11-12-1985 Council Packet - Laserfiche WebLink

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removal of 5,000 cubic yards of dirt together with the attendant vegetation. The design as presented shows a smell alteration in the storm water ponding characteristics to improve retention time and while removing some of the vegetation to create an open water area, the designed sloping will allow vegetation such as purple loosestrife, cattails, sedge grasses around the border to provide both a sediment and nutrient trap. Side benefit of the proposal is to improve the wildlife habitat for principally migratory water fowl. ENVIRONMENTAL RISK Due to the fact that there is a smal 1 improvement in the storm water retention characteristics in this design, the principal concern is that in either short or long term this alteration will result in increasing either the amount of sediment or nutrients. Given the flow and size characteristics of this pond in relationship to the bay to which it would outflow (even if there were an extrodinary increase in the amount of principally nutrient materials that may occur because of hitting a nutrient rich pocket in a sedimentation removal go into Lake Minnetonka; it would be of such a small lake input), probably under 1% that it would have no significant impact. The short term concern is '.hat there would be both substantially increased sediment and nutrient outflow during construction. Since the construction of the wildlife pond will. be done principally during winter together with the fact that the outflow structure will be physically blocked up, this should effectively mitigate any prcblems associated with this phase. Over t'e long term because of the flow size characteristics of this pond t t sediment increase should not be a problem, however the nutrient increase may happen due to a possible exposure to nutrient rich soils. It is ,nticipated that this has a small probability of happening and when .:onsidered against the expected improvements including improved nutrient trapping by the alteration, the size of the pond, and the fact that over the long term (3 to 5 years) that this would probably seal. itself off, that this is not considered a significant concern. ENVIRONMENTAL BENEFITS Mature Wetlands - While mature wetlands which have grassy growth etc. do pick up nutrients and trap sediment during the growing season, once they die out these nutrients are released back into the pond and generally over the year term there is no net removal of nutrients from the water in the pond. Given the fact that the principal flow from this pond is in the spring during the run off, nutrients released by the dying material may or may not have settled out into the soil from the previous fall when the spring runoff occurs. Vegetation - As mentioned much of the existing vegetation includes willows and other woods which are not good at nutrient absorption. It is anticipated the altoration wili improve both the sediment and nutrient trapping characteristics of the wetlands and also by removing the nutrient rich materials may very possibably allow for additional absorbtion capacity fromm they wetlands. Additionally besides the "out of water" growth (cattails, (-.-tc.) that wi 1 1 be on the slopes there will be underwater aquatic growth in the open water areas but that will. not be as qood a nutrient plant as emergent aquatic PlA nts such as cattails.