Stormwater wetlands are similar in design to stormwater ponds and mainly differ by their variety of water depths and associated vegetative complex. They require slightly more surface area than stormwater ponds for the same contributing drainage area. Stormwater wetlands are constructed stormwater management practices, not natural wetlands. Like ponds, they can contain a permanent pool and temporary storage for water quality control and runoff quantity control.
Wetlands are widely applicable stormwater treatment practices that provide both water quality treatment and water quantity control. Stormwater wetlands are best suited for drainage areas of at least 10 acres. When designed and maintained properly, stormwater wetlands can be an important aesthetic feature of a site.
1.2 Function Within Stormwater Treatment Train
Stormwater wetlands are typically installed at the downstream end of the treatment train (they are considered an end-of-pipe BMP). Stormwater wetland size and outflow regulation requirements can be significantly reduced with the use of additional upstream BMPs. However, when a stormwater wetland is constructed, it is likely to be the only management practice employed at a site, and therefore must be designed to provide adequate water quality and water quantity treatment for all regulated storms.
1.3 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. The permit and related documentation can be found online at www.pca.state.mn.us/water/stormwater/stormwater-c.html. These standards must be applied in all projects in which at least one acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, stormwater wetlands currently fall under the “Wet Sedimentation Basin” category described in Part III.C.1 of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given as described in Part III.C.4. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the wetland will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice. Of note, the MPCA will evaluate the need to keep stormwater wetlands under the “wet sedimentation basin” category in future CGP revisions and consider it as a bioretention system instead. Input to MPCA on this would be welcomed.
The following terms are used in the text to distinguish various levels of stormwater wetland design guidance:
Of course, there are situations, particularly retrofit projects, in which a stormwater pond is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular stormwater wetland, depending on where it is situated both jurisdictionally and within the surrounding landscape.
1.4 Design Variants
Stormwater wetlands are constructed with varying amounts of the following three components:
The amount of each of the components named above depends on the desired type of stormwater wetland (e.g., shallow wetland). Figure 12.WETL.1 shows the three major types of stormwater wetlands presented in this Manual.
Stormwater wetland design must be tailored to site characteristics; however, some general RECOMMENDED design criteria for shallow wetland, ED shallow wetland, and pond/wetland design are presented in Table 12.WETL.1.
The proportions of deep water, low marsh, high marsh, and semi-wet marsh are presented above in Table 12.WETL.1 and are defined as:
Deepwater zone – From 1.5 to 6 feet deep. Includes the outlet micropool and the deepwater channels through the wetland facility. This zone supports little emergent wetland vegetation but may support submerged or floating vegetation. It is consistent with the Cowardin Wetland Classification of palustrine aquatic beds.
Low marsh zone – From 6 to 18 inches below the normal permanent pool or water surface elevation. This zone is suitable for the growing of several emergent wetland plant species.
High marsh zone – From 6 inches below the pool to the normal pool elevation. This zone will support a greater density and diversity of wetland species than the low marsh zone. The high marsh zone should have a higher surface area to volume ratio than the low marsh zone.
Semi-wet zone – Those areas above the permanent pool that are inundated during larger storm events. This zone supports a number of species that can survive flooding.
1.5 Retrofit Suitability
As a retrofit, stormwater wetlands have the advantage of providing both educational and habitat value. One disadvantage of wetlands, however, is the difficulty in storing large amounts of runoff without consuming a large amount of land. Therefore, the most common type of wetland retrofit involves the modification of an existing dry or wet pond.
1.6 Special Receiving Waters Suitability
The following Table 12.WETL.2 provides guidance regarding the use of wetlands in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated (Chapter 10). The corresponding information about other BMPs is presented in the respective sections of this Manual.
1.7 Cold Climate Suitability
Wetland performance can be decreased in spring months when large volumes of runoff occur in a relatively short time carrying the accumulated pollutant load from the winter months. Because stormwater wetlands are relatively shallow, freezing of the shallow pool can occur. Also, freezing of inlet and outlet structures can occur, which will reduce performance of the stormwater wetland. To avoid these problems, the CWP (Caraco and Claytor, 1997) made some general design suggestions, which are adapted as follows:
Inlet pipes should not be submerged, since this can result in freezing and upstream damage or flooding.
Burying all pipes below the frost line can prevent frost heave and pipe freezing. Wind protection can also be an important consideration for pipes above the frost line. In these cases, designs modifications that have pipes “turn the corner” are helpful.
Increase the slope of inlet pipes to a minimum of 1% to prevent standing water in the pipe, reducing the potential for ice formation. This design may be difficult to achieve at sites with flat local slopes.
If perforated riser pipes are used at the outlet, the minimum opening diameter should be ½”. In addition, the pipe should have a minimum 6” diameter.
When a standard weir is used, the minimum slot width should be 3", especially when the slot is tall.
Baffle weirs can prevent ice reformation during the spring melt near the outlet by preventing surface ice from blocking the outlet structure.
Alternative outlet designs that have been successful include using a pipe encased in a gravel jacket set at the elevation of the aquatic bench as the control for water quality events. This practice was both avoids stream warming and is also a non-freezing outlet.
Trash racks should be installed at a shallow angle to prevent ice formation.
''1.8 Water Quantity Treatment
Stormwater wetlands are well-suited to provide channel protection and overbank flood protection. As in ponds, this is accomplished with live storage (extended detention) above the permanent pool. It is HIGHLY RECOMMENDED that when providing water quantity control in stormwater wetlands, the smallest possible bounce (vertical water level fluctuation) be designed for in order to limit the amount of stress on the vegetation.
1.9 Water Quality Treatment
Pollutants are removed from stormwater runoff in a wetland through uptake by wetland vegetation and biota (algae, bacterial), vegetative filtering, soil adsorption, and gravitational settling in the slow moving marsh flow. Volatilization and chemical activity can also occur, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons.
Pollutant removal efficiencies for selected parameters are provided in Table 12.WETL.3. Optimum effluent concentrations for select parameters are provided in Table 12.WETL.4.
The following general limitations should be recognized when considering installation of stormwater wetlands:
2. Major Design Elements
2.1 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 BMPs.
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 ground water table is intercepted and a water balance indicates that a permanent pool can be sustained.
Space Required – Approximately 2-4% of the tributary drainage area is RECOMMENDED for 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 ground water 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.
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.
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 karst 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.
2.2.1 Inflow Points
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%, 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.
'2.2.2 Adequate Outfall Protection
Stormwater wetland 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 fps).
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.
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.
2.4.1 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.
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:
1. Check maximum drawdown during periods of high evaporation and during an extended period of no appreciable rainfall to ensure that wetland vegetation will survive.
2. The change in storage within a wetland = inflows – outflows.
3. Potential inflows: runoff, baseflow and rainfall.
4. Potential outflows: Infiltration, surface overflow and evapotranspiration.
5. 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.
6. 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.
2.4.2 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% 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% of the total surface area of stormwater wetlands should have a depth of six inches or less, and at least 65% of the total surface area shall be shallower than 18 inches (see mosquito control discussion in Chapter 6).
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 (four 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.
2.5.1 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 Appendix E as well as to 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.
If a minimum coverage of 50% is not achieved in the planted wetland zones after the second growing season, a reinforcement planting is required.
It is RECOMMENDED that a landscape architect or another landscape professional be consulted in selection of wetland plants.
2.6 Constructed Wetlands Buffers and Setbacks
It is REQUIRED (Minnesota Department of Health Rule 4725.4350) that a 50’ 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.
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.
3. Construction Details and Specifications
CADD-based details for pond and wetland systems are contained in Appendix D. The following details, with specifications, have been created for stormwater ponds/wetlands:
4. Operation and Maintenance
Maintenance is necessary for a stormwater wetland to operate as designed on a long-term basis. The pollutant removal, channel protection, and flood control capabilities of stormwater wetlands will decrease if:
Stormwater wetland maintenance activities range in terms of the level of effort and expertise required to perform them. Routine stormwater wetland maintenance, such as mowing and removing debris or trash, is needed multiple times each year, but can be performed by citizen volunteers. More significant maintenance, such as removing accumulated sediment, is needed less frequently but requires more skilled labor and special equipment. Inspection and repair of critical structural features such as embankments and risers, needs to be performed by a qualified professional (e.g., structural engineer) that has experience in the construction, inspection, and repair of these features.
4.2 Design Phase Maintenance Considerations
The following references may be consulted for more information on stormwater wetland maintenance:
Implicit in the design guidance in the previous sections, many design elements of stormwater wetland systems can minimize the maintenance burden and maintain pollutant removal efficiency. Primarily, providing easy access (typically 8 feet wide) to stormwater wetlands for routine maintenance is REQUIRED.
Mosquito control is of particular concern in the case of stormwater wetlands. They can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations, but no design will eliminate tem completely. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat. See Chapter 6 for an in-depth discussion of mosquito concerns in stormwater management.
4.3 Construction Phase Maintenance
The construction phase is another critical step where O&M issues can be minimized or avoided.
Inspections during construction are needed to ensure that the stormwater wetland is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided in Appendix D.
4.4 Post Construction Operation and Maintenance
Proper post-construction maintenance is important to the long-term performance of a stormwater wetland. Potential problems due to lack of maintenance include:
A clogged outlet structure can increase water levels, killing vegetation and reducing the wetland’s ability to attenuate and store floods. Water quality can be compromised by not providing adequate storage time.
Excess sediment can reduce storage volumes leading to many of the problems outlined above.
Nuisance issues such as beaver and muskrat burrows/dens can threaten the integrity of embankments.
Some important post construction maintenance considerations are provided below. A more detailed checklist of maintenance activities and associated schedules is provided in Appendix D. More detailed maintenance guidance can be found in the Pond and Wetland Maintenance Guidebook (CWP, 2004).
It is REQUIRED that a legally binding and enforceable maintenance agreement be executed between the practice owner and the local review authority.
Adequate access must be provided for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles. It is RECOMMENDED that a maintenance right of way or easement extend to ponds from a public or private road.
It is HIGHLY RECOMMENDED that stormwater wetlands be inspected annually during winter freeze periods to look for signs of improper operation.
It is HIGHLY RECOMMENDED that sediment removal in the forebay occur every 2 to 7 years or after 50% of total forebay capacity has been lost. In areas where road sand is used, an inspection of the forebay and stormwater wetland should be scheduled after the spring melt to determine if clean-out is necessary.
Sediments excavated from stormwater wetlands that do not receive runoff from confirmed hotspots are generally not considered toxic or hazardous material, and can be safely disposed by either land application or land filling. Sediment testing may be necessary prior to sediment disposal when a confirmed hotspot land use is present (see also Chapter 13).
Periodic mowing of the stormwater wetland buffer is HIGHLY RECOMMENDED along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow (mowing every other year), prairie, or forest.
General maintenance activities and schedule are provided in Table 12.WETL.5.
5. Cost Considerations
Cost factors for stormwater management ponds are extremely sensitive to site conditions. Availability of materials for embankment construction, outlet protection, cost of excavation, liner materials, and land costs are significant factors. Maintenance and inspection costs for mowing and periodic dredging are post-development factors. Other technologies such as infiltration trenches may be more cost-effective in smaller drainage areas due to construction and long-term maintenance costs (Young et al., 1996). 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 may 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. (FHWA, 1997).
5.1 Detailed Cost Estimate
The most appropriate technique for determining the cost to construct and maintain a specific BMP will be to apply unit costs to each component of construction, operation and/or maintenance. Table 12.WETL.6 represents the typical components for stormwater wetlands. This table presents those components of a construction project that are unique to this best management practice. Costs that would be associated with all aspects of a construction site, such as erosion and sediment control, mobilization, or traffic control, are not presented as unique costs.
Designers are encouraged to use the cost worksheet included in Appendix D to estimate their BMP construction cost.
6. 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.
6.1 Step-by-Step Design Procedure:
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:
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).
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 as:
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:
For special waters (see Chapter 10):
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.
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 Figures 12.WETL.2 and 12.WETL.3.
Additional details on the Unified Stormwater Sizing Criteria are found in Chapter 10.
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% or more of the total design inflow, with a RECOMMENDED volume equal to 10% 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% of the required permanent pool volume has already been allocated to the pre-treatment forebay, the remaining required volume may be allocated between marsh, micropool, and ED volumes using the recommendations presented in Table 12.WETL.1 to meet the CGP or local requirements.
''''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 Table 12.WETL.1. Other guidelines for constructed wetland layout are:
Provide maintenance access (10’ width for trucks/machinery)
Length to width ratios as presented in Table 12.WETL.1.
Step 8. 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 (see discussion in Chapter 9).
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 maps have been prepared in Chapter 2 (Figures 2.5-2.7) 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 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:
t = the detention time in seconds determined above
Check to determine if Qcp is less than or equal to 5.66 cfs 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 cfs, 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:
ELcp = the elevation of the channel protection volume and ELwp is the wetland pool elevation.
3. Given the design release rate, estimated in #1 above, 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:
t = the detention time in seconds determined above
Check to determine if Qwq is less than or equal to 5.66 cfs 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 cfs, the release time should be increased.
3. The average head is calculated as:
where ELwq is the elevation of the water quality volume elevation and ELwp is the 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:
where t is the time in seconds determined above in 2. Check to determine if Qcp meets all design requirements.
7. The average head is calculated as:
where ELcp is the elevation of the channel protection volume and ELwq 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:
where g [ft/s/s] is the gravitational constant equal to 32.2 ft/s2,
The discharge coefficient, C, can be conservatively estimated to be 0.6.
The diameter of the opening can then be solved for:
9. The discharge from the wetland can then be computed for any elevation above the water quality elevation as:
Qcp = Kh0.5
Step 9. 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 10. Design embankment(s) and spillway(s).
Size emergency spillway, calculate 100-year water surface elevation, set top of embankment elevation, and analyze safe passage of the Extreme Flood Volume (Vp100).
At final design, provide safe passage for the 100-year event. Attenuation may not be required.
The following guidelines should also be followed (see NRCS Practice Standard 378 for further guidance):
Embankments should be stabilized with vegetation (no trees) or riprap.
Embankments may require a core-trench if geotechnical considerations warrant.
Embankment side slopes should not be steeper than 1V:3H on the front, 1V:3H on the back (impounded side).
Minimum embankment top width is 6’ (8’ if equipment access is necessary).
Material consolidation and shrinkage needs to be factored into embankment design.
Emergency overflows must be stabilized
Step 11. 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% if conditions permit.
Step 12. 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 13. 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 14. Design maintenance access and safety features.
Maintenance access to the pond, forebay, and inlet and outlet structures is REQUIRED. The access routes should be designed with a minimum 10’ width and maximum 15% 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
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. Use the following link to research safety measures for excavation sites:
OSHA has prepared a flow chart which will help site owners and operators determine if the site safety plan must address confined space procedures:
Permit-required Confined Space Decision Flow Chart - 1910.146 App A
Step 15. Check expected pond performance against regulatory requirements.
Check that Vwq is detained for an average of 12 hours.
Check that the Vwq release rate does not exceed 5.66 cfs/acre of pond area.
Determine applicable requirements for Vcp volume and release rate, and verify that the constructed wetland performs adequately for the appropriate design event.
Determine applicable requirements for Qp10 and Qp100 release rates (e.g., pre-development rates), and check release rates (and freeboard) for the appropriate design events.
'Step 16. 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 17. Prepare Operation and Maintenance (O&M) Plan.
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 18. Prepare cost estimate.
Refer to the Cost Considerations section and Appendix D for guidance on preparing a cost estimate for constructed wetlands.
ASCE/EPA International Stormwater Best Management Practices (BMP) Database. www.bmpdatabase.org
Center for Watershed Protection, 2004. Stormwater Pond and Wetland Maintenance Guidebook.
(FHWA Ultra Urban BMP Manual) Ultra-Urban Best Management Practices. Federal Highway Administration, Washington, D.C., 1997 www.fhwa.dot.gov/environment/ultraurb/index.htm
MPCA, 2000. Protecting Water Quality in Urban Areas.
Natural Resources Conservation Service. Conservation Practice Standard 378, Pond.
Caraco, D. and R. Claytor. 1997. Stormwater BMP Design Supplement for Cold Climates. Center for Watershed Protection. Ellicott City, MD.
Shaw, D. and R. Schmidt. 2003. Plants for Stormwater Design. Minnesota Pollution Control Agency, St. Paul, MN.
Winer, R. 2000. National Pollutant Removal Performance Database. Center for Watershed Protection, Ellicott City, MD.
Young, G Kenneth, Stuart Stein, Pamela Cole, Traci Kammer, Frank Graziano, Fred Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. Office of Environment and Planning, Federal Highway Administration, Washington, D.C.