2002-06-26 CC Packet - Laserfiche WebLink

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, . Retention-Svstems - "Werponcts Limitations (continued) . The local climate during winter may affect the biological removal of pollutants in the pond. (Lower tempera- tures decrease the rate of biological activity). Also, formation of an ice layer may reduce the pond's treatment efficiency. . Concern for mosquitoes and maintaining oxygen in ponds. . Cannot be placed on steep unstable slopes. . Depending on volume and depth, pond designs may require approval from dam safety authorities. Reauirements lilslgl'l Pond Volume and Surface Area Designing the volume of the permanent pool usually involves defining a rainstorm event called the "water quality event". For wet ponds, the runoff volume generated from the water quality event (called the "water quality volume") is equal to the volume ofthe wet pond. Local authorities will likely have their own definitions for the water quality event or the runoff volume that it generates. However, two published water quality event recom- mendations endorsed by the MPCA are the I-year, 24-hour storm event (roughly 2 to 2.4 inches in Minnesota) and the 2.5-inch, 24-hour storm event (Walker, 1987). If the I-year, 24-hour storm event is used, an additional volume for sediment storage must be added to the permanent pool design. If Walker's 2.5" storm is used, 25 years of sediment accumulation has already been incorporated into the pond volume. The volume of runoff from the water quality event is best predicted by a combination of monitoring existing conditions and modeling future conditions. However, local authorities will likely have their own methods for calculating runoff volume from storm events. For design purposes, the water quality volume should be considered an instant flow to the pond, not an inflow-outflow calculation. In other words, this volume should be considered to arrive at the pond all at once, rather than over the course of several hours or days. The assumption of instant runoff is conservative, but it accounts for a great deal ofthe variability that occurs in both storm events and runoff conditions. The 25-year sediment volume needed for the pond (if the I-year, 24-hour storm is used as the water quality event) can be calculated with NRCS sediment storage calculation methods (see Sediment Management, below). A minimum pool surface area of 0.25 acres is recommended based on the typical drainage area size required to sustain a permanent pool during summer months. ?f \) Pond Depth: Pool depth is an important design factor, especially for sediment deposition. An average pool depth of 3 to 6 feet is recommended. Settling column studies and modeling analyses have shown that shallow ponds have higher solids removal than deeper ones. However, resuspension of settled materials by wind may be a problem in shallow ponds that are less than 2 feet in depth. Depths in excess of 10 feet may result in thermal stratification. Stratified pools tend to become anoxic (low or no oxygen) more often than shallower ponds. For wetpool depths in excess of 10 feet, it is re ommended that some form of recirculation be provided in the summer, such as a fountain or aerator t prevent stagnation and low dissolved oxygen con 1 IOns. --- 3-258 Minnesota Urban Small Sites BMP Manual