This document combines several documents related to stormwater ponds. Individual documents can be viewed by clicking on the appropriate link below.
Stormwater ponds articles

# Overview

Photo of a wet pond.

This section provides an overview of stormwater . 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 ponds are typically installed as an end-of-pipe BMP at the downstream end of the . Stormwater pond size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, due to their size and versatility, stormwater ponds are often 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 (or common area of development) is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, stormwater ponds fall under the category Wet Sedimentation Basin described in the CGP. If used in combination with other practices, for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA CGP, 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 pond will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.

Of course, there are situations, particularly retrofit projects, in which a stormwater pond is constructed without being subject to the conditions of the MPCA CGP. 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 pond, depending on where it is situated both jurisdictionally and within the surrounding landscape.

## Retrofit suitability

Ponds are widely used for stormwater retrofits and have two primary applications as a retrofit design. In communities where dry detention ponds (see Types of stormwater ponds) were designed for flood control in the past, these facilities can be modified by adding a permanent pool for water quality treatment and adapting the outlet structure for channel protection. This is desirable because dry ponds have limited effectiveness for pollutant removal. Alternatively, new ponds can be installed in available open areas as a part of a comprehensive watershed retrofit inventory.

Note that the MPCA CGP permanent pool specifications do not apply to retrofit ponds that serve an existing developed area unless new impervious acreage occurs as part of the retrofit project. Therefore, any of the aforementioned pond variants may be considered, along with other alternative approaches to treatment basin design.

## Special receiving waters suitability

The table below provides guidance regarding the use of stormwater ponds in areas upstream of special .

Design restrictions for special waters - constructed ponds and wetlands

BMP
Watershed Management Category
A
Lakes
B
Trout Waters
C
Drinking Water*
D
Wetlands
E
Impaired Waters
Constructed 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
Wet Extended Detention Pond RECOMMENDED Some variations NOT RECOMMENDED due to pool and stream warming concerns RECOMMENDED RECOMMENDED (alteration of natural wetlands as stormwater wetlands not allowed) RECOMMENDED

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

## Cold climate suitability

Plan and profile view of a wet detention pond. Click on image to enlarge.

One of the biggest problems associated with proper pond operation during cold weather is the freezing and clogging of inlet and outlet pipes. 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.
• Incorporating winter operating levels as part of the design to introduce available storage for melt events (see figure at right and Cold climate impact on runoff management).
• 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, the minimum opening diameter should be ½ inch. In addition, the pipe should have a minimum 8 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.
• In cold climates, riser hoods should be oversized and reverse slope pipes should draw from at least 6 inches below the typical ice layer.
• 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 both avoids stream warming and serves as a non-freezing outlet.
• Trash racks should be installed at a shallow angle to prevent ice formation.

## Water quantity treatment

Ponds are one of the best and most cost-effective stormwater treatment practices for providing runoff detention storage for channel protection and overbank flood control (see Unified sizing criteria). These goals are achieved with the use of extended detention storage, where runoff is stored above the permanent pool and released at a specified rate through a control structure. Wherever an embankment is constructed to store water at a level higher than the surrounding landscape, dam safety regulations must be followed to ensure that downstream property and structures are adequately protected.

Ponds are primarily detention practices and therefore do not retain significant amounts of water. There is some loss to evapotranspiration and seepage through the bottom of the pond.

## Water quality

Information: The discussion of water quality credits applies only to wet ponds. Dry ponds do not receive credit for volume or pollutant removal

Ponds 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 pond. Another mechanism for the removal of pollutants (particularly nutrients) is uptake by algae and aquatic vegetation. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons.

The longer the runoff remains in the pond, the more settling (and associated pollutant removal) and other treatment can occur, and after the particulates reach the bottom of the pond, the permanent pool protects them from resuspension when additional runoff enters the basin. For these reasons, because they lack the crucial permanent pool, dry extended detention ponds are not considered an acceptable option for meeting water quality treatment goals; however, they may be appropriate to meet water quantity criteria (Vcp, Vp10, Vp100; see Unified sizing criteria). It should again be noted that the only type of pond complying with the MPCA CGP is the wet extended detention pond (or wet sedimentation basin) constructed according to the minimum standards outlined in the permit.

The long detention or retention time associated with stormwater ponds can be problematic in coldwater fisheries due to the potential increase in water temperature. In these situations, detention times should be limited to a maximum of 12 hours or other treatment alternatives () should be explored.

Removal efficiencies and typical stormwater pond effluent concentrations for key pollutants for wet extended detention ponds are provided in the following two tables.

Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.

Practice TSS TP PP DP TN Metals1 Bacteria Hydrocarbons
Infiltration2 3 3 3 3 3 3 3 3
Biofiltration and Tree trench/tree box with underdrain 80 link to table link to table link to table 50 35 95 80
Sand filter 85 50 85 0 35 80 50 80
Iron enhanced sand filter 85 65 or 746 85 40 or 606 35 80 50 80
Dry swale (no check dams) 68 link to table link to table link to table 35 80 0 80
Wet swale (no check dams) 35 0 0 0 15 35 35 NSD
Constructed wet ponds4, 5 84 50 or 685 84 8 or 485 30 60 70 80
Constructed wetlands 73 38 69 0 30 60 70 80
Permeable pavement (with underdrain) 74 41 74 0 NSD NSD NSD NSD
Green roofs 85 0 0 0 NSD NSD NSD NSD
Vegetated (grass) filter 68 0 0 0 NSD NSD NSD NSD
Harvest and reuse Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.

TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
1Data for metals is based on the average of data for zinc and copper
2BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
3Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
4Dry ponds do not receive credit for volume or pollutant removal
5Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
6Lower values are for Tier 1 design. Higher values are for Tier 2 design.

Typical pollutant concentrations leaving stormwater pond BMPs. Concentrations are in milligrams per liter (ppm). Note that a range of values, from low to high, is provided for TSS and TP

Practice TSS Low-Med-High TP Low-Med-High TN Cu Zn
Stormwater Ponds 10-19-30 0.10-0.17-0.25 1.3 0.005 0.030

## Limitations

The following general limitations should be recognized when considering installation of stormwater ponds. Ponds generally

• consume a large amount of space,
• tend to increase water temperature and may cause downstream thermal impact,
• have the potential for nuisance insects or odor,
• are problematic for areas of low relief, high water table, or near-surface bedrock, and
• pose safety concerns

# Types of stormwater ponds

Photo of a wet detention pond
Photo of a dry pond

Several distinct pond design variants (see CADD designs) are typically described in current stormwater management literature. While it is possible that any one of these pond types could be beneficially implemented somewhere in Minnesota, both the climatic conditions and the applicable regulations prevalent throughout the state strongly favor the use of one of them in particular, namely the wet extended detention pond (wet pond). Indeed, the is the only design variant fitting the description of a Wet Sedimentation Basin as described in the MPCA Construction General Permit (CGP). For this reason, much of this discussion focuses on wet extended detention ponds; however, all four main design variants are presented here for the sake of completeness.

## Flow-through pond (no extended detention) design

Often called a “wet pond” in other literature, a pond that has an essentially unrestricted spillway as its primary outlet, with its crest at the elevation of the permanent pool. It provides water quality treatment by holding a volume of stormwater equal to the permanent pool volume, permitting settling to occur. The water stored in the pond is later displaced by new runoff. Note that “wet sedimentation basin” in the MPCA CGP is not a flow-through pond (“wet pond”) but rather a wet extended detention pond. The flow-through pond is generally not a good design option for Minnesota, because the storage volume allocated for treatment is entirely below the permanent pool, making it inaccessible to new runoff during frozen conditions (see Cold climate impact on runoff management).

Information: All pond designs should incorporate an access bench

## Wet extended detention pond

The wet sedimentation basin referenced in the MPCA CGP falls under this category. This indicates a combination of permanent pool storage and extended detention storage above the permanent pool to provide additional water quality or rate control.

## Micropool extended detention pond

This variation of the wet extended detention pond has a markedly smaller permanent pool at the pond outlet to prevent resuspension. Typically, the permanent pool in a micropool extended detention pond will not be large enough to satisfy the requirements of the MPCA CGP.

## Dry pond

Information: Dry ponds do not receive credit for volume or pollutant removal

This pond has no permanent pool; it relies only upon extended detention storage for its treatment volume. It is highly susceptible to sediment resuspension and generally only useful for rate control.

# Design criteria for stormwater ponds

This section describes information on design of stormwater ponds.

The following terminology is used throughout this "Design Section":

Warning: Required - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

Highly recommended - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the constructed pond, but not specifically required by the MPCA CGP.

Recommended - Indicates design guidance that is helpful for constructed performance but not critical to the design.

## Summary of permit requirements

Permit requirements are included throughout this page. A summary of these requirements is provided below.

• Stormwater ponds must not be located in, nor drain water from, wetlands unless mitigated for
• Stormwater ponds must not be located within surface water bodies or any buffer zones required under Section 23.11 of the CSW permit
• The Required minimum permanent pool volume, or dead storage (Vpp below the outlet elevation), is 1800 cubic feet of storage below the outlet pipe for each acre that drains to the pond
• The Required minimum water quality volume, or live storage (Vwq), is 1.0 inch of runoff from the net increase in impervious surfaces created by the project. This should be calculated as an instantaneous volume
• The CGP requires that the Vwq is discharged at no more than 5.66 cubic feet per second per acre of surface area of the pond. The surface area of the pond is calculated at the elevation that results from the Vwq being dropped into the pond instantaneously
• Permanent pool depths must be a minimum of 3 feet and maximum of 10 feet at the deepest points
• Basin outlets must have energy dissipation
• Adequate maintenance access, typically with a minimum width of 8 feet, must be provided. Where a forebay is installed, direct vehicle/equipment access should be provided to the forebay for sediment removal and other maintenance activities. The maintenance access should extend to the forebay, access bench, riser, and outlet, and allow vehicles to turn around
• An emergency spillway must be provided to pass storms in excess of the pond hydraulic design. The spillway must be stabilized to prevent erosion and designed in accordance with applicable dam safety requirements (NRCS Pond Standard 378 and Mn/DNR dam safety guidelines). The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. If the spillway crosses the maintenance access, materials meeting the appropriate load requirements must be selected
• The riser must be located so that short-circuiting between inflow points and the riser does not occur
• Basin outlets must be designed to prevent discharge of floating debris
• Permittees must design basins using an impermeable liner if located within active karst terrain
• Inlet areas should be stabilized to ensure that non-erosive conditions exist during high-flow events
• All pond designs should incorporate an access bench
• (Minnesota Department of Health Rule 4725.4350) states that a minimum horizontal distance between a water-supply well and the ordinary high water level of a storm water retention pond is 35 feet
• Public safety must be considered in every aspect of pond design

## Physical feasibility initial check

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

Warning: It is Required that stormwater ponds not be located in, nor drain water from, wetlands unless mitigated for
Warning: It is Required that stormwater ponds not be located within surface water bodies or any buffer zones required under Section 23.11 of the CSW permit
Warning: It is Required that the minimum horizontal distance between a water-supply well and the ordinary high water level of a storm water retention pond is 35 feet
Warning: It is Required that CSW permittees must design basins using an impermeable liner if located within active karst terrain.
• Drainage area – 10 acres minimum Recommended, to ensure 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. See [1]
• Space required – Approximately 1 to 3 percent of the tributary drainage area is Recommended for the pond footprint.
• Site topography and slope – It is Highly Recommended that slopes immediately adjacent to ponds be less than 25 percent but greater than 0.5 to 1 percent to promote flow towards the pond.
• Minimum head – The elevation difference Recommended at a site from the inflow to the outflow is 6 to 10 feet, but lower heads will work at small sites.
• Minimum depth to water table – In general, there is no minimum separation distance required with ponds. Intercepting the groundwater table can help sustain a permanent pool. However, some source water protection requirements may dictate a separation distance or an impervious liner if there is a sensitive underlying aquifer and the bottom material of the pond allows for infiltration.
• Soils – Underlying soils of hydrologic group C or D should be adequate to maintain a permanent pool. A liner may be needed for most group A soils and some group B soils, in order to maintain a permanent pool. A site specific geotechnical investigation should be performed. Also, if earthen embankments are to be constructed, it will be necessary to use suitable soils and to follow guidance in NRCS Pond 378 or other guidelines from the Dam Safety Section of the Minnesota Department of Natural Resources.
• Groundwater protection – It is Highly Recommended that ponds treating runoff from potential stormwater hotspots (PSHs) have excellent pretreatment practices provided. In some cases (depending on the land use and associated activities), lining the pond may be necessary to protect groundwater, particularly when the seasonally high groundwater elevation is within three feet of the pond bottom.
• Separation distance - The minimum horizontal distance between a water-supply well and the ordinary high water level of a storm water retention pond is 35 feet ([2]).
• Karst – It is Recommended that ponds not be used in karst areas, due to the long term implication of having deep ponded water. If ponds are used in karst areas, impermeable liners and a minimum 3 foot vertical separation from the barotic rock layer are Recommended. Geotechnical investigations are necessary in karst areas.
• Cold water fisheries – Ponds may not be appropriate practices where receiving waters are sensitive cold water fisheries, due to the potential for stream warming from pond outflows. If ponds are used, it is Highly Recommended that the 1-year, 24-hour storm be detained for no longer than 12 hours. If regulatory provisions allow, a smaller permanent pool with more extended detention storage should be considered.
• Shallow soils and bedrock – For situations with shallow bedrock and ground water, pond use is limited due to the available depth, affecting the surface area required as well as the aesthetics of the pond. Consider stormwater wetlands as an alternative.

## Conveyance

### Inflow points

• It is Highly Recommended that pretreatment be provided to reduce the future pond maintenance burden. If pretreatment has not been provided in the contributing watershed, then it is Recommended that a forebay be provided at each inlet contributing greater than 10 percent of the total design storm inflow to the pond.
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).
Warning: It is Required that basin outlets have energy dissipation
• It is Highly Recommended that rip-rap or other channel liners be extended below the permanent pool elevation.
• It is Highly Recommended that inlet pipe inverts be located at the permanent pool elevation. Submerging the inlet pipe can result in freezing and upstream damage.
• 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 where open channels are used to convey runoff to the pond, the channels be stabilized to reduce the sediment loads.

Pond outfalls should be designed to not 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).
Warning: It is Required in the CGP that the Vwq is discharged at no more than 5.66 cubic feet per second per surface area of the pond.
• 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 rip-rap 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.

## Pretreatment

Construction of pretreatment measures immediately upstream of the main pond is Highly Recommended, to reduce the maintenance requirements and increase the longevity of a stormwater treatment pond. A large portion of the overall sediment load (the heavier sediments) can be captured by relatively small (and therefore relatively easy to clean and maintain) BMPs. The larger pond area can thus be devoted to the settling of finer sediments, allowing it to fill more slowly and therefore requiring less frequent maintenance.

It is therefore Highly Recommended that each pond have a sediment forebay or equivalent upstream pretreatment (non-pond BMPs may serve as pretreatment) at each inflow point that contributes greater than 10 percent of the inflow volume. A sediment forebay is a small pool, separated from the permanent pool by barriers such as earthen berms, concrete weirs, or gabion baskets, where initial settling of heavier particulates can occur.

Warning: It is Required that where a forebay is installed, that direct vehicle/equipment access be provided to the forebay for sediment removal and other maintenance activities.
• It is Highly Recommended that flows from forebays enter the permanent pool area with non-erosive outlet conditions.
• It is Recommended that the forebay(s) be sized to contain 10 percent of the water quality volume (Vwq; see Unified sizing criteria) in a pool that is four to six feet deep. The forebay storage volume counts toward the total permanent pool requirement.
• It is Recommended that the forebay(s) be designed with a surface area equivalent to 10 percent of the pond permanent pool surface area or equivalent to 0.1 percent of the drainage area.
• It is Recommended that a fixed vertical sediment depth marker be installed in the forebay to measure sediment deposition over time. The marker should be sturdy and placed deep enough into the bottom of the forebay so that ice movement does not affect its position.
• It is Recommended that the bottom of the forebay be hardened, using concrete, asphalt, or grouted riprap, to make sediment removal easier.
Information: Pretreatment is an alternative to the preferred active management strategy of drawing down the permanent pool seasonally to provide detention while the permanent pool is frozen.

## Treatment

### Permanent pool and water quality volume (Vwq) sizing for new impervious area

• Under the MPCA Permit, it is Required that stormwater ponds have permanent pool volume (dead storage) equal to at least 1800 cubic feet per acre of drainage to the pond. For example, a 30-acre drainage area requires a permanent pool volume of at least 54,000 cubic feet or 1.24 acre-feet.
Warning: 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 new impervious acre.
Warning: It is Required that permanent pool depths be a minimum of 3 feet and maximum of 10 feet at the deepest points.
• Where phosphorus load reductions are a priority, it is Recommended that a maximum depth of 8 feet be used, to limit the likelihood of stratification and the potential for bottom sediment to release phosphorus.
• If extended detention storage for the Channel Protection Volume (Vcp; see Unified sizing criteria) is provided, it is Recommended that the Vwq be computed and routed through the outlet for Vcp.
Warning: It is Required that the Vwq be released at a rate not to exceed 5.66 cubic feet per second per acre of permanent pool surface area.
• It is Highly Recommended that the Vcp be released over a minimum 24 hour period.
• Where phosphorus load reductions are a priority, permanent pool volumes as large as 3600 cubic feet per acre of drainage are Recommended for enhanced removal.
• To compensate for ice build-up on the permanent pool, it is Highly Recommended that 12 inches (or a volume equal to the average snow melt) of additional storage be provided. This is an alternative to the preferred active management strategy of drawing down the permanent pool seasonally to provide detention while the permanent pool is frozen.
• Using pumps or bubbling systems can reduce ice build-up and prevent the formation of an anaerobic zone in pond bottoms. Caution must be exercised, however, because of the possibility of thin or no ice cover.
• A water balance is Recommended to document sufficient inflows to maintain a constant permanent pool during prolonged dry weather conditions. 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 pond = (inflows – outflows).
• Potential inflows include runoff, baseflow and rainfall (groundwater and surface water).
• Potential outflows include infiltration, surface overflow and evapotranspiration.
• Assume no inflow from baseflow, no outflow losses for infiltration and because only the permanent pool volume is being evaluated, no outflow losses for surface overflows. The validity of these assumptions need to be verified for each design.

### Pond liners

It is Highly Recommended that pond liners be considered in circumstances where a permanent pool is needed but difficult to maintain due to site conditions, or where seepage from the pond into the groundwater would otherwise occur but must be avoided. This includes:

If geotechnical tests confirm the need for a liner, see the section on liner specifications.

The site layout and pond grading affect the pollutant removal capability of the pond as well as the ease of maintenance. Performance is enhanced when multiple treatment pathways are provided by using multiple cells, longer flowpaths, high surface area to volume ratios, complex microtopography, and/or redundant treatment methods (combinations of pool, extended detention, and marsh). It is Recommended that a berm or simple weir be used instead of pipes to separate multiple ponds, because of the higher freezing potential of pipes. Specific guidelines are provided below:

• It is Highly Recommended that pond side slopes within the permanent pool (below the aquatic bench) not exceed 1:2 (V:H).
• It is Highly Recommended that side slopes to the pond should be 1:3 (V:H) or flatter, and that they terminate on an access bench.
• It is Recommended that approximately 15 percent of the permanent pool surface area be allocated to a shallow (i.e., less than or equal to 18 inches in depth) zone along the perimeter to promote a shallow marsh littoral zone.
• It is Recommended that the minimum length to width ratio for ponds be 1.5:1.
• It is Recommended that the maximum drainage area to surface area ratio be 100:1.
• It is Recommended that to the greatest extent possible, ponds should be irregularly shaped and long flow paths should be maintained.

### Pond benches

All pond designs should incorporate an access bench (a shallow slope area adjacent to the pond, providing equipment access and preventing people from slipping into the water) and a submerged aquatic bench (a shallow slope area just inside the pond perimeter, facilitating the growth of aquatic plants). This is a Highly Recommended design practice that may be required by local authorities. Mosquito breeding concerns exist along bench areas. Therefore, it is Highly Recommended that designers follow recommendations from the Metropolitan Mosquito Control District.

• Access Bench: It is Highly Recommended that an access bench extending 10 feet outward from the permanent pool edge to the toe of the pond side slope be provided. Narrower benches may be used on sites with extreme site limitations. The maximum cross-slope of the access bench should be 0.06:1 (V:H), or 6 percent. Access benches are not needed when the pond side slopes are 1:4 (V:H) or flatter.
• Aquatic Bench: It is Highly Recommended that an irregularly configured aquatic bench, extending up to 10 feet inward from the normal shoreline and graded no more than 18 inches below the permanent pool water surface elevation, be incorporated into the pond.
Information: All pond designs should incorporate an access bench

### Maintenance access

Warning: It is Required that adequate maintenance access, typically with a minimum width of 8 feet, be provided.

If feasible, it is Recommended that the access be 10 feet wide, have a maximum slope of 0.15:1 (V:H) or 15 percent, and be appropriately stabilized for use by maintenance equipment and vehicles. Steeper grades may be allowable if designed using appropriate materials for the grade.

Warning: It is Required that the maintenance access extends to the forebay, access bench, riser, and outlet, and allows vehicles to turn around.

### Riser in embankment

Warning: It is Required that the riser be located so that short-circuiting between inflow points and the riser does not occur.

It is Recommended that the riser be located within the embankment for maintenance access, prevention of ice damage, and aesthetics.

### Spillway design

The principle spillway (riser) should be designed for the desired release rates while keeping the future maintenance needs in mind. Lessening the potential for clogging and freezing, creating safe access paths for inspection and maintenance, barring access to children and vandals, and allowing safe draw down of the permanent pool, when necessary, are goals of riser design that consider long-term maintenance needs.

### Non-clogging low flow orifice

It is Highly Recommended that the low flow orifice be adequately protected from clogging by either an acceptable external trash rack (recommended minimum orifice of 3 inches) or by internal orifice protection that may allow for smaller diameters (recommended minimum orifice of 1 inch). The Recommended method is a submerged reverse-slope pipe that extends downward from the riser to an inflow point at least one foot below the normal pool elevation (see CADD designs). This should also draw from at least 6 inches below the typical ice layer. To avoid release of deposited sediment, the pipe should not be installed on the pond floor.

Alternative methods are to employ a broad crested rectangular, V-notch, or proportional weir, protected by a half-round CMP that extends at least 12 inches below the normal pool. It is Highly Recommended that the minimum weir slot width be 3 inches, especially when the slot is tall. It is Recommended that hoods over orifices be oversized to account for ice formation.

### Trash racks

Warning: It is Required that basin outlets be designed to prevent discharge of floating debris.

It is Highly Recommended that the principal spillway openings be equipped with removable trash racks to prevent clogging by large debris and to restrict access to the interior for safety purposes. US EPA guidance on control of floatables suggests that openings in the range of 1.5 inches are both cost-efficient and effective in removing floatables and large solids.

It is Recommended that trash racks be installed at a shallow (~15°) angle to prevent ice formation.

Baffle weirs (essentially fences in the pond) can prevent ice reformation during the spring melt near the outlet by preventing surface ice from blocking the outlet structure.

### Pond drain

It is Highly Recommended that each pond be equipped with a drain that can dewater the pond to the maximum extent possible within 24 hours. The drain pipe should have an elbow or protected intake extending at least 1 inches above the bottom of the permanent pool to prevent deposited sediment from clogging the pipe or being re-released while the pond is being drained.

It is Recommended that the pond drain and possibly the low flow orifice be equipped with an adjustable gate valve (typically a handwheel activated knife gate valve). These valves should be located inside the riser, where they (a) will not normally be inundated and (b) can be operated in a safe manner. To prevent vandalism that alters the pond level, the handwheel should be chained to a ringbolt, manhole step or other fixed object.

It is Recommended that both the low flow orifice pipe and the pond drain be sized one pipe size greater than the calculated design diameter and the gate valve be installed and adjusted to an equivalent orifice diameter.

### Riser access

It is Recommended that lockable manhole covers and manhole steps within easy reach of valves and other controls be installed, to allow for maintenance access and prevent vandalism.

### Emergency spillway

Warning: It is Required that an emergency spillway should be provided to pass storms in excess of the pond hydraulic design.
Warning: It is also Required that the spillway be stabilized to prevent erosion and designed in accordance with applicable dam safety requirements (NRCS Pond Standard 378 and Mn/DNR dam safety guidelines). The emergency spillway must be located so that downstream structures will not be impacted by spillway discharges. If the spillway crosses the maintenance access, materials meeting the appropriate load requirements must be selected.

### Temperature control

Caution: The use of wet ponds in watersheds containing trout streams is strongly discouraged, because the discharge can cause stream temperature warming.

The Permittee(s) must design the Permanent Stormwater Management System such that the discharge from the project will minimize any increase in the temperature of trout stream receiving waters resulting from the one (1)-and two (2)-year 24-hour precipitation events. This includes all tributaries of designated trout streams within the Public Land Survey System (PLSS) Section that the trout stream is located. Projects that discharge to trout streams must minimize the impact using one or more of the following measures, in order of preference:

a. Minimize new impervious surfaces.
b. Minimize the discharge from connected impervious surfaces by discharging to vegetated areas, or grass swales, and through the use of other non-structural controls.
c. Infiltration or other volume reduction practices evapotranspiration of to reduce runoff in excess of pre-project conditions (up to the two (2)-year 24-hour precipitation event).
d. If ponding is used, the design must include an appropriate combination of measures such as shading, filtered bottom withdrawal, vegetated swale discharges or constructed wetland treatment cells that will limit temperature increases. The pond should be designed to draw down in 24 hours or less.
e. Other methods that will minimize any increase in the temperature of the trout stream.

The following recommendations, from the North Carolina design Manual, C-3, Wet Ponds, pertain to reducing the warming of stormwater in a wet pond:

• Trees and shrubs can be planted to maximize pond shading, primarily along the south, east, and west sides of the basin to reduce temperature impacts.
• The outlet structure can be modified to withdraw from a deeper point in the permanent pool to reduce temperature impacts.

## Landscaping

### Landscaping plan

It is Highly Recommended that a landscaping plan for the stormwater pond and the surrounding area be prepared to indicate how aquatic and terrestrial areas will be stabilized, and established with vegetation (see vegetation for guidance on vegetation). Landscaping plans should also include maintenance schedules. It is Highly Recommended that the plan be prepared by a qualified professional. The following guidance suggests how landscaping can be incorporated into pond design.

Woody vegetation should not be planted or allowed to grow within 15 feet of the toe of the embankment or 25 feet from the inlet and outlet structures.

Wherever possible, wetland plants should be encouraged in a pond design, either along the aquatic bench (fringe wetlands), the access bench and side slopes (ED wetlands) or within shallow areas of the pool itself.

The best elevations for establishing wetland plants, either through transplantation or volunteer colonization, are within six inches (plus or minus) of the normal pool.

The soils of a pond buffer are often severely compacted during the construction process to ensure stability. The density of these compacted soils can be so great that it effectively prevents root penetration, and therefore, may lead to premature mortality or loss of vigor. Consequently, it is advisable to excavate large and deep holes around the proposed planting sites, and backfill these with uncompacted topsoil or other organic material.

As a rule of thumb, planting holes should be three times deeper and wider than the diameter of the rootball (of balled and burlap stock), and five times deeper and wider for container grown stock. This practice should enable the stock to develop unconfined root systems.

Species that require full shade, are susceptible to winterkill, or are prone to wind damage should be avoided. Extra mulching around the base of the tree or shrub is strongly recommended as a means of conserving moisture and suppressing weeds.

### Pond buffers and setbacks

Warning: It is Required (Minnesota Department of Health Rule 4725.4350) that a minimum horizontal distance between a water-supply well and the ordinary high water level of a storm water retention pond is 35 feet.

It is Highly Recommended that a pond buffer extending 25 feet outward from the maximum water surface elevation of the pond be provided. Permanent structures (e.g., buildings) should not be constructed within the buffer. This distance may be greater under local regulations.

The pond 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 pond 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 pond.
• Warning signs prohibiting swimming, skating, and fishing should be posted.
• Pond fencing is generally not encouraged because the fence limits access to emergency personnel. A preferred method is to grade the pond to eliminate steep drop-offs or other safety hazards. Designers should check local requirements since fencing is required by some municipalities.
• Dam safety regulations should be strictly followed with pond design to ensure that downstream property and structures are adequately protected.

## Design considerations for ponds used for harvest and use

Photo of pond used for capturing stormwater and irrigating Eagle Valley and Prestwick Golf Club. Photo courtesy of Emmons and Olivier Resources.

Stormwater ponds can be used as the storage component of a stormwater harvest and use system. These ponds are multi-purpose, providing stormwater retention, sedimentation, and storage for later use. In this way, stormwater harvest and use systems can be part of a treatment train approach for stormwater management. Existing ponds can be retrofitted to serve as a water source for a harvest and use system.

The first question to ask before selecting a constructed stormwater pond as the BMP is whether a pond is the most appropriate BMP. If the goal is to meet a volume retention requirement and the retention requirement can be met through infiltration of stormwater, then stormwater infiltration practices should be considered. On soils conducive to infiltration and where site constraints do not exist, infiltration will typically be the most appropriate BMP. However, if site goals include other factors, such as replacing a water supply or irrigation of vegetation, harvest and use is an appropriate BMP.

Example pond design for a harvest and use/reuse system.

Ponds should be designed following guidance in pond design guidance section of this manual. However, there are or may be specific design considerations for stormwater ponds used in harvest and use/reuse systems. These design considerations are summarized below.

• Reuse of stormwater from a pond treating runoff from potential stormwater hotspots may pose a public safety & welfare concern, as well as may be cost prohibitive to pre-treat if special filter devices are required.
• Stormwater and rainwater harvest and use/reuse systems may require a water-supply well to supplement irrigation needs when runoff is not available. A minimum horizontal distance of 35 feet may apply.
• Important to maintain a permanent pool depth below which no pumping occurs to prevent resuspension of sediment.
• Multiple aquatic benches may be necessary for ponds that experience repeated bounce or drawdown due to irrigation reuse. An alternative to multiple aquatic benches would be mild side slopes of 5:1 from the bench downward to the permanent pool elevation, then grade downward as necessary. These considerations are dependent on aesthetics, adjacent land use (residential vs. commercial, etc.), and objectives for operations and maintenance.
• Ponds that experience repeated bounce or drawdown due to irrigation reuse may create an environment for invasive vegetation species. Some of these species may include an abundance of volunteer sandbar willow and cottonwood, which may need to be removed.
• Pump house, control panels, intake and discharge pumps, electrical controls, etc. need to be secure to prevent public access.
• The operator will need to access the reuse system for operations & maintenance, therefore a well thought out landscape plan needs to be prepared.

Below is a list of additional considerations that are not specifically addressed above.

• Some stormwater pond owners prefer inlet pipe inverts be submerged to reduce erosion. Consideration should be given to pipe material and diameter. Reinforced concrete pipe (RCP), with tied joints may last longer than high density polyethylene pipe (HDPE) or corrugated metal pipe (CMP), which both can become buoyant when submerged, and even damaged from repetitive ice heave. In addition, the diameter of the pipe entering the pond may be oversized to account for submerged inverts, and reduced capacity. This is applicable to all constructed ponds.
• Adequate sediment storage must be provided to preserve reservoir capacity for intended use(s)
• Pond must be properly designed. This includes consideration of local codes and watershed district rules, water quality targets for both the intended use, and water quality goals for water captured by the pond but not used for the intended use (e.g. water discharged to a surface water body via the storm sewer system).
• Is lining the pond with topsoil or clay necessary to hold water for use or prevent infiltration in Groundwater Protection Areas? If soil infiltration rates are high in the underlying soils, have infiltration BMPs been considered for stormwater management?
• What is the depth of the pond at normal water level (NWL) and after drawdown for irrigation?
• Does a drawdown limit need to be set with a flow to maintain sufficient water levels for pond aesthetics? Can a buffer of tall, native vegetation be used to improve aesthetics during drawdown?
• Will there be limitations to vegetation established due to water level bounce and drawdown?
• Consider the potential for erosion from inlets and sideslopes under low pond water levels.
• To discourage the growth of algae and other microorganisms, ponds should be sized such that detention times are not excessive during warm weather. As temperatures increase, the recommended maximum detention time decreases (Met Council, 2011). This is not likely to be a concern for reuse ponds since detention times are typically short. The following table is from the Met Council Reuse Guide Storage Systems Toolbox I.2, originally adapted from New South Wales Department of Environment and Conservation, Managing Urban Stormwater, Harvesting and Reuse, April 2006:

Maximum Detention Time - Average Daily Temperature

Maximum Detention Time (days) to limit algae blooms: Average Daily Temperature (F)
50 59
30 68
20 77

## Design procedure

The following steps outline a recommended design procedure for a wet extended detention pond (wet sedimentation basin) in compliance with the MPCA CGP for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.

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

Warning: For sites covered under the MPCA Construction Stormwater General Permit or under the MPCA Municipal Separate Storm Sewer System General Permit, the permittee must first consider implementing on-site volume control practices.

A. Make a preliminary judgment as to whether site conditions are appropriate for the use of a stormwater pond, and identify the function of the pond in the overall treatment system. A. Consider basic issues for initial suitability screening, including:

• Site drainage area
• Depth to water table
• Depth to bedrock
• Presence of wetlands
• Soil characteristics
• Receiving water(s)

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

• Decide whether the pond is the only BMP to be employed, or if are there other BMPs (including other ponds) addressing some of the treatment requirements.
• Determine whether the pond needs to treat water quality (Vwq), quantity (Vcp, Qp, Qf), or both (see Unified sizing criteria).
• Determine whether the pond is being designed as a wet sedimentation basin under the MPCA General Stormwater Permit for Construction Activities (CGP).
• Decide where on the site the pond is most likely to be located.

### Step 2. Confirm design criteria and applicability.

A. Determine whether the pond must comply with the MPCA CGP.

B. Check with local officials, watershed organizations, 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 pond would be a good BMP for the site, it is Recommended that one boring per acre with a minimum of three soil borings or pits be dug in the same location as the proposed pond to verify soil types and to determine the depth to ground water and bedrock.
• It is Recommended that the minimum depth of the soil borings or pits be five feet below the bottom elevation of the proposed pond.
• 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 Pool Volume (Vpp), Water Quality Volume (Vwq)

$V_{pp} = 1800A$

where

A = the drainage area of the stormwater pond (acres)

or

$V_{pp} = 0.0417 A$

where

A = the drainage area of the stormwater pond (square feet)
Warning: If the pond is being designed as a wet detention pond for new construction under the MPCA CGP Permit, then a permanent pool volume equal to 1800 cubic feet for each acre draining to the pond is Required.

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

Warning: It is also Required that sediment deposited during construction be removed before normal operation begins (refer to MPCA Permit for additional design requirements).

The water quality volume, Vwq, in cubic feet is given by

$V_{wq} = 1inch * IC * \frac{43,560 ft^2}{12 inches}$

where

IC = new impervious area (acres)

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 pond. It is assumed that the pond will be the only BMP used for rate control for larger storms. If this is the case, the pond should be designed to treat the entirety of these runoff control volumes.

### Step 5. Determine location and preliminary geometry.

The preliminary grading plan can be developed with the following procedure:

1. Locate the pond in the lowest elevation area of the site (not in a jurisdictional wetland) and provide space around the pond for maintenance access (10 foot minimum width is Recommended).
2. Establish a primary outlet elevation (normal water level) and a pond bottom elevation.
3. Provide storage for the permanent pool below the primary outlet elevation in the main pond area.
4. Include an aquatic bench extending into the permanent pool and an access bench extending out from the permanent pool.
5. Considering the desired pond footprint during the Vwq design storms, allocate storage volume above the primary outlet elevation for Vwq. While developing the grading plan, consider the desired (or required) length to width ratio and side slopes detailed earlier in this section (or in applicable regulations).
6. Once the preliminary grading plan has been developed, determine the associated stage-storage relationship for water surface elevations through the maximum expected levels.
Warning: Adequate maintenance access (typically 8 feet) is Required.

The approximate storage corresponding to a given stage (elevation) can be determined using the average end area method. The area within each of the closed contour lines on the grading plan representing the pond is measured, and the average area of each set of adjacent contours is computed. The approximate volume between the two contours is then calculated as the average area multiplied by the elevation difference.

$V_{1-2} = ((A_1 + A_2)/2) (E_2 - E_1)$

where:

V1-2 = the volume between contour 1 and contour 2;
A1 and A2 = the areas within closed contours 1 and 2, respectively; and
E2 and E1 = the elevations of contours 1 and 2, respectively.

Cumulative volume above the bottom of the pond, or above the normal water surface elevation, can be calculated by adding subsequent incremental volumes. This is readily accomplished with the use of a spreadsheet prepared as follows in the table below (the first row of the table below contains the spreadsheet column header, the second row is column description, and the third, fourth, and fifth rows provide an example, with a permanent pool elevation of 902).

The table below is an example spreadsheet - cumulative volume above normal surface elevation.

This table shows an example spreadsheet - Cumulative volume above normal surface elevation.

Spreadsheet Column Header Elevation Area Average Area Depth Volume Cumulative Volume Volume Above Permanent Pool
Spreadsheet Column Description Elevation of Pond Contour Line Area enclosed by Contour Line Average area of current and previous rows Elevation difference between current and previous rows Volume between current and previous contour Volume between current and lowest contour Volume between current and permanent pool contour
Example value 900 1000 N/A N/A N/A 0 N/A
Example value 902 1200 1100 2 2200 2200 0
Example value 904 1600 1400 2 2800 5000 2800

The stage-storage relationship will be used to develop a stage-storage-discharge table as outlet structures are designed. This is an iterative process that may include revising the preliminary grading plan and subsequently redetermining the stage-storage relationship (or using an acceptable model to check).

### Step 6. Determine pretreatment (sediment forebay) volume

In the absence of adequate upstream treatment by other BMPs, it is Highly Recommended that a sediment forebay or similarly effective pretreatment 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 permanent pool volume in a pool 4 to 6 feet deep (at shallower depths, the risk of sediment resuspension in the pretreatment area increases). The forebay storage volume counts toward the total permanent pool requirement. The storage volumes from other BMPs used upstream in the treatment train count toward the water quality volume (Vwq) requirement and thus may be subtracted from it.

### Step 7. Consider water quality treatment volume variations for frozen conditions

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.

This schematic illustrates seasonal operation for snowmelt runoff management.

The average snowmelt volume can be computed from the following equation

$A_{sv} = (A_{sd} S_{nw}) - I_{vol}$

where:

Asv = Average snowmelt volume (depth/unit area)
Asd = Average snowpack depth at the initiation of the snowmelt period
Snw = Typical snowpack water at time of melt
Ivol = Estimated infiltration volume likely to occur during a 10-day melt period.

A series of maps that 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 8. Size and design outlet structures

Warning: It is Required in the CGP that the Vwq is discharged at no more than 5.66 cubic feet per second per surface area of the pond.

The following outlet stages should be included in the pond design. It is possible to design one device to meet all stages. Equations included in this step are based on certain assumptions about which types of outlet structures will be used to control the various stages. If the designer chooses to use different structure types, the specific equations used to determine stage-discharge relationships will change, but the general approach will remain the same.

Emergency drain: a drawdown pipe sized to drain the pond within 24 hours to allow access for riser repairs and sediment removal, or to lower the permanent pool in late fall (to provide additional storage during frozen conditions)
Water quality (low flow) outlet: an outlet (typically an orifice) designed to release Vwq with an average detention time of 12 hours. After designing the orifice, a check should be made to verify that the release rate is no greater than 5.66 cfs/acre of pond surface area. (Calculation steps adapted from Vermont Stormwater Management Manual.)

The average release rate for Vwq is computed as

$Q_{wq_avg} = V_{wq} / t_{wq}$

where:

twq = the intended Vwq detention time.

From the stage-storage table, find the elevation associated with Vwq. Calculate the approximate average head on the water quality outlet as

$h_{wq_avg} = (E_{wq} - E_{PermPool}) / 2$

where:

Ewq = the Vwq pool elevation; and
EPermPool = the elevation of the permanent pool (the invert of the water quality orifice)

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

$Q_{wq_avg} = CA_{wq} \sqrt{2gh _{wq avg}}$

where:

C = the orifice coefficient (0.6 is typically used, but may not apply in all cases);
Awq = the orifice area; and
g = gravitational acceleration.

The diameter of the orifice is then

$d_{wq} = 2 \sqrt{A_{wq} / \pi}$

The rate of discharge from the orifice for any head value hwq on the orifice can then be computed as

$Q_{wq} = CA_{wq} \sqrt{2gh_{wq}}$

Use this equation to check that your discharge rate meets the CGP.

Using the determined size information, incorporate the outlet structures into the pond design. Be aware of concerns associated with frozen conditions, particularly the risk of clogging or blockage of outlet structures with ice and the importance of burying pipes below the frost line.

Warning: A skimmer or similar device is Required to prevent the discharge of floating debris.

### Step 9. Design spillway and embankments

The NRCS has compiled additional design guidance and requirements for spillways and embankments (NRCS Pond 378 Conservation Practice Standard for Minnesota. The following items are some of the key guidelines to adhere to in the design of spillways and embankments.

• It is Required that the emergency overflow be stabilized.
• It is Required that embankments be overfilled by at least 5 percent to account for settling.
• The Required minimum embankment width is 6 feet (wider for embankment heights greater than 10 feet or if maintenance access will be required).
• It is Required that embankments be adequately stabilized with vegetation or other measures.
• It is Highly Recommended that side slopes be no steeper than 1:3 (V:H).

To prevent freezing and blockage of inflow, it is Highly Recommended that inlet pipes not be fully submerged and that they be buried below the frost line. The Minnesota Department of Transportation has developed frost and thaw depths for several Minnesota sites.

It is also Highly Recommended to design the inlet to reduce or prevent scour, by including riprap or flow diffusion devices such as plunge pools or berms.

### Step 11. Design sediment forebay

The size of the sediment forebay was determined in Step 6. It is Recommended that a sediment marker be included in the forebay to indicate the need for sediment removal in the future. A hard bottom surface in the forebay is also Recommended in order to make sediment removal easier.

As discussed in Step 6, a weir structure added to the forebay will ensure that some pretreatment storage is available, even when the normal forebay is frozen.

### Step 12. Design maintenance access and safety features

Warning: Adequate maintenance access (typically 8 feet) is Required.

The access routes should be designed with a minimum 8 feet 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 on safety for construction sites is available from OSHA. OSHA has prepared a flow chart which will help site owners and operators determine if the site safety plan must address confined space procedures.

### Step 13. Check expected pond performance against regulatory requirements.

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

Check that the Vpp is at least 1800 cubic feet per acre that drains to the pond.

Check that the permanent pool is between 3 feet and 10 feet deep.

Check that Vwq is equal to at least 1.0 inch of runoff from the net increase in impervious surfaces created by the project.

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

### Step 14. 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.

### Step 15. Prepare operation and maintenance plan.

Preparation of a plan for operation and maintenance of the pond and associated structures and landscaping is Highly Recommended. See the Operation and Maintenance section for further details.

### Step 16. Prepare cost estimate.

Refer to the Cost Considerations section for information on preparing a cost estimate for stormwater ponds.

See CADD designs for design details for pond systems. The following details, with specifications, have been created for stormwater ponds:

# Construction specifications for stormwater ponds

The following details, with specifications, have been created for stormwater ponds (see CADD images for individual best management practices):

• stormwater pond plan;
• stormwater pond profile;
• pond inlet; and
• riser pipe outlet structure.

# Design considerations for constructed stormwater ponds used for harvest and irrigation use/reuse

Photo of pond used for capturing stormwater and irrigating Eagle Valley and Prestwick Golf Club. Photo courtesy of Emmons and Olivier Resources.

Stormwater ponds can be used as the storage component of a stormwater harvest and use system. These ponds are multi-purpose, providing stormwater retention, sedimentation, and storage for later use. In this way, stormwater harvest and use systems can be part of a treatment train approach for stormwater management. Existing ponds can be retrofitted to serve as a water source for a harvest and use system.

The first question to ask before selecting a constructed stormwater pond as the BMP is whether a pond is the most appropriate BMP. If the goal is to meet a volume retention requirement and the retention requirement can be met through infiltration of stormwater, then stormwater infiltration practices should be considered. On soils conducive to infiltration and where site constraints do not exist, infiltration will typically be the most appropriate BMP. However, if site goals include other factors, such as replacing a water supply or irrigation of vegetation, harvest and use is an appropriate BMP.

Example pond design for a harvest and use/reuse system.

Ponds should be designed following guidance in pond design guidance section of this manual. However, there are or may be specific design considerations for stormwater ponds used in harvest and use/reuse systems. These design considerations are summarized below.

• Reuse of stormwater from a pond treating runoff from potential stormwater hotspots may pose a public safety & welfare concern, as well as may be cost prohibitive to pre-treat if special filter devices are required.
• Stormwater and rainwater harvest and use/reuse systems may require a water-supply well to supplement irrigation needs when runoff is not available. A minimum horizontal distance of 35 feet may apply.
• Important to maintain a permanent pool depth below which no pumping occurs to prevent resuspension of sediment.
• Multiple aquatic benches may be necessary for ponds that experience repeated bounce or drawdown due to irrigation reuse. An alternative to multiple aquatic benches would be mild side slopes of 5:1 from the bench downward to the permanent pool elevation, then grade downward as necessary. These considerations are dependent on aesthetics, adjacent land use (residential vs. commercial, etc.), and objectives for operations and maintenance.
• Ponds that experience repeated bounce or drawdown due to irrigation reuse may create an environment for invasive vegetation species. Some of these species may include an abundance of volunteer sandbar willow and cottonwood, which may need to be removed.
• Pump house, control panels, intake and discharge pumps, electrical controls, etc. need to be secure to prevent public access.
• The operator will need to access the reuse system for operations & maintenance, therefore a well thought out landscape plan needs to be prepared.

Below is a list of additional considerations that are not specifically addressed above.

• Some stormwater pond owners prefer inlet pipe inverts be submerged to reduce erosion. Consideration should be given to pipe material and diameter. Reinforced concrete pipe (RCP), with tied joints may last longer than high density polyethylene pipe (HDPE) or corrugated metal pipe (CMP), which both can become buoyant when submerged, and even damaged from repetitive ice heave. In addition, the diameter of the pipe entering the pond may be oversized to account for submerged inverts, and reduced capacity. This is applicable to all constructed ponds.
• Adequate sediment storage must be provided to preserve reservoir capacity for intended use(s)
• Pond must be properly designed. This includes consideration of local codes and watershed district rules, water quality targets for both the intended use, and water quality goals for water captured by the pond but not used for the intended use (e.g. water discharged to a surface water body via the storm sewer system).
• Is lining the pond with topsoil or clay necessary to hold water for use or prevent infiltration in Groundwater Protection Areas? If soil infiltration rates are high in the underlying soils, have infiltration BMPs been considered for stormwater management?
• What is the depth of the pond at normal water level (NWL) and after drawdown for irrigation?
• Does a drawdown limit need to be set with a flow to maintain sufficient water levels for pond aesthetics? Can a buffer of tall, native vegetation be used to improve aesthetics during drawdown?
• Will there be limitations to vegetation established due to water level bounce and drawdown?
• Consider the potential for erosion from inlets and sideslopes under low pond water levels.
• To discourage the growth of algae and other microorganisms, ponds should be sized such that detention times are not excessive during warm weather. As temperatures increase, the recommended maximum detention time decreases (Met Council, 2011). This is not likely to be a concern for reuse ponds since detention times are typically short. The following table is from the Met Council Reuse Guide Storage Systems Toolbox I.2, originally adapted from New South Wales Department of Environment and Conservation, Managing Urban Stormwater, Harvesting and Reuse, April 2006:

Maximum Detention Time - Average Daily Temperature

Maximum Detention Time (days) to limit algae blooms: Average Daily Temperature (F)
50 59
30 68
20 77

# Assessing the performance of stormwater ponds

Green Infrastructure: Constructed basins (ponds and wetlands) can be an important tool for retention and detention of stormwater runoff. Because they utilize vegetation, bioretention practices provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

Constructed basins ( and ) are designed to retain solids and associated pollutants by settling. A typical method for assessing the performance of of constructed basins is therefore measuring and comparing pollutant concentrations at the influent and effluent.

An online manual for assessing BMP treatment performance was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory. The manual advises on a four-level process to assess the performance of a Best Management Practice.

• Level 1: Visual Inspection. This includes assessments for infiltration practices and for filtration practices. The website includes links to a downloadable checklist.
• Level 2: Capacity Testing. Level 2 testing can be applied to both infiltration and filtration practices.
• Level 3: Synthetic Runoff Testing for infiltration and filtration practices. Synthetic runoff test results can be used to develop an accurate characterization of pollutant retention or removal, but can be limited by the need for an available water volume and discharge.
• Level 4: Monitoring for infiltration or filtration practices

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management . 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:

Additional information on designing a monitoring network and performing field monitoring are found at this link.

Case studies include the following.

# Operation and maintenance of stormwater ponds

Information: For additional information on pond maintenance, link to Maintenance for Sedimentation Practices (U of M)

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

• Permanent pool elevations fluctuate;
• Debris blocks the outlet structure;
• Pipes or the riser are damaged;
• Invasive plants out-compete the wetland plants;
• Sediment accumulates in the pond, reducing the storage volume;
• Slope stabilizing vegetation is lost; or
• The structural integrity of the embankment, weir, or riser is compromised.

Pond maintenance activities range in terms of the level of effort and expertise required to perform them. Routine pond and wetland maintenance, such as mowing and removing debris or trash, is needed multiple times each year. Owners may consider an “adopt-a-pond” program in which properly trained citizen volunteers perform basic landscape maintenance activities (the City of Plymouth, for example, has instituted such a program). 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.

The following terminology is used throughout this section:

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP) or other consistently applicable regulations

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the bioretention practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for bioretention practice performance but not critical to the design.

## Design phase maintenance considerations

Implicit in the design guidance in the previous sections, many design elements of pond systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key maintenance considerations are providing access for inspection and maintenance, and designing all outlets and the principal spillway to minimize clogging.

Warning: Providing easy access (typically 8 feet wide) to all pond components for routine maintenance is Required.

Stormwater ponds can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat (see section on mosquito control).

## 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 practice is built in accordance with the approved design 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

Information: For additional information on pond maintenance, link to Maintenance for Sedimentation Practices (U of M)

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

Warning: It is Required that a maintenance agreement be executed between the BMP owner and the local review authority. It is Highly Recommended that this agreement be legally binding and enforceable.
• 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 ponds be inspected annually during winter freeze periods to look for signs of improper operation.
• It is Highly Recommended that sediment removal in the forebay and permanent pool occur every 2 to 7 years or after 50 percent of total forebay or permanent pool capacity has been lost. In areas where road sand is used, an inspection of the forebay and permanent pool should be scheduled after the spring melt to determine if clean-out is necessary.
• Sediments excavated from stormwater ponds 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 (see also the section on Case studies).
• Periodic mowing of the pond 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.
• Ponds should not be drained during the spring, as temperature stratification and high chloride concentrations at the bottom can occur, which could result in negative downstream effects.
• Care should be exercised while draining the pond to prevent rapid release and minimize the discharge of sediments or anoxic water. The approving jurisdiction should be notified before draining a pond.

Example operation and maintenance checklist here.

Warning: It is Required that OSHA safety procedures be followed for maintenance activities within enclosed areas, such as outlet structures.

### Operation to address frozen conditions

It is Highly Recommended that the Operation and Maintenance plan include a provision to lower the level of the permanent pool in the late fall, to provide additional retention storage for snowmelt runoff and ensure that some permanent pool storage is available above the ice (the permanent pool should not be completely eliminated nor allowed to freeze through completely).

# Calculating credits for stormwater ponds

 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 Design level TSS TP PP DP TN Metals Bacteria Hydrocarbons 1 60 34 60 0 or 401 30 60 70 80 2 84 50 84 8 or 481 30 60 70 80 3 90 60 90 23 or 631 30 60 70 80 1 If iron or another amendment to retain phosphorus has been incorporated into the design, the dissolved phosphorus removal is 40 percent. With no amendment, removal is 0 percent. Note that only iron enhanced pond benches are discussed in this manual as a mechanism for retaining dissolved phosphorus. -

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.
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

This page provides a discussion of how constructed basins ( and ) 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
 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 60 70 80

Stormwater ponds and 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 detention ponds, , wet sedimentation basins, , constructed 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 practices, such as oil/water separators, swirl concentrators, and other manufactured devices, that have 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 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, (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.

### Wet pond design levels

Wet ponds have many potential designs. Credits vary with design. Below are minimum requirements for three design levels used to credit constructed wet ponds.

• Design Level 1: must meet the following criteria
• Dead (or permanent) storage of at least 1800 cubic feet per acre (=1/2 inch of impervious area) that drains to the pond
• The pond’s permanent storage volume must reach a minimum depth of at least 3 feet and must have no depth greater than 10 feet. The basin must be configured such that scour or resuspension of solids is minimized.
• Flow path length to pond width ratio less than 1:1 or greater than 10:1 (scouring occurs at ratios greater than 10:1)
• Design Level 2: Meets all of the requirements for Design Levels 1 and 2 (except flow path) and does not meet all design requirements for Design Level 3
• (flood pool volume) >= 1 inch of impervious area
• Discharge rate of water quality volume does not exceed 5.66 cubic feet per second per acre of surface area of the pond.
• Flow path length to pond width ratio = 1:1 to 3:1. A ratio of 3:1 is recommended.
• Design Level 3: Must meet all of the following design requirements
• Discharge rate of water quality volume does not exceed 5.66 cubic feet per second per acre of surface area of the pond
• Water quality volume (flood pool volume) > 1.5 inch of impervious area
• Wet extended detention or multi-cell system
• Sediment forebay at all major inflows
• Flow path length to pond width ratio 3:1 to 10:1

### Iron-enhanced sand filtration bench in wet ponds

Iron enhanced sand bench, Prior Lake, MN. Photo courtesy of Ross Bintner.
Iron-enhanced sand filter bench schematic.

An iron-enhanced sand filtration bench in a wet pond is essentially a wet extended detention pond with a permanent pool and a flood pool. The outlet structure of the pond is designed such that the water in the flood pool during and after a storm event is held above the elevation of the iron-enhanced sand filter bench, thereby allowing water to filter through the bench. The basic design elements of an iron-enhanced sand filter basin include the following.

• An iron-enhanced sand filter of desired width and length sited along the perimeter of the wet pond (iron-enhanced sands filters should be no less than 5 percent but no greater than 8 percent iron by weight to prevent clogging, see Erickson et al., 2010 and Erickson et al., 2012. The 5 to 8 percent range is based upon iron filing material that is approximately 90 percent elemental iron with a size distribution approximately equal to that of sand.
• An outlet structure that controls the flood pool elevation and can receive the filter bed drain.
• Subsurface drains at the filter bed bottom to drain the bed. The outlet of these subsurface drains should be exposed to the atmosphere and above the downstream high water level to allow the filter to fully drain.
• An impervious barrier (typically geotextile liner, for example HDPE) between the pond and the trench to minimize seepage from the pond into the trench.
• Filter draw down within 48 hours of storm completion to avoid filter fouling and to prepare the filter for next storm event.
• An underdrain that consists of corrugated polyethylene pipe with slits not holes to prevent loss of sand and minimize clogging. If holes are used, the pipe should be covered with pea gravel.

### 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).

Ponds constructed under the Construction Stormwater General Permit (CGP) must meet the following conditions.

• 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.

Constructed ponds

• Design Level 1 TSS removal = 60%
• Design Level 2 TSS removal = 84%
• Design Level 3 TSS removal = 90%

Design Level 2 is the most common design level, with a median removal of 84 percent

Constructed wetlands: median removal rate of 73 percent.

For a discussion of the principles of sedimentation, see Weiss et al..

The (VWQ), which is equivalent to Vpp, is delivered as an instantaneous volume 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 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.

In the Minimal Impact Design Standards (MIDS) Calculator, phosphorus in runoff is assumed to be 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). Using these values, 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.

The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

For wet ponds, removal rates for PP and DP vary with design level. Assuming PP removal is 55% of TP, the removal rates are given below.

• Design Level 1 removal rates: DP = 0%, PP =60%, TP = 34%
• Design Level 2 removal rates: DP = 8%, PP = 84%, TP = 50%
• Design Level 3 removal rates: DP = 23%, PP = 90%, TP = 60%

The MIDS Calculator gives no credit for DP unless an amendment to retain phosphorus is incorporated into the pond design. Data from the International BMP Database indicates constructed basins with no P-retaining amendment typically provide no credit for DP. Information on phosphorus removal fractions (percentages) can be found here. PP removal rates for pond Design Level 2, the most common design, are 0.84 for constructed ponds and 0.69 for constructed wetlands.

Assuming PP is 55 percent of TP, 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 0.85. 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 * 0.84) + (0.45 * 0)) * 0.3 * 11.72 = 4.42 pounds

If the BMP was a constructed wetland the removal efficiency for particulate phosphorus would be 0.68 instead of 0.85 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
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 event mean concentration 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 (EPA 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 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, 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.

Caution: 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.

### 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.

### Credits based on field monitoring

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

This manual contains the following guidance for monitoring.

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:

• Chapter 2: Developing a monitoring plan. Describes a seven-step approach for developing a monitoring plan for collection of data to evaluate BMP effectiveness.
• Chapter 3: Methods and Equipment for hydrologic and hydraulic monitoring
• Chapter 4: Methods and equipment for water quality monitoring
• Chapters 5 (Implementation) and 6 (Data Management, Evaluation and Reporting)
• Chapter 7: BMP Performance Analysis
• Chapters 8 (LID Monitoring), 9 (LID data interpretation]), and 10 (Case studies).
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

# Cost-benefit considerations for stormwater ponds

Costs for ponds typically include costs for embankment, riser and spillway structures, outfall protection, vegetative stabilization, excavation, and grading. Additional costs for site preparation can include soil amendments, precision grading, plant materials and creation of occluding layers in coarse-textured soil types if wetlands systems must be created on upland sites due to project constraints. Project costs can be lowered if existing pre-construction site conditions are carefully considered and isolated areas with hydric soils contained within the footprint of the project are utilized as stormwater management facilities.

Additional maintenance costs will be incurred until the establishment of the wetland ecosystem. Invasive plants must be culled and dead plants replaced. The outlet structure may have to be adjusted, based on seasonal observations, to achieve the proper water surface in the pond.

## Construction and maintenance costs

The Introduction to stormwater BMPs section outlines a cost estimation method which site planners could use to compare the relative construction and maintenance costs for structural best management practices. These curves are excellent for purposes of comparison; however, it is recommended that construction and maintenance budgets should be based on site specific information. Utilizing the table below and the cost estimation worksheet for stormwater ponds, will allow designers to avoid over or under estimation of fixed costs. The table below lists the specific site components that are specific to stormwater ponds. Not included in this table are those cost items that are common to all construction projects, such as mobilization, traffic control, erosion and sediment control, permitting, etc.

This table shows Cost components for stormwater ponds

Implementation Stage Primary Cost Components Basic Cost Estimate Other Considerations
Site Preparation Tree & plant protection Protection Cost ($/acre) x Affected Area (acre) Removal of existing structures, topsoil removal and stockpiling Topsoil salvage Salvage cost ($/acre) x Affected Area (acre)
Clearing & grubbing Clearing Cost ($/acre) x Affected Area (acre) Site Formation Excavation / grading 8-ft Depth Excavation Cost ($/acre) x Area (acre) Soil & rock fill material, tunneling
Hauling material offsite Excavation Cost x (% of Material to be hauled away)
Structural Components Inlet structure ($/structure) Pipes, catchbasins, manholes, valves Outlet structure ($/structure)
Site Restoration Seeding or sodding Seeding Cost ($/acre) x Seeded Area (acre) Tree protection, soil amendments, seed bed preparation, trails Planting / transplanting Planting Cost ($/acre) x Planted Area (acre)
Annual Operation, Maintenance, and Inspection Debris removal Removal Cost ($/acre) x Area (acre) x Frequency (2 / 1yr) Vegetation maintenance, cleaning of structures Sediment removal Removal Cost ($/acre) x Area (acre) x Frequency (1 / 5yr)
Gate / valve operation Operation Cost ($) x Operation Frequency (2 / 1 yr) Inspection Inspection Cost ($) x Inspection Frequency (2 / 1 yr)
Mowing Mowing Cost ($) x Mowing Frequency (4 / 1 yr) Stormwater pond cost estimate worksheet Link to this table 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 - 8' depth square yard$10.00 $0.00 Grading square yard$1.50 $0.00 Hauling off-site - 8' depth square yard$10.00 $0.00 Inlet structure each$2,000.00 $0.00 Overflow structure each$3,500.00 $0.00 Sod - above vegetative bench square yard$4.50 $0.00 Soil preparation square yard$5.00 $0.00 Seeding - vegetative bench square yard$0.50 $0.00 Mulch square yard$2.00 $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 visitt $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

# Occurrence and mechanisms of constructed stormwater ponds that do not effectively retain phosphorus

Contributors and Acknowledgements
• Anthony Aufdenkampe, Ph.D., LimnoTech
• Dendy Lofton, Ph.D, LimnoTech
• Ben Crary, EIT, LimnoTech
• Hans Holmberg, P.E., LimnoTech
• Jeremy Walgrave, P.E., LimnoTech
• John Gulliver, Ph.D., P.E., University of Minnesota
• Ben Janke, Ph.D, University of Minnesota
• David Fairbairn, Ph.D., Minnesota Pollution Control Agency
• Jacques Finlay, Ph.D. University of Minnesota
• Bruce Wilson, Ph.D., P.E., University of Minnesota
• Kerry Holmberg, University of Minnesota

This page provides a brief summary of findings from a study to

• estimate the extent and occurrence of constructed stormwater ponds that do not effectively retain phosphorus,
• identify conditions likely to contribute to phosphorus export from constructed stormwater ponds,
• compile information for constructed stormwater ponds identified as potentially exporting phosphorus into a spreadsheet or database, and
• conduct a high level assessment of characteristics, trends, and patterns for ponds that potentially export phosphorus.

## Occurrence of constructed stormwater ponds that do not effectively retain phosphorus

Literature reviewed for this task included the following.

In addition, University of Minnesota researchers, Jacques Finlay and Ben Janke, contributed data and expertise to this effort.

Comparison of pond TSS removal compared to designed removal efficiency. TSS is used as a surrogate for TP removal. (Lake Simcoe Region Conservation Authority)

General conclusions included the following.

• Of the over 500 ponds in this initial compilation, the dataset was narrowed to include 240 ponds in the Upper Midwest or Canada that had at least one surface total phosphorus (TP) measurement
• Of 66 ponds with paired TP measurements (influent and effluent samples collected at approximately the same time), 8 which were in the Midwest, 17 percent of the effluent measurements exceeded the paired influent measurement, and six of the eight ponds in the upper-Midwest had at least one effluent exceeding the influent (International BMP Database). The median effluent exceedance was 13 percent for both the national and upper-Midwest dataset. Higher effluent concentrations do not occur in all paired samples within a pond, highlighting the variable nature of TP export. For the upper-Midwest dataset, these exceedances occurred throughout the year, suggesting multiple possible drivers of TP export. Further, concentration data do not indicate the overall load retained (i.e. difference between influent and effluent loads).
• Riley Purgatory Bluff Creek Watershed District found that in 2013 (the year with the broadest sampling effort), 71 percent of the ponds surveyed had annual average TP concentrations that were equal to or greater than the MPCA’s threshold for effluent water (0.25 mg/L), and suggested that these ponds may not be retaining phosphorus. LimnoTech’s own review of this data found that 34 percent of the ponds had an average TP concentration greater than 0.50 mg/L over the 2010-2013 period, which is the upper 95th percentile confidence limit of Twin Cities stormwater runoff measurements (n=19 sites; Janke et al., 2017).
• From the Lake Simcoe study, of 74 ponds analyzed, 28 were estimated to be meeting design phosphorus removal efficiency. The average removal efficiency was 59 percent, compared to the average design efficiency of 76 percent,. The figure on the right shows that 7 ponds were not retaining phosphorus (i.e. potentially exporting phosphorus).

Overall the literature has not come to a conclusion on the extent of phosphorus release from stormwater ponds, but recent efforts have begun to shed light on the processes. RPBCWD revealed a large proportion of ponds have surface concentrations higher than estimated stormwater influent, and LSRCA identified that many are not operating as efficiently as designed. LSRCA speculated that anoxic conditions (which were documented in nearly half of their 98-pond survey) may be contributing to internal phosphorus loading. This speculation has been supported by recent literature highlighting the conditional influences on internal phosphorus dynamics.

## Mechanisms for phosphorus release

This section provides a very brief summary of major biogeochemical and geochemical mechanisms driving phosphorus release from the sediments and retention by the sediments in constructed stormwater ponds. For a detailed discussion of these mechanisms, see the technical report: File:P in ponds final report.docx.

For over three decades, constructed stormwater ponds have been designed and maintained to maximize sedimentation and minimize scour during storm periods. The design of stormwater ponds for water quality has focused nearly entirely on the goal of maximizing sedimentation of phosphorus bound to particles and minimizing scouring of these particles by subsequent storms. Other physical, geochemical, and biological processes have not adequately been considered.

A conceptual box model of a stormwater pond composed of three reservoirs of phosphorus (upper water column, lower column, and sediment)) and seven major fluxes of phosphorus (arrows).

The figure on the right shows a conceptual model illustrating phosphorus reservoirs and fluxes in a typical stormwater pond. While each of the fluxes shown are potentially important, the following are likely to be of greatest importance for ponds that potentially release phosphorus.

• Input/external load. Ponds receiving heavy phosphorus and sediment loads or large quantities of labile phosphorus are more likely to eventually export phosphorus. Large sediment loads ultimately reduce pond efficiency and create a large phosphorus reservoir in benthic sediment. Generally, more labile forms of phosphorus are associated with organic material, such as leaves.
• Dissolution. If phosphorus concentrations in benthic sediment are high and anoxic conditions occur, phosphorus will be released from iron-bearing minerals.
• Mixing and resuspension. Phosphorus released through dissolution can be brought to the upper water column through mixing. It may then be exported from the pond.

Although phosphorus dynamics in ponds are complicated and highly variable, ponds susceptible to phosphorus export are those receiving high inputs, particularly of organic material, and that undergo stratification, which allows anoxic conditions to occur within the pond. The issue of stratification may be exasperated by chemostratification resulting from road deicers accumulating in ponds (Taguchi et al., 2018; McDivitt, 2019)

## List of ponds with phosphorus data

A list of over 500 stormwater ponds in the upper Midwest was compiled in order to identify characteristics that may be indicative of phosphorus release in this general region. This list includes approximately 400 ponds in Minnesota and 98 that are located in Ontario, Canada. The dataset was narrowed to 240 ponds that had at least one surface TP measurement.

The following information was targeted for inclusion in the dataset, although much of this information was not available for many of the ponds.

• Pond name
• Pond location
• Year constructed
• Maintenance record
• As built information (if available)
• Monitoring data (if available)
• Chemistry (i.e. TP, SRP, iron, DO) data in the pond (if available)
• Chemistry in any inflows or outflows (if available)
• The occurrence of stratification of temperature, conductivity, and DO in the pond (if available)
• Soil/sediment chemistry information (if available)
• Contact information

To access the file with the pond information: File:Pond data.xlsx. The technical report provides some analysis of the data: File:P in ponds final report.docx.

## High level assessment of phosphorus release

Analysis of the data attempted to identify patterns or trends that would help understand why some ponds are more susceptible to releasing phosphorus. The analysis is in the technical report: File:P in ponds final report.docx.

Overall, the assessment revealed that the data are not sufficiently detailed to tell the story of internal phosphorus cycling. Linear correlations suggest that ponds built more recently perform more effectively, but positive correlations between estimated P removal efficiency and surface TDP highlight that there is more to be learned about the internal mechanisms beyond estimated efficiency.

Although more rigorous research is needed to gain a fuller understanding of phosphorus dynamics in ponds, some general observations included the following.

• Categorical analysis indicated Minnesota ponds designed to NURP guidance have higher surface TP concentrations than non-NURP ponds.
• While data was limited, ponds in residential and forested drainage areas were among the ponds with highest surface TP concentrations.

## Conclusions

• Previously published studies demonstrate that for many stormwater ponds water column phosphorus concentrations are higher than water quality standards or regionally calculated event mean concentrations, and that many ponds appear to operate at removal efficiencies that are lower than targeted during pond design.
• Previously published studies typically do not present sufficiently detailed data to independently assess phosphorus removal performance, as most only present average phosphorus concentrations in the pond water column and sometimes in the influent waters to the pond. Calculations of the difference of these averages between influent and presumed effluent concentrations, especially when sample numbers are not equal and collections known to be paired, can not provide robust estimates of removal due to differing hydrological conditions.
• The design of stormwater ponds over the last three decades has focused nearly entirely on the goal of maximizing water quality via sedimentation of phosphorus bound to particles and minimizing scouring of these particles by subsequent storms.
• The full suite of physical, geochemical, and biological processes, mechanisms, drivers and factors that control phosphorus cycling and fluxes within stormwater ponds, and their temporal dynamics, should be considered when evaluating and managing phosphorus retention by ponds.
• Useful information can be gleaned from the compilation and reanalysis of existing metadata on pond characteristics and summary data on pond phosphorus concentrations and previously estimated removal efficiency. However, these summary datasets are insufficiently detailed to support robust statistical analyses that might best highlight the characteristics of ponds that are best and worst at retaining phosphorus during storms or over an annual cycle.

## On-going or recent studies - Minnesota

There are several on-going or recent studies examining phosphorus dynamics in and release from constructed stormwater ponds. Below is a review of some of that work, including links to reports and presentations.

• Characterization of Phosphorus Release from Ponds. This is a study of the P dynamics of stormwater ponds to understand the factors that control retention and release of P. Because ponds rely on sedimentation for P removal, they are most effective when a high percentage of P is bound to suspended solids and less effective when input P is in the soluble form. More generally, the study will represent the first stage in research to understand the functioning of stormwater ponds with respect to P removal. This information is crucial for the development of guidelines for the management of ponds for improved P retention.
• Detecting phosphorus release from stormwater ponds to guide management and design. There is growing concern that aging stormwater retention ponds may become net sources of phosphorus (P) to receiving waters. Release of P previously deposited in sediments (i.e. internal loading) is a major contributor to eutrophication in lakes. Stormwater ponds often have high external P loading, and other characteristics that may increase the likelihood of internal loading as ponds age. However, stormwater ponds have received comparatively little research attention, even though they are widely used with the intended goal of permanent immobilization of phosphorus. The ability of these systems to retain phosphorus over their lifespan is essentially unknown. The proposed research will build understanding necessary to assess the capacity of stormwater ponds to retain or release phosphorus in Minnesota’s stormwater pond infrastructure. The projects aim to develop methods for rapid and efficient identification of pond phosphorus release, to guide management of existing ponds, and to reveal factors that underlie poor performance for P removal. The results of this project will be used to inform and improve pond maintenance, pond design and decision making around construction of new ponds, and to ultimately improve the water quality of our lakes, rivers and wetlands.
• Research team: John Gulliver, Jacques Finlay, Ben Janke, Poornima Natarajan, Shahram Missaghi
• Project duration: January 1, 2019-June 30, 2020
• Pond Treatment with Spent Lime to Control Phosphorus Release from Sediments. Sedimentation ponds that accumulate particles and phosphorus in stormwater runoff are a standard and widely applied storm water best management practice. However, just as internal loading occurs in lakes during warm summer periods when the potential for oxygen depletion is greatest, aging ponds have the potential to release more phosphorus than is captured during summer months (Watershed Protection Techniques, Technical Note 102). Dredging is a potential, but expensive, option to improve pond performance, but phosphorus release may occur long before a pond is filled with sediment. Areal applications of alum and iron can control phosphorus release, but incur raw material production costs. In cooperation with SPRWS, City of White Bear Lake, RWMWD, and VLAWMO staff, Barr Engineering proposes this study to evaluate the application of spent lime (amorphous calcium carbonate from drinking water treatment) to pond sediments to reduce phosphorus release during warm summer months. Spent lime can reduce phosphorus release by forming calcium phosphate and potentially by increasing the pH of the treated sediments to facilitate iron and aluminum phosphate binding. This study includes a laboratory and a field component and is intended to validate large-scale applications. The laboratory component includes the addition of spent lime at a range of doses to phosphorus rich pond sediment to determine optimal spent lime dosing. The field component involves the addition of spent lime to two ponds and monitoring to determine the magnitude of reduced phosphorus release, evaluate cost-effective methods for areal application and quantify the benefits of this water treatment byproduct.
• Research team: Greg Wilson, Keith Pilgrim, Erin Anderson-Wenz, Kevin Menken, Tyler Olsen
• Use of alum in stormwater ponds

## Other recent studies

• Mobility and Bioavailability of Sediment Phosphorus in Urban Stormwater Ponds. Paul C. Frost, Clay Prater, Andrew B. Scott, Keunyea Song, Marguerite A. Xenopoulos. 2019. Water Resources Research, Volume55, Issue5, May 2019. Abstract: "Stormwater ponds can serve as retention hotspots for phosphorus (P) moving out of the urban environment. This retention may be reduced by P speciation that reduces the bioavailability of P to primary producers and alters its mobility in sediments. Here we examined the mobility and fate of dissolved P in urban stormwater ponds with a set of complementary field measurements and short‐term laboratory and field experiments. We measured the types and amount of P in water column and sediments of urban stormwater ponds. We further assessed the mobility of different P types in pond sediments in the field and rates of P release from sediment cores maintained under laboratory conditions. Finally, we assessed P uptake rates by pond algal communities using short‐term bioassay experiments. We found that dissolved organic P was highly prevalent in urban pond water and sediments and that this type of P was mobile within sediments and could be released under high or low O2 conditions. We also found highly variable P demand by algae among stormwater ponds and that algal growth responses to P was correlated to water column N:P ratios. Altogether, our results indicate an important role for organic phosphorus cycling in urban stormwater ponds, which likely constrains the overall retention efficiency in these aquatic ecosystems."