Design criteria for stormwater wetlands

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 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 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.

Conveyance

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.

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 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.

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% 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.

Landscaping

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.

''2.7 Safety

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.