Stormwater Wetlands The sections in this chapter may be viewed as a single page.
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.
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.