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Minnesota Stormwater Manual

Design criteria for stormwater wetlands

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

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

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

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

Physical feasibility initial check

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

Warning: Groundwater Protection – It is Required that stormwater wetlands treating runoff from Potential Stormwater Hotspots (PSHs) provide excellent treatment capabilities. In some cases (depending on the land use and associated activities), lining the stormwater wetland may be necessary to protect groundwater, particularly when the seasonally high groundwater elevation is within three feet of the practice bottom.
  • Drainage Area – 25 acres minimum Highly Recommended, ensuring hydrologic input sufficient to maintain permanent pool; 10 acres (or less) may be acceptable, particularly if the groundwater table is intercepted and a water balance indicates that a permanent pool can be sustained.
  • Space Required – Approximately 2 to4 percent of the tributary drainage area is Recommended for the wetland footprint.
  • Minimum Head – The elevation difference Recommended at a site from the inflow to the outflow is a minimum of 2 feet. The relatively small head requirement makes stormwater wetlands a feasible practice in areas with shallow soils.
  • Minimum Depth to Water Table – In general, there is no minimum separation distance required with stormwater wetlands. In fact, intercepting the groundwater table is common and helps sustain a permanent pool. However, some source water protection requirements may dictate a separation distance if there is a sensitive underlying aquifer, which means that a liner might be required for portions of the wetland with standing water.
  • Soils – Underlying soils of hydrologic group “C” or “D” should be adequate to maintain a wetland. Most group “A” soils and some group “B” soils may require a liner. A site specific geotechnical investigation should be performed. Also, if earthen embankments are to be constructed,it will be necessary to use suitable soils.
  • Karst – Stormwater wetlands are a preferred management technique over stormwater ponds in karst areas, but it is Recommended that maximum pool depths be 3 to 5 feet. If stormwater wetlands are used in areas, impermeable liners may be needed.
  • Cold Water Fisheries – Stormwater wetlands may not be appropriate practices where receiving waters are sensitive cold water fisheries due to the potential for stream warming from wetland outflows. Suitable vegetative canopy may lessen potential negative effects.

Conveyance

Inflow Points

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

Adequate outfall protection

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

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

Pre-treatment

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

Treatment

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

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

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

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

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

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

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

Landscaping plan

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

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

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

Constructed wetlands buffers and setbacks

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

Safety

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

Design procedure

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

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

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

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

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

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

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

Step 2: Confirm local design criteria and applicability

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

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

Step 3: Confirm site suitability

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

It is Recommended that the soil borings or pits be five feet below the bottom elevation of the proposed stormwater wetland.

It is Highly Recommended that the field verification be conducted by a qualified geotechnical professional.

B. Perform water balance calculations if needed.

Step 4: Compute runoff control volumes and permanent pool volume

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

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

This can be calculated by

\(V_{pp} = 1800 A\)

or

\(V_{pp} = 0.0417 IC\)

where:

A = total watershed area in acres draining to the pool

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

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

For normal waters:

\(V_{wq} = 0.0417 IC\)

For special waters see Unified sizing criteria:

\(V_{wq} = 0.0833 IC\)

where:

Ai = the new impervious area in acres.

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

Schematic of shallow wetland profile
Schematic showing a shallow wetland profile

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

Additional details can be found in Unified sizing criteria

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

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

Step 6: Allocate the remaining Vpp and Vwq volumes

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

Design restrictions for special water or other sensitive receiving watersheds.
Link to this table

BMP
Watershed Management Category
A
Lakes		 B
Trout Waters		 C
Drinking Water*		 D
Wetlands		 E
Impaired Waters
Infiltration basin/trench RECOMMENDED RECOMMENDED ACCEPTABLE with cautions for [Potential stormwater hotspots PSH] RECOMMENDED RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride
Filtration Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments RECOMMENDED RECOMMENDED ACCEPTABLE RECOMMENDED for non-nutrient impairments
Filtration (including green roofs) ACCEPTABLE if P removal in media is considered RECOMMENDED RECOMMENDED RECOMMENDED RECOMMENDED for non-nutrient impairments
Iron enhanced filters RECOMMENDED RECOMMENDED RECOMMENDED RECOMMENDED RECOMMENDED
Wetlands Some variations NOT RECOMMENDED due to poor P removal, combined with other treatments. NOT RECOMMENDED
except for wooded wetlands		 RECOMMENDED RECOMMENDED
but no use of natural wetlands		 RECOMMENDED

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


Step 7. Determine wetland location and preliminary geometry

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

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

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

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

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

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

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

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

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

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

1. The desired release rate may then be calculated by

\(Q_{cp} = V_{cp} / t\)

where:

t = the detention time in seconds determined above.

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

2. The average head is calculated as

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

where:

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

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

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

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

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

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

2. The release rate may then be calculated by\[Q_{wq} = V_wq} / t\]

where:

t = the detention time in seconds

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

3. The average head is calculated as\[h_{avg} = (EL_{WQ} - EL_{WP}) / 2\]

where:

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

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

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

6. The release rate may then be calculated by

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

where:

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

7. The average head is calculated as

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

where:

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

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

\(A_{CP} = Q{CP} / (C (2g / s^2) h_avg^0.5)\)

where:

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

The diameter of the opening can then be solved for

\(d_{CP} = 2 (A_{CP} / π)^0.5\)

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

\(Q_{cp} = Kh ^0.5\)

where:

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

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

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

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

Size emergency spillway, calculate 100-year water surface elevation, set top of embankment elevation, and analyze safe passage of the Extreme Flood Volume \((V_{p100})\).

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

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

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

Step 12. Design inlets

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

Step 13. Design sediment forebay

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

Step 14. Design outlet structures

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

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

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

Outlet pipes should be buried below the frost line to the extent possible. Information on frost depths can be found from the Minnesota Department of Transportation at: www.mrr.dot.state.mn.us/research/seasonal_load_limits/thawindex/frost_thaw_graphs.asp

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

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

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

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

Step 15. Design maintenance access and safety features

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

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

Aesthetic enhancements such as trails or benches can also be included

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

Additional information can be obtained from OSHA.

Step 16. Check expected pond performance against regulatory requirements.

Check that \(V_{wq}\) is detained for an average of 12 hours.

Check that the \(V_{wq}\) release rate does not exceed 5.66 cubic feet per second per acre of pond area.

Determine applicable requirements for \(V_{cp}\) volume and release rate, and verify that the constructed wetland performs adequately for the appropriate design event.

Determine applicable requirements for \(Q_{p10}\) and \(Q_{p100}\) release rates (e.g., pre-development rates), and check release rates (and freeboard) for the appropriate design events.

Step 17. Prepare vegetation and landscaping plan.

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

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

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

See the Operation and Maintenance section for further details.

Step 19. Prepare cost estimate.

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

Stormwater wetland cost estimate worksheet.
Link to this table

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

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


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