07-22-1985 Council Packet - Laserfiche WebLink

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Hennepin Conservation District 12450 Wayzata Boulevard. Suite 205 - Minnetonka; Minnesota 55343 Telephone (612) 544 8572 April 3, 1985 Ms. Jeanne Mabusth Zoning Administrator City of Orono P.O. Box 66 Crystal Bay, MN 55323 rear Jeanne: I have reviewed the grading and wetland alteration plan for the Gregory residence as you requested. I hope that the following comments are helpful. Cattail -sedge wetlands (Tyne 3) are the most efficient nutrient traps for treating surface water runoff. Artificial ponds and open water wetlands (Type 5) are less effective dve to reduced nutrient uptake by rooted aquatic plants. The efficacy of shrub swamps (Type 6) as nutrient filters is not as well. docurcnted. Nutrient uptake in Type 6 wetlands is believed to be less than in Type 3 w- H ands due r.o the slower growth of woody vegetation. The conversion of part of the cattail -sedge -shrub wetland into a wildlife pond will reduce the nutrient retention capability of this basin. The reduction however, will not significantly impact the water quality of Brown's Bay. The large off -site wetland that is upstream of the proposed alteration will continue to treat the majority of the runoff. The proposed wildlife pond should provide adequate treatment for the localized runoff. The invert elevation of the culvert under County Road 15 further ensures an adequate retention (treatment) time in the wildlife pond during most flow conditions. The followin^ recommendations are intended to minimize any potential impact of this wetland alteration on the quality of water discharged to Brown's Bay. 1. The areas of the wildlife pond nearest the pond's outlet and the inlet from the formal pond should have a 7:1 slope (horizontal:vertical) rather than the approximate 4:1 slope as shown in :he plan. This slope and the resulting increase in shallow water area will provide a better habitat for re- establishing a fringe of emergent vegetation. This fringe of emergent vegetation would provide a buffer between the open w.3ter areas and the outlet. This fringe would also serve as a filter strip for runoff from the formal pond.