. Not included in this BMP category are dry basins without a permanent pool. Also not included are oil/water separators, swirl concentrators, and other manufactured devices with a permanent pool of water in the device.

### Pollutant Removal Mechanisms

Constructed basins rely on physical, biological, and chemical processes to remove pollutants from incoming stormwater runoff. The primary treatment mechanism is gravitational settling of particulates and their associated pollutants as stormwater runoff resides in the permanent pool. Stormwater wetlands provide an additional mechanism for the removal of nutrient and other pollutants through the uptake by algae and aquatic vegetation. Volatilization and chemical activity can also occur in both ponds and wetlands, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons (WEF, ASCE/EWRI, 2012).

The longer stormwater runoff remains in the permanent pool, the more settling (and associated pollutant removal) and other treatment will occur. After the particulates settle to the bottom of a pond, a permanent pool provides protection from re-suspension when additional runoff enters the pond during and after a rain event (WEF, ASCE/EWRI, 2012).

### Location in the Treatment Train

Stormwater treatment trains are comprised of multiple Best Management Practices (BMPs) that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Constructed basins are typically located at the end of the stormwater treatment train, capturing all the runoff from the site.

## Methodology for calculating credits

This section describes the basic concepts used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Constructed basins generate credits for TSS and TP. They do not substantially reduce the volume of runoff. Constructed basins are effective at reducing concentrations of other pollutants associated with sediment, including metals and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for Other pollutants.

### Assumptions and approach

In developing the credit calculations, it is assumed the constructed basin is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the Manual (pond design, construction, maintenance; wetland design, construction, maintenance).

The approach in the following sections is based on the following general design considerations, which are consistent with the Construction Stormwater General Permit (CGP).

• It is REQUIRED in the CGP that the water quality volume (Vwq) is discharged at no more than 5.66 cubic feet per second per acre surface area of the pond.
• The REQUIRED total storage volume (Vts) equals the sum of the volume in the permanent pool (Vpp below the outlet elevation) plus live storage allocation for water quality volume (Vwq). Vwq equals 1.0 inch of runoff per impervious acre.
• If the pond is being designed as a wet detention pond for new construction under the MPCA CGP Permit, then a permanent pool volume (Vpp) equal to 1,800 cubic feet for each acre draining to the pond is REQUIRED.
• It is REQUIRED in the CGP that permanent pool depths be a minimum of 3 feet and maximum of 10 feet at the deepest points.
• It is REQUIRED in the CGP that the riser be located so that short-circuiting between inflow points and the riser does not occur.
• The constructed basin must be situated outside of surface waters and any buffer required under Appendix A, Part C.3

If any of these assumptions are not valid, the credit will be reduced.

### Volume credit calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff, and serve the purpose of reducing peak stormwater flows for channel protection and overbank flood control. Pollutant removal is accomplished by the maintenance of a permanent pool of water that serves to both settle and store the particulates. The necessity of the permanent pool negates the ability to infiltrate runoff; therefore no volume credit is obtained for basins and wetlands.

### Total suspended solids (TSS) calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff. No credits associated with volume reduction are available.

The event-based TSS credit for constructed basins, MTSS in pounds, is given by

$M_{TSS} = 0.0000624\ R_{TSS}\ EMC_{TSS}\ V_{pp}$

where

RTSS is the TSS removal fraction for the constructed basin;
EMCTSS is the event mean concentration of TSS in runoff, in milligrams per liter;
Vpp is the volume treated by the BMP, in cubic feet; and
0.0000624 is a conversion factor.

TSS removal for constructed ponds and wetlands varies with the design. Median removal rates in this Manual are 84 percent for constructed ponds and 73 percent for constructed wetlands. For a discussion of the principles of sedimentation, see Weiss et al..

The water quality volume (VWQ), which is equivalent to Vpp, is delivered instantaneously to the BMP. The VWQ can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch times the new impervious surface area. For MIDS, the VWQ is 1.1 inches times the new impervious surface area.

The annual TSS credit, in pounds, is given by

$M_{TSS} = 2.72\ R_{TSS}\ EMC_{TSS}\ F\ V_{annual}$

where

F is the fraction of annual runoff treated by the BMP,
Vannual is annual runoff in acre-feet, and
2.72 is a conversion factor.

For a constructed pond or wetland, the fraction of annual runoff treated by the BMP is assumed to be 1, meaning all runoff from the contributing area passes through and is treated by the BMP.

Example calculation

Assume a constructed pond is designed to treat 5 acres of impervious surface and 5 acres of forested land on B (SM) soils. The TSS concentration in runoff is 54.5 milligrams per liter. Annual runoff, calculated using the MIDS calculator, is 11.72 acre-feet. The annual TSS reduction is 2.72 * 0.84 * 54.5 * 11.72 = 1459 pounds. If the BMP was a constructed wetland instead of a constructed pond, the removal efficiency would be 0.73 instead of 0.84 and the TSS reduction would be 1268 pounds.

### Total phosphorus (TP) calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff. No credits associated with volume reduction are available.

Phosphorus in runoff is assumed to be 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). The event-based TP removal, MTP in pounds, is given by

$M_{TP} = 0.0000624\ ((0.55\ R_{PP})\ + (0.45\ R_{DP}))\ EMC_{TP}\ V_{pp}$

where

• RPP is the removal fraction for particulate phosphorus;
• RDP is the removal fraction for dissolved phosphorus; and
• EMCTP is the event mean concentration for total phosphorus in runoff, in milligrams per liter.

Constructed basins typically receive no credit for DP. Information on phosphorus removal fractions (percentages) can be found here. Recommended values for RPP are 0.91 for constructed ponds and 0.69 for constructed wetlands.

The annual TP credit, in pounds, is given by

$M_{TP} = 2.72\ ((0.55\ R_{PP})\ + (0.45\ R_{DP}))\ EMC_{TSS}\ F\ V_{annual}$

where

• F is the fraction of annual runoff treated by the BMP;
• Vannual is annual runoff in acre-feet; and
• 2.72 is a conversion factor.

For a constructed pond or wetland, the fraction of annual runoff treated by the BMP is assumed to be 1, meaning all runoff from the contributing area passes through and is treated by the BMP.

Example calculation

Assume a 10 acre site with 5 acres of impervious and 5 acres of forested land. Annual rainfall is 31.9 inches and the soil is B (SM) with an infiltration rate of 0.45 inches per hour. The TP EMC is 0.3 milligrams per liter and the removal efficiency of the BMP for particulate phosphorus is 0.91. No dissolved phosphorus is removed. The MIDS calculator was used to calculate an annual runoff of 11.72 acre-feet delivered to the BMP. The annual TP reduction is therefore

2.72 * ((0.55 * 0.91) + (0.45 * 0)) * 0.3 * 11.72 = 4.79 pounds

If the BMP was a constructed wetland the removal efficiency for particulate phosphorus would be 0.68 instead of 0.91 and the total phosphorus removed would be 3.58 pounds.

## Methods for calculating credits

This section provides specific information on generating and calculating credits from constructed basins for total suspended solids (TSS) and total phosphorus (TP). Stormwater runoff pollution reductions (“credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
2. The Simple Method and MPCA Estimator
3. MIDS Calculator
4. Quantifying volume and pollution reductions based on values reported in literature
5. Quantifying volume and pollution reductions based on field monitoring

The techniques described in this article assume that volume credit cannot be obtained for stormwater ponds and wetlands. This is based on an overall assumption that ponds and wetlands have insignificant losses related to seepage, evaporation, and transpiration. Stormwater pond and wetland designers that suspect significant volume losses from a specific BMP are encouraged to quantify these volume losses through field measurements.

Ponds and wetlands are also effective at reducing concentrations of other pollutants including nitrogen and metals. This article does not provide information on calculating credits for pollutants other than TSS and phosphorus, but references are provided that may be useful for calculating credits for other pollutants; see Other Pollutants and References for more information.

### Credits Based on Models

Warning: The model selected depends on your objectives. For compliance with the Construction Stormwater permit, the model must be based on the assumption that an instantaneous volume is captured by the BMP.

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for stormwater ponds and wetlands. The available models described in this section are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

• Model name and version
• Date of analysis
• Person or organization conducting analysis
• Detailed summary of input data
• Calibration and verification information
• Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for constructed basin BMPs. In using this table to identify models appropriate for constructed ponds and wetlands, use the sort arrow on the table and sort by Constructed Basin BMPs. Models identified with an X may be appropriate for using with constructed basins.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SELECT X X X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.

### The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of mean pollutant concentrations and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load. Default TSS removal fractions are 0.84 for wet basins and 0.73 for constructed wetlands. Default removal fractions for TP are 0.50 for wet basins and 0.38 for constructed wetlands.

Warning: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

A quick guide for the estimator is available Quick Guide: MPCA Estimator tab.

### MIDS calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various BMPs, including constructed ponds and constructed wetlands. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including constructed ponds and constructed wetlands. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

### Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the constructed pond or constructed wetland device. A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page.

Designers may use the pollutant reduction values in the Minnesota Stormwater Manual or may research values from other databases and published literature. Designers who opt for this approach should

• select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database;
• select a pollutant removal reduction from literature that studied a stormwater pond or wetland device with site characteristics and climate similar to the device being considered for credits;
• when using data from an individual study, review the article to determine that the design principles of the studied stormwater pond or wetland are close to the design recommendations for Minnesota and/or by a local permitting agency; and
• give preference to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed stormwater pond, considering such conditions as watershed characteristics, pond sizing, and climate factors.

• International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals
• Compilation of BMP performance studies published through 2011
• Provides values for TSS, Bacteria, Nutrients, and Metals
• Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels
• Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon
• Appendices L and M contain Excel spreadsheets of structural and non-structural BMP performance evaluations
• Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics
• Applicable to flters, wet ponds, porous pavements, soakage trenches, flow through stormwater planters, infiltration stormwater planters, vegetated infiltration basins, swales, and treatment wetlands
• The Illinois Green Infrastructure Study
• Figure ES-1 (page 9) summarizes BMP effectiveness
• Provides values for TN, TSS, peak flows / runoff volumes
• Applicable to permeable pavements, constructed wetlands, infiltration, detention, filtration, and green roofs
• New Hampshire Stormwater Manual.
• Volume 2, Appendix B summarizes BMP effectiveness
• Provides values for TSS, TN, and TP removal
• Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
• BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA
• Appendix B provides pollutant removal performance curves
• Provides values for TP, TSS, and Zn
• Pollutant removal broken down according to land use
• Applicable to infiltration trench, infiltration basin, bioretention, grass swale, wet pond, and porous pavement
• Watershed Protection Techniques, Technical Note #114. Pollutant Removal Dynamics of Three Wet Ponds in Canada. 2000
• Provides values for TSS, phosphorus, nitrogen, metals, bacteria, pentachlorophenol and oil/grease
• Applicable to wet ponds
• Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report
• Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
• Applicable to wet basins, stormwater wetlands, bioretention filter, sand filter, infiltration trench, and filter strips/grass swales
• Semadeni‐Davies, Annette. "Winter performance of an urban stormwater pond in southern Sweden." Hydrological processes 20.1 (2006): 165-182
• Provides removal efficiencies in cold-weather climates for TSS and metals, and reports influent/effluent vales of pH
• Applicable to stormwater ponds

### Credits based on field monitoring

In the event that a credit is being calculated for an existing stormwater pond or wetland installation, field monitoring may be made in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
1. Which pollutants will be measured?
2. Will the monitoring study the performance of a single BMP or multiple BMPs?
3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
4. Will the results be compared to other BMP performance studies?
5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
2. Plan the field activities. Field considerations include
1. equipment selection and placement;
2. sampling protocols including selection, storage, and delivery to the laboratory;
3. laboratory services;
4. health and Safety plans for field personnel;
5. record keeping protocols and forms; and
6. quality control and quality assurance protocols
3. Execute the field monitoring
4. Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.

• Chapter 4: Stormwater Characterization
• 4.2: General Characteristics and Pollutant Sources
• 4.3: Sources of Stormwater Quality data
• Chapter 8: Performance Evaluation
• 8.1: Methodology Options
• 8.5: Evaluation of Quality Performance for Individual BMPs
• 8.6: Overall Hydrologic and Water Quality Performance Evaluation
• Chapter 10: Hydrologic Evaluation
• 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices
• In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).
Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include.

• Chapter 4: Monitoring Methods and Equipment
• Chapter 5: Analytical Methods and Laboratory Selection
• Chapter 6: Monitoring Site Selection
• Chapter 8: Equipment Installation and Maintenance
• Chapter 10: Pre-Storm Preparation
• Chapter 11: Sample Collection and Handling
• Chapter 12: Quality Assurance / Quality Control
• Chapter 13: Laboratory Reports and Data Review
• Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

## Other Pollutants

In addition to TSS and phosphorus, constructed basins can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that constructed basins are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD). A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Other Pollutants Reduced by Constructed Basins: Stormwater Ponds

Pollutant Category Constituent Treatment Capabilities (Low = < 30%;

Medium = 30-65%; High = 65 -100%)

Metals1, 2 Cd, Cr, Cu, Zn Medium/High
As, Fe, Ni, Pb
Nutrients Total Nitrogen, Medium
TKN Low
Organics High

1 Results are for total metals only
2 Information on As was found only in the International Stormwater Database where removal was found to be low