2002-06-26 CC Packet - Laserfiche WebLink

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. . Retention-Svstems - wert'onC1s Varying depths throughout the pond are recommended. Intermittent benches around the perimeter of the pond are recommended to enhance public safety and to promote the growth of aquatic vegetation. Six to eighteen inches of water are needed for optimum wetland vegetation growth. Deeper depths near the riser will yield cooler water bottom discharges, which may mitigate downstream thermal effects. Avoidance of short-circuiting and the promotion of plug flow: To prevent short-circuiting, water is forced to flow, to the extent practical, to all potentially available flow routes, avoiding "dead zones" and maximizing the time water stays in the pond during the active part of a storm. Design features that encourage plug flow and avoid dead zones are: Providing a broad surface for water exchange across cells rather than a constricted area. . Maximizing the flowpath between inlet and outlet, including the vertical path, also enhances treatment by increasing residence time. Baffles or islands can be added within the permanent pool to increase the flow path. The ratio of flowpath length to width from the inlet to the outlet should be at least 3: I. The flowpath length is defined as the distance from the inlet to the outlet, as measured at mid-depth. The width at mid-depth can be found as follows: width = (average top width + average bottom width)/2. . All inlets should enter the first cell. Ifthere are multiple inlets, the length-to-width ratio should be based on the average flowpath length for all inlets. Using a teardrop shape (as opposed to a rectangular one), as it minimizes dead zones caused by comers. Pond Slopes: The side slopes ofthe permanent pool should be no steeper than 3: I. Flatter slopes help to prevent erosion ofthe banks during larger storms and make routine bank maintenance tasks, such as mowing, easier. Flat slopes also provide for public safety, and allow easier access. Furthermore, the sides of the pool that extend below the safety and aquatic benches to the bottom of the pool should be at a slope that will remain stable, usually no steeper than 2: I (horizontal: vertical). Slopes leading to the wet pond should be no steeper than 3: I. Jt II Sediment Management: A sediment forebay or similar pretreatment device is highly recommended to enhance pollutant removal and to prolong pond effectiveness in larger (>4,000 cubic feet) facilities. The original design volume of the wet pond should take into account gradual sediment accumulation. An access for maintenance, minimum width of 10 feet and a maximum slope of 15%, must be provided by public 'Or private right-of-way. This access should never cross the emergency spillway, unless the spillway has been designed for that purpose. An emergency drain (with a pipe sized to drain the pond in less than 24 hours) should be installed in all ponds to allow access for riser repairs and sediment removal (Schueler, 1987). Metropolitan Council I Barr Engineering Co. 3-259