ILLINOIS URBAN MANUAL PRACTICE STANDARD Forebay (no.)

CODE 914

Source: County Management Commission

DEFINITION BOTTOM 811, A sediment forebay (forebay) is a pool or Practice Standard WETLAND and constructed at the DETENTION BASIN 812. incoming points of a 2. A sediment forebay may be stormwater best management practice appropriate for other existing (BMP). and retention basins where improved water quality is desired. PURPOSE 3. Forebays shall be for locations which The purpose of this practice is to are readily accessible for provide retention for a portion of the first maintenance (sediment removal). flush stormwater runoff and allow 4. Forebays shall not be constructed sediment to settle out from the incoming over existing underground utilities. stormwater before it reaches the larger BMP. Forebays help isolate sediment CRITERIA and facilitate maintenance Since forebays provide a dual function: and effectiveness of the larger BMP. dissipation of energy from incoming stormwater and a place for sediment to CONDITIONS WHERE PRACTICE accumulate the design of forebays shall APPLIES include zones for each of these 1. Where stormwater is entering a functions. stormwater management BMP such as Practice Standard DRY Forebays shall be installed for each DETENTION BASIN 809, Practice incoming discharge point in a BMP or as Standard EXTENDED DETENTION required in the relevant BMP practice BASIN 810, Practice Standard WET standard. material, influent pipe diameter or Forebay sizing criteria shall include dimensions, peak discharge, evaluation of the watershed size and and height of influent pipe invert or the land use/cover of that watershed. channel above substrate. The area required for the energy dissipation zone The total forebay volume shall be of the forebay shall be optimized in the approximately 20% of the total detention site-specific design based on these basin (BMP) volume for the 2-year, 24- factors. A safety factor of additional hour storm event for a single inlet. If depth shall be included to accommodate multiple inlets are planned, the total volume/depth loss due to sediment forebay volume requirement shall be accumulation between maintenance proportionally distributed to each inlet cycles. based on the drainage area to each inlet. If there are numerous inlets Sedimentation Zone with forebays, they shall be sized to The forebay’s sedimentation zone provide effective protection at each inlet isolates the deposition of sediment from given the expected inflow velocities and the adjacent BMP. To create this volumes on a site-specific basis even if isolation, a spillway such as a stabilized that total volume exceeds the 20%. earthen berm, concrete structure, gabions, or riprap can be used. If an Any live storage provided by the earthen berm is used, it shall be fully forebays shall not be included toward stabilized before accepting flow. the required volume of the associated BMP (basin), since at any given time a Exit velocities from the forebay shall be portion of that volume may be filled by non-erosive to the adjacent BMP. A accumulated in the forebay. designed overflow section shall be constructed on the top of the spillway Forebays and their associated BMP (berm). (basin) may be subject to local, county, state, or federal rules and regulations CONSIDERATIONS which may have more restrictive Planning should be the first step in any requirements than those within this successful project. Forebays should be standard. integrated into the design of the BMP where required or optional in the BMP Plans and specifications shall be standard for maximum performance. followed by the site superintendent and field personnel during the construction Energy Dissipation process. Energy dissipation zones can be designed by specifying the influent pipe Energy Dissipation or channel height above the pool Energy dissipation zones shall be substrate, the substrate composition, designed as deeper pools to dissipate and the plunge pool depth. As influent inflow energy and reduce resuspension pipe height above the substrate of sediments in the forebay. Sizing the increases, plunge pool depth and width plunge pool is a function of substrate increase, and plunge pool length

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decreases. Depending on the size, design, and life expectancy of the forebay, the use of a Research has shown that energy bench or shelf with emergent vegetation dissipation zones in forebays with an around the perimeter will help with water armored bottom, a permanent pool quality, provide a safety feature, and between 2 and 18 inches deep, and a improve aesthetics. pool diameter at least as large as the influent culvert diameter is effective. PLANS AND SPECIFICATIONS

Sedimentation Zone Plans and specifications for installing Sedimentation zones should be a sediment forebay adjacent to a designed with as elongated a flow path larger BMP shall be consistent with as feasible, be shallow enough to this standard and shall describe the reduce sediment settling distance, and requirements for applying the practice be vegetated to promote sedimentation to achieve its intended purpose. and stabilization. Where forebays are required as part Research has shown that unstable of an adjacent BMP, the design watersheds need larger sedimentation should be integrated into the required zones in forebays, while stable construction documents for that BMP. watersheds may need a smaller Forebays can also be used at any forebay, but they can lose sediment if inlet to an existing or lake. the energy dissipation zone Construction documents shall include accumulates sediment. at a minimum:

A measuring device should be 1) Forebay location, alignment, incorporated into the forebay design to dimensions and geometry of the determine the depth of sediment energy dissipation and accumulation and help assess sedimentation zones of the maintenance needs and capacity. forebay.

Forebays help reduce the sediment load 2) Material specifications especially reaching the adjacent BMP, and for the spillway barrier used to therefore will typically require separate the forebay from the maintenance more frequently than the adjacent BMP and the dissipation adjacent BMP. zone substrate.

To reduce the need for dewatering 3) Soil and sediment control sediment removed from the forebay plan. during maintenance, a flashboard riser or other structure may be incorporated 4) Maintenance plan and schedule. into the design to draw down the Note that sediment forebays are permanent pool in the forebay prior to included in the Practice Standards for sediment removal. DRY DETENTION BASIN 809 (consideration), EXTENDED

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DETENTION BASIN 810, WET Manual 2C-11 Inlet Sediment Forebays. BOTTOM DETENTION BASIN 811 (consideration), and WETLAND Federal Highway Administration. 2006. DETENTION BASIN 812. Hydraulic Engineering Circular 14 – “Energy Dissipaters”. PDF available at OPERATION AND MAINTENANCE fwha.dot.gov/engineering/hydraulics/pub Forebays shall be inspected at least s/06086/hec14ch05.cfm annually, preferably quarterly. Johnson, Jennifer Lee. 2006. Remove any accumulated floatable Evaluation of Stormwater Wetland and debris at each inspection. Wet Pond Forebay Design and Stormwater Wetland Pollutant Removal Ensure utility locates (JULIE) are Efficiency. Unpublished master’s thesis, prior to sediment removal from the North Carolina State University. forebay. http://repository.lib.ncsu.edu/ir/bitstream /1840.16/1590/1/etd.pdf Remove and dispose of accumulated sediment as needed after annual or Massachusetts Department of quarterly inspections based on sediment Environmental Protections and Mass accumulation to maintain performance Highways. 2004. MassHighway Storm of the forebay. Water Handbook for Highways and Bridges. Energy Dissipation Sediment should be removed from the Virginia Department of Environmental energy dissipation zone of the forebay in Quality. 2011. Stormwater Design a typical mixed used watershed every 3- Specification, Appendix D: Sediment 5 years, or as needed to maintain Forebay. design geometries and function. Sherrod, T. 2011. Innovative Designs Sedimentation Zone for Erosion, Sediment and Turbidity Sediment in the sedimentation zone Control. Environmental Connection should be removed when the sediment 2011, EC 6210. International Erosion exceeds 50% of the volume capacity of Control Association. the sedimentation zone within the forebay. Virginia Department of Transportation. Adopted 2002, revised 2014. VDOT The forebay spillway separation barrier Drainage Manual, Chapter 11 and dewatering device, if any, shall be Stormwater Management. inspected for any breaches or other problems that would compromise its US Environmental Protection Agency. performance and appropriate repairs 1999. Stormwater Technology Fact made. Sheet: Wet Detention Ponds.

REFERENCES August 2015 Iowa Department of Natural Resources. 2008. Iowa Stormwater Management

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