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Download Detailed Design Specifications for Gravel Wetland 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. Gravel Wetland Design Specifications Page 2 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org 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. Gravel Wetland Design Specifications Page 3 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org 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. Gravel Wetland Design Specifications Page 4 of 10 Stormwater Manual - City of South Portland, Maine www.southportland.org 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.
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