Agenda - Environmental Policy Board - 07/20/2009 - Special Meeting

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Below Normal Water Elevation Restoration below the normal water elevation will be done if groundwater does not maintain the normal water level. When restoration is required, all areas of the 10: 1 shelf above the groundwater and below normal elevation will be restored with annual fast-germinating seed. The recommended pond design criteria in order of importance are as follows: (1) The permanent pool is important because it provides storage and treatment of runoff during and between storm events. Permanent pool volume should be greater than or equal to the volume of runoff resulting from 2.5-inches of rain (based on the SCS runoff equation) under complete watershed development. This value has been derived from design criteria developed in Nation-wide Urban Runoff Program (NURP), with a 25 percent increase in volume to allow for roughly 25 years of sediment accumulation. (2) To promote settling and provide space for sediment accumulation, the mean depth of the permanent pool (volume/surface area) should be greater than or equal to 4 feet. This constraint may be infeasible for small ponds (< approx. 3 acre-feet in volume, see below), where mean depths of 3 to 4 feet may be used. (3) To prevent development of thermal stratification, loss of oxygen and nutrient recycling from bottom sediments, the maximum depth of permanent pool should be less than or equal to 10 feet. (4) To promote plug flow behavior, the ratio of maximum length to maximum width (Lc/Wc) should be greater than or equal to 3. Expected performance is less sensitive to the length/width ratio than to volume or depth. This constraint may be infeasible for some site plans or for small ponds. In such situations, baffles may be installed to isolate the inflow area from the remainder of the pond. A desirable alternative (for all pond sizes) is to construct two or more separate ponds in series with a total volume equal to that specified above Item (1). (5) For safety purposes and to provide suitable habitat for rooted aquatic plants, the bench width should be at least 15 feet and the bench slope should not be steeper than 10: 1 (horizontal vertical). The bench slope begins at the normal pool elevation and includes lower elevations until the minimum length criteria is met. (6) To provide stability, the side slopes below the bench should not be steeper than 4 feet horizontal to 1 foot vertical. Shallower slopes may be appropriate, depending upon soil engineering properties. Shallower slopes are more feasible for larger ponds. Newly constructed detention basins shall provide additional storage volume below the outlet to allow for reasonable accumulation of sediment. Access to the area to allow for sediment removal is required. Water Management Plan P:\Mpls\23 MN\02\2302047\WorkFiles\3rd Gen WMP\LRRWMO _3rd Gen WMP _July 2009 _Draft.doc 82