City of South Portland Stormwater Manual Design Specifications Gravel Wetland Adapted from RI Stormwater Design Manual and UNH Design Specifications. Overview

HOW IT WORKS Gravel wetlands (GW) are horizontal flow retention and filter systems. The GW utilizes temporary storage for solids settling and soil media filtration as the primary mechanisms for pollutant removal. Gravel Wetlands maintain a saturated gravel bed and provide treatment by stormwater movement through the gravel bed and plant/soil treatment processes. Gravel Wetlands are well suited for poorly draining subsoils. These systems are well suited because: 1) there is a limited hydraulic head requirement, and 2) does not require separation from groundwater due to lack of infiltration. This is one of the University of New Hampshire Stormwater Center’s most successful systems for overall pollutant removal. Hydraulic head requirements for gravel wetlands are approximately four inches, whereas underdrained filtration systems may require as much as three feet or more. Because the Gravel Wetland is a horizontal porous media flow system it does require a hydraulic head to drive the water through the system. At a minimum, the driving head is the difference between the vertical distance from the ponded water level above the wetland surface and the invert of the primary outlet. To maintain the system in its saturated condition, it must be situated in low hydraulic conductivity soils or lined below the gravel layer. Because infiltration is not designed to occur, separation from groundwater is not required and the GWs are sited much like stormwater ponds.

Figure 1: Diagram of a gravel wetland. Source: UNH Stormwater Center.

Gravel Wetland Design Specifications Page 1 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org

Gravel wetland basins are primarily suited for soils that do not provide infiltration; however, an impermeable liner may be required to protect the groundwater from contamination in some soil types. Gravel wetlands must be planted or seeded with plant species that are tolerant of saturated soils and periodic inundation.

What it Does These systems can also provide peak flow attenuation and, through evapotranspiration, can reduce stormwater volumes. These systems are especially well‐suited on poorly draining soils or in locations with limited hydraulic head. Gravel wetland systems have the potential to reduce overall stormwater volumes and peak flows and have been shown to be effective at removing a wide range of pollutants from stormwater runoff. Gravel wetlands contain an aerobic and anaerobic zone and therefore are particularly well suited for nutrient removal and sequestration.

University of New Hampshire Stormwater Center shows that gravel wetlands do a good job of treating nitrogen pollution. Nitrogen reduction is important if sensitive coastal bays or estuaries are downstream. Gravel wetland systems reduce overall stormwater volumes, delay peak flows, and are effective at removing a wide range of pollutants.

Difficulty Level Gravel wetlands can be designed and constructed using common materials. Wetland soil media composition is critical to the function of the systems and careful attention should be made with construction composition and installation. GW systems should be designed by a professional engineer and are applicable to post‐construction stormwater management plans and permits.

For more information, see UNH Stormwater Center’s specifications for gravel wetlands.

POST‐CONSTRUCTION STORMWATER MANAGEMENT REQUIREMENTS In South Portland, GW systems must detain a runoff volume equal to 0.5 inch times the subcatchment's impervious area plus 0.2 inch times the subcatchment's landscaped developed area. The peak storage depth within the filter structure may exceed 18 inches, but provisions should be made for adequate drain down in these situations.

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Figure 2: Large Gravel Wetland installed in Greenland NH.

Site Suitability Criteria Gravel wetland systems are primarily constructed as filter systems depending on site soils; however, an impermeable liner may be required to protect the groundwater from contamination in some soil types. Site soils should be characterized for infiltration potential, seasonal high water table and bedrock depth prior to construction.

Drainage Area The surface size of the gravel wetland and the storage capacity above the wetland soil is based on the size and the land use within the area draining to the structure. Depth to Groundwater Gravel wetland systems are particularly well suited to areas with seasonally high groundwater tables. The outlet invert should be above the seasonally high water table.

Bedrock Bedrock close to the surface may require blasting or an impermeable liner to prevent fast infiltration and the potential of contaminating groundwater.

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General Design Criteria The following design criteria apply to all soil filter systems.

Required Treatment Volume A gravel wetland must detain and filter a runoff volume equal to 0.5 inch times the subcatchment's impervious area plus 0.2 inch times the subcatchment's landscaped developed area. The required volume must be stored in the depression area above the wetland soil media.

Construction Components Gravel wetlands are constructed in excavated holes that are at least three feet deep and consist of, from bottom up:  A geotextile fabric between natural soils and constructed components. An impermeable membrane may be required if groundwater contamination is a concern.  A 24 inch base of coarse clean stone.  A 3 inch layer of pea gravel transition course.  A 8 inch layer of wetland soil.  A depression for surface stormwater storage above the wetland media.

Impoundment Depth The peak storage depth within the filter structure should generally not exceed 18 inches. Some vegetation may be harmed by periodic inundation. A landscape architect should be consulted for vegetation selection in cases of maximum storage depths.

Outlet The treatment volume must be discharged solely through the horizontal media components via a network of underdrain pipe having a single outlet. Each system must discharge to an area capable of withstanding concentrated flows and saturated conditions without eroding.

Sediment Pretreatment Pretreatment devices such as grassed swales, grass or meadow filter strips and sediment traps shall be provided to minimize the discharge of sediment to the gravel wetland system.

Access Where needed, a maintenance access shall be planned for and maintained that is at least 10 feet wide with a maximum slope of 15% and a maximum cross slope of 3%. This access should never cross the emergency spillway, unless the spillway has been designed for that purpose. An easement for long‐term access may be needed.

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Specific Design Criteria Inlet and Conveyance Design  Inlet areas should be stabilized to ensure that non‐erosive conditions exist for at least the 1‐year frequency storm event.  A GW designed with an organic soil layer at the surface should have vertical perforated riser pipes that deliver stormwater from the surface down to the subsurface perforated distribution lines. These risers shall have a maximum horizontal spacing of 15 feet. Oversizing of the perforated vertical risers is useful to allow a margin of safety against clogging with a minimum recommended diameter of 12” for the central riser and 6” for end risers. The vertical risers shall not be capped, but rather covered with a beehive inlet grate to allow for an overflow when the water level exceeds the intended storage depth.  Flow paths from the inflow points to the outflow points of GRAVEL WETLAND shall be maximized through the use of BMP geometry and features such as berms and islands.

Outfall  The channel immediately below a GW outfall shall be modified to prevent erosion and conform to natural dimensions in the shortest possible distance, typically by use of appropriately sized riprap placed over filter cloth.  A stilling basin or outlet protection shall be used to reduce flow velocities from the principal spillway to non‐erosive velocities (3.5 to 5.0 fps).  A subsurface water level must be maintained in the GW through the design of the outlet elevation (invert just below the surface). Care should be taken to not design a siphon that would drain the GW: the outlet invert location must be open or vented.  The GW outlet structure should be based on a calculated release rate by orifice control to drain the proposed storage volume over 24 hrs.

Sediment Forebay  A sediment forebay is important for maintenance and longevity of a GW. Each GW shall have a sediment forebay or equivalent upstream pretreatment.  A forebay shall be provided at each inlet.  The forebay shall be designed with non‐erosive outlet conditions.  Direct access for appropriate maintenance equipment shall be provided to the forebay.  The bottom of the forebay may be hardened (i.e., concrete, asphalt, grouted riprap) to make sediment removal easier.

Gravel Wetland Geometry Design Guidance  Flow paths from the inflow points to the outflow points of the Gravel Wetland shall be maximized through the use of basin geometry and features such as berms and islands. A GW length to width ratio of 1:1 (L:W) or greater is needed for each treatment cell with a minimum flow path (L) within the gravel substrate of 15 feet.

Gravel Wetland Design Specifications Page 5 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org

 Berms and weirs separating the forebay and treatment cells should be constructed with clay, or non‐conductive soils, and/or a fine geotextile, or some combination thereof, to avoid water seepage and soil piping through these earthen dividers.  The bed of a GW should be graded to create maximum internal flow path and microtopography. Microtopography (complex contours in the wetland layer surface, providing greater depth variation) is encouraged to enhance habitat diversity.

Treatment Design  At least 10% of the treatment volume shall be provided in a sediment forebay or other pretreatment practice (if practicable). The remaining 90% of the treatment volume shall be provided in some combination of one or more basins or chambers filled with a minimum 24‐inch gravel layer and the open, storage volume above the wetland soil. The portion of the treatment volume provided above the permanent pool within the stone layer shall drain over 24 hours.  It is generally desirable to provide water quality treatment off‐line when topography, head, and space permit.  Water quality storage should be provided in multiple cells where possible. Performance is enhanced when multiple treatment pathways are provided by using multiple cells, longer flow paths, high surface area to volume ratios, complex microtopography, and/or other redundant treatment methods.  For gravel wetland cross‐section, a layer of organic wetland soil is used as a top layer and should have a minimum thickness of 8”, should have a surface slope of zero (but may contain microtography features), and should be underlain by 3” minimum thickness of an intermediate layer of a graded aggregate filter (i.e. pea gravel) to prevent the organic soil from moving down into the stone sublayer. The wetland soil shall meet the following material specifications.  This soil can be manufactured using organic material, sand, and some fine soils to blend to a high % organic matter content soil (>15% organic matter). Organic material should be non‐agricultural in origin. Avoid using clay contents in excess of 15% because of potential migration of fines into subsurface gravel layer. Do not use geotextiles between the horizontal layers of this system as they will clog due to fines and may restrict root growth.  An intermediate layer of a graded aggregate filter (i.e., pea gravel) is needed to prevent the wetland soil from migrating down into the crushed‐stone (gravel) sub‐layer. This is to prevent migration of the finer setting bed (wetland soil) into the coarse sublayer.  Below the wetland soil and pea gravel is a crushed stone sublayer with a 24 in. (0.6 m) minimum thickness. Angular crushed stone is needed with a minimum size ~3/4–in (2‐ cm). Large particle, angular coarse to very coarse gravel is needed to maintain system longevity.  A geotextile fabric with suitable characteristics shall be placed between the sides of the GW components and adjacent soil. The fabric will prevent the surrounding soil from migrating into and clogging the filter and clogging the outlet. Overlap seams must be a minimum of 12 inches. Do not wrap fabric over the top of pipe bedding as it will cause

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clogging and will prevent flows out of the filter. The geotextile fabric shall be Mirafi 170n or equivalent.

8” (20 cm) minimum thickness of wetland soil

3” (8 cm) minimum thickness of graded filter (i.e., pea gravel) if needed

24” (60 cm) minimum thickness of 3/4” (2 cm) crushed stone (gravel)

Low permeability soil or liner if underlying soils are very permeable

Figure 3: Diagram of typical gravel wetland construction.

Gravel Wetland Drain Design Except where local slopes (e.g., coastal areas) prohibit this design, each GW shall have a drain pipe that can completely or partially drain wetland soil layer. The drain pipe shall have an elbow or protected intake within the GW to prevent sediment deposition, and a diameter capable of draining the permanent pool within 24 hours. Access to the drain pipe shall be secured by a lockable structure to prevent vandalism and/or accidental draining of the pond, which could pose a safety hazard due to high drainage velocities.

Safety Features Design  The Bypass outlet (emergency spillway, or secondary spillway) is sized to pass designs flows for the 25‐year 24 hours storm event and/or in accordance with City requirements. This outlet is sized by using conventional routing calculations of the inflow hydrograph through the surface storage provided by the subsurface gravel wetland system.  Proposed graded side slopes to the GW shall not exceed 3:1 (H:V), and shall terminate on the safety bench.  The principal spillway opening shall not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.  “Token” or emergency spillways (those placed above the water elevation of the largest managed storm) may be required if not already provided as part of the conveyance of

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the 100‐year storm event and must be a minimum 8 ft wide, 1 ft deep, with 2:1 channel side slopes.  Both the safety bench and the aquatic bench may be landscaped to prevent access to the permanent pool.  Warning signs prohibiting swimming and skating may be posted.  Fencing is generally not encouraged, but may be required by some owners and/or agencies. A preferred method is to manage the contours of the GW to eliminate dropoffs or other safety hazards.

Planting Plan Design  A planting plan for a GW and its setback shall be prepared to indicate how aquatic and terrestrial areas will be stabilized and established with vegetation. Minimum elements of a plan include: delineation of pondscaping zones, selection of corresponding plant species, plant locations, sequence for preparing GW bed (including soil amendments, if needed), and sources of plant material.  Donor soils for GW shall not be removed from natural wetlands.  The best elevations for establishing emergent plants, either through transplantation or volunteer colonization, are within six inches (plus or minus) of the normal pool.  The soils of a GW setback are often severely compacted during the construction process to ensure stability. The density of these compacted soils is so great that it effectively prevents root penetration, and therefore, may lead to premature mortality or loss of vigor. Consequently, it is advisable to excavate large and deep holes around the proposed planting sites, and backfill these with uncompacted topsoil.  A GW should be planted to achieve a rigorous root mat with grasses, forbs, and shrubs with obligate and facultative wetland species. The GW shall be planted with plants that are tolerant of periodic inundation. Plants should be chosen for their tolerance to urban runoff, pollutant loading, temperature and pH.  Planting holes should be the same depth as the root ball and two to three times wider than the diameter of the root ball. In addition, the root ball of container‐grown stock should be gently loosened or scored along the outside layer or roots to stimulate new root development. This practice should enable the stock to develop unconfined root systems. Avoid species that require full shade or are prone to wind damage. Extra mulching around the base of the tree or shrub is strongly recommended as a means of conserving moisture and suppressing weeds. Acceptable mulch must be well aged, uniform in color, and free of foreign material including plant root material. Lightweight bark mulches have a tendency to float in the soil filter treatment system. Stump grindings or other stringy, woody mulches are more effective in these systems.  Structures such as fascines, coconut rolls, or carefully designed stone weirs can be used to create shallow cells in high‐energy flow areas of the GW.

Gravel Wetland Setback Design  Woody vegetation shall not be planted or allowed to grow on a dam, or within 15 feet of a dam or toe of the embankment, or within 25 feet of a principal spillway outlet.

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 Existing trees should be preserved in the setback area during construction as practicable. It is desirable to locate forest conservation areas adjacent to a GW. To help encourage reforestation and discourage resident geese populations, the setback can be planted with trees, shrubs and native ground covers.  Annual mowing of the GW setback is only required along maintenance rights‐of‐way and the embankment. The remaining setback can be managed as a meadow (mowing every other year) or forest.

Maintenance  Maintenance responsibility for a GW and its setback shall be vested with a responsible authority by means of a legally binding and enforceable maintenance agreement that is executed as a condition of plan approval.  General inspections shall be conducted on an annual basis.  A maintenance and operation plan must specify that sediment removal in the forebay shall occur every 5 years or after 50% of total forebay capacity has been lost, whichever occurs first.  An operation and maintenance plan shall specify that if a minimum vegetative coverage of 50% is not achieved in the planted areas after the second growing season, a reinforcement planting is required.  Sediment and organic build‐up shall be removed from a GW every 2 years, as needed.  Inspection shall consist of verifying that the GW is draining to the permanent pool elevation within the 24‐hour design requirement and that potentially clogging material, such as accumulation of decaying leaves or debris, does not prevent the discharge through the gravel. When clogging occurs, at least the top 8 inches of gravel shall be replaced over with new material. Sediments shall be disposed of in an acceptable manner.  Areas with a permanent pool should be inspected on an annual basis. The maintenance objectives for these practices include preserving the hydraulic and removal efficiency of the GW and maintaining the structural integrity.  Sediment removed from forebays shall be disposed according to state solid waste standards.  The slopes of the basin or GW should be inspected for erosion and gullying. Reinforce existing riprap if riprap is found to be deficient, erosion is present at the outfalls of any control structures, or the existing riprap has been compromised. Re‐vegetate slopes as necessary for stabilization.  All structural components, which include, but are not limited to, trash racks, access gates, valves, pipes, weir walls, orifice structures, and spillway structures, should be inspected and any deficiencies should be reported. This includes a visual inspection of all stormwater control structures for damage and/or accumulation of sediment.  All dead or dying vegetation within the extents of the GW should be removed, as well as all herbaceous vegetation rootstock when overcrowding is observed and any vegetation that has a negative impact on stormwater flowage through the facility. Any invasive vegetation encroaching upon the perimeter of the facility should be pruned or removed

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if it is prohibiting access, compromising sight visibility and/or compromising original design vegetation.  A maintenance right of way or easement shall extend to a GW from a public or private road.  Maintenance access should be at least 10 feet wide, have a maximum slope of no more than 15%, and be appropriately stabilized to withstand maintenance equipment and vehicles.  The maintenance access should extend to the forebay(s), safety bench, emergency spillway, outlet control structure, and outlet and be designed to allow vehicles to turn around.  The grass around the perimeter of the GW should be mowed at least annually.

Gravel Wetland Design Specifications Page 10 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org