Agenda Packets - 2021/07/06 - Laserfiche WebLink

Home Browse Search

3.2 Stormwatelr Pond Treatment Capacity Considerations The extent of sediment deposition in the stormwater pond was evaluated by comparing the existing permanent pool volume to the estimated "original" permanent pool volume for the pond. Determining the original permanent pool volume of a pond can be a challenge, as accurate data on the "as -built - construction of the pond is not often readily available. In this case, information from a 1989 pond management plan (developed a few years after the initial construction of the pond) and information on the current configuration of the pond were used to estimate the `original" permanent pool volume for the pond. Information on the pond depth in 1989 was input into AutoCA❑ Civil 3D software. The software was then used to create pond bottom contours based on the depth measurements and the current side slopes of the pond. The digital contours were then measured and compared to existing conditions. The total storage volume needed to meet MPCA guidelines (based on a drainage area of 196.8 acres and an impervious fraction of 32%) was computed to be 10.82 acre-feet if the entire tributary area to the pond was considered, and 7.84 acre-feet if only the previously untreated tributary area to the pond (138.5 acres and an impervious fraction of36%) was considered. The estimated existing storage volume of the pond is 45.7 acre-feet. However, even considering the full loading from the entire watershed tributary to the pond, Silver View Pond still contains nearly four times the MPCA"s minimum recommended volume for water treatment if it were to be constructed today. None -the -less. 38% of the pond's original volume is tilled in with sediment. A summary of this data is provided in Table 3. Silver View Pond was also analyzed for its current potential to treat stormwater for total phosphorus (TP) and total suspended solids (TSS) removal. TP removal was calculated using Figure 5 from Lake ami Reservoir Management: 17ol (me Ill, p 314-326 (..Phosphorus Removal by Urban Runoff Detention Basins'-) by William W. Walker, Jr., 1987. Using that document, Silver View Pond currently has the potential to remove about 7594) of the total phosphorus load entering the pond when considering the entire tributary area. These results and calculation parameters can be found in Table 4. Similarly, the potential TSS removal for the pond was calculated using Figure 13 in Merhodologi br Analv.vis cif-Delen ion -Basins for Control of'Urhtm RunoffQuality, by the United States Environmental Protection Agency, 1986. The potential TSS removal efficiency was predicted to be 95% when considering the entire tributary area. Due to the pond`s depth and large surface area. the pond has the potential to be a highly functioning treatment basin. However, since there are multiple inlet pipes located relatively close to the nutlet pipe of the pond, some of the water entering I'- Mrls 23 AIN 62 2162I082 silver View Pond Assmmneni W';orkrilea Relretrl Silver View Pond Assesctnent-finai.dncx 12