2002-06-26 CC Packet - Laserfiche WebLink

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i . Retention_S\lStems - "Werponc1s Sediment Forebays The settling area for incoming sediments can be increased through the addition of a sediment forebay. The use of a sediment forebay, however, is only recommended for wet ponds larger than 4,000 cubic feet. The forebay is an excavated settling basin or a section separated by a low weir at the head of the primary impoundment. Forebays serve to trap sediments before the runoff enters the primary pool, effectively enhancing removal rates and mini- mizing long term operation and maintenance problems. Periodic sediment removal from the forebay is easier and less costly than removal from the primary wet pond pool. Sediment forebays should be designed for ease of maintenance. Hard bottom forebays make sediment removal easier, and forebays should be accessible by heavy machinery, if necessary. About 10 to 25 percent of the surface area of the wet pond should be devoted to the forebay The forebay can be distinguished from the remainder of the pond by one of several means: a lateral sill with rooted wetland vegetation, two ponds in series, differential pool depth, rock-filled gabions or retaining wall, or a horizontal rock waJI filter placed laterally across the pond. Energy dissipation techniques should be used at the inlet to the sediment forebayto avoid erosion, to promote settling, and to minimize short-circuiting offlows. The length to width ratio of tile forebay should be at least 2:1 to minimize short-circuiting. Figures 3a and 3b show a schematic of a wet pond that has a separate sediment fore bay. Pond Shape To avoid reducing the pollutant removal capability and to maximize travel distance, the inflow points of the wet pond should be as far from the outlet as possible. To maximize storm water contact and residence time in the pool, a length to width ratio of 3: 1 is recommended. A minimum pool surface area of 0.25 acres is recommended. Performance of the wet pond may be enhanced by enlarging the surface area to increase volume, as opposed to deepening the pool. Multi-Stage Outlets Wet ponds may be designed with a multi-stage outlet structure to control discharges from different size storms. Usually the pond is designed to control multiple design storms (e.g. 2- and/or 10-year storms) and safely pass the 100-year storm event. However, the design storm may vary depending on local conditions and requirements. Chemical Treatment Addition of chemicals to precipitate phosphorus within the permanent pool (e.g. alum) can further enhance the removal of both particulate and dissolved forms of phosphorus entering the wet pond. Aesthetic Enhancements Many design features can be incorporated to enhance aesthetics where possible, such as: . Providing pedestrian access to shaJIow pool areas enhanced with emergent wetland vegetation. This allows the pond to be more accessible without incurring safety risks. Providing side slopes that are sufficiently gentle to avoid the need for fencing (3:1 or flatter). . Creating flat areas overlooking or adjoining the pond for seating that can be used by residents. IncorporatingwaIking or jogging trails into the pond design. Including fountains or integrated waterfall features for privately maintained facilities. Providing visual enhancement with clusters of trees and shrubs. Metropolitan Council I Barr Engineering Co. 3-253 L--____________