Agenda Packets - 2021/07/06 - Laserfiche WebLink

Home Browse Search

In such instances, traditional open -water water quality treatment basins can be supplanted by an extended detention basin which has a significantly different outlet structure than a normal wet detention basin outlet. And, in instances where it is desired to further improve on an existing basin's treatment efficiency, the basin's outlet structure can be modified or fitted with an extended detention basin -type outlet structure. Such a structure tnodifrcation would detain flows from a 1-year to 2-year storm event for up to 24 hours. With the vast amount of sediment and pollutants that enter a pond coming from storms of that magnitude, if flows from those storms can be held in the basin over a longer period of time, significantly improved water quality treatment can be achieved. in the case of the city's current water quality treatment ponds. there may be reason and opportunity to retrofit certain basin outlet structures with a new extended detention basin -type structure. Such a structure modification is recommended for Silver View Pond. Following are a few situations where the city may want to analyze performing such modifications: 1. Where an existing basin is also a historic wetland and where excavation of the sediments may prove to be very costly, because wetland replacement will be required by the local governmental unit (LGU) that administers the Wetland Conservation Act. In this case, the city may be able to forego or significantly delay the point in time where excavation is needed by simply retrofitting, the pond's existing outlet structure with an extended detention basin - type structure. 2. Where a basins inlet(s) and outlet are in close proximity, which enables inflows to short- circuit and not use the full extent of the basin. In this case, the extended detention structure can force waters to be detained long enough to utilize the full treatment capacity of the basin, 3. When higher treatment efficiencies are required due to regulations, MS4 permits, or Total Maximum Daily Load (TMDL) requirements. 4. When the city decides on its own to provide even better treatment than its current pond is providing,. This may he based on monitoring results or other decision factors. The cost for adding an extended detention nutlet structure to Silver View Pond is included in Table 1. Because extended detention basins and their structures are less well-known, the following paragraphs are intended to provide the reader of this document a better understanding of how such facilities function. I' MpI-23 MN 62 'iW SiIker Vie" fond Assessment Workriks keporl NiIVcI Vie" I'nnri Aisevinnent !innI,daca 22