Agenda Packets - 2021/07/06 - Laserfiche WebLink

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the pond is likely short-circuiting, and the pond is not achieving the high pollutant removal rates that the generic tables would indicate it has the potential to attain_ Since Silver View Pond currently exceeds MPCA minimum pond sizing guidelines, no urgent action is needed. However, since the pond does have significant sedimentation (approximately 39% of the estimated constructed desi(n volume has been removed by accumulated sediment), the city should consider (a) removing sediment to return the pond to its original estimated design conditions or (b) modifying the pond's current outlet structure to serve as an extended detention basin and to help prevent short-circuiting of flaws. While these improvements are not urgent. they would improve the pond's functionality and water quality treatment potential and they would enable the city to demonstrate compliance with its MS4 permit requirements. There are several issues to be considered and understood to determine if removal of sediments is feasible at this time including but not limited to: the level of contamination in the sediment to be removed, options for sediment disposal, availability of land for sediment disposal, and/or wetland mitigation requirements. For stormwater ponds that were constructed in upland areas and were not previously wetlands, sediment may be removed and the permanent pool volume expanded by dredging or excavating without any major concerns for wetland issues. However, for stormwater ponds that were wetlands or partial wetlands prior to the original construction, the Wetland Conservation Act (WCA) requirements are applicable and wetland mitigation may be required, especially if the pond is to he expanded beyond its original construction. In cases where excavation to remove accumulated sediment is proposed, but only to the extent that the excavation recreates the original excavation contours, a ,permit is required but mitigation is not typically a permit requirement. If expansion of the pond surface area or pond volume is desired in a pond that was originally constructed in an existing wetland, the city would need to obtain a permit and would likely be required to perform wetland mitigation. Further discussion of wetland permitting is included below. In those instances the city could opt for an alternative to pond expansion or sediment removal, Such an alternative would involve modifying the pond's outlet structure to force the pond to act as an extended detention basin for frequently occurring storms such as a I -year or ?-year event. The outlet structure modification option is only possible if additional land surrounding the pond is available to enable a slightly higher I00-year flood bounce than currently exists. Typically extended detention modifications only increase the I00-year flood level by a few inches. But if that increase would cause the I00-year flood to encroach on homes or buildings, an extended detention modification may not be viable. WCA requires mitigation/replacement for excavation in Type 3. 4, or 5 wetlands, but provides an exemption for maintenance of wetland stormwater treatment ponds. 'Typically. proof that the V %Ipls'.? MN n� :;(I l] '_ Si IN Ur Vic" hili! Ntics,wenl WorkFlies litport SONer View Panl Assec:rncnr-i-hi aLdon 13