02-13-1992 Planning Packet - Laserfiche WebLink

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■ Zoning File #1715 February 13, 1992 Page 2 Mark Gronberg, the City's consultant engineer for this project, has prepared the application before the Watershed District. He has been asked to provide two optional drainage plans for the City's consideration. Option 1 involves three retention ponds involving alterations within the designated wetland area. Option 2 involves a single pond located within the wetland just involving the western portion. Although Option 2 shows a singi.e pond, it has been noted that ponding and sediment treatment area will have to be extended closer to the baffle weir that carries water directly to the east. Both plans provide adequate separation setback from the proposed structures; Option 1 at approximately 35+' and Option 2 at approximately 40+'. Both plans will meet the detention and storage requirements of the Watershed District. The road providing access to the water tower will require special construction per Minnehaha Creek Watershed District standards. A culvert under the roadway will not be allowed. The District will require that the road be constructed with a swale design allowing overflow at the 1017.9 elevation. Erosion control has already been established on the site. Silt fencing has been placed between the construction area and the designated wetlands and low retention areas to the central portion of the property and will remain until groundcover is stabilized and restored. Steve McComas, limnologist with Bonestroo Anderlik & Rosene, has been asked to comment on the environmental superiority of one plan over the other. Please review Exhibit D. He concludes that both proposals have environmentally attractive features but that Option 1 with the three ponds has an edge over the proposal with the single pond and is recommended as the first choice. The three pond plan offers greater diversity as each of the ponds could have a different wildlife emphasis based on the type of upland and aquatic vegetation to be planted. Most important, the three pond plan offers more potential for water quality improvements of storm water run-off. The main receiving pond, or the one closest to the baffle weir, in its current configuration can be easily maintained for silt and sand removal. Option 2 would not be as easily maintained at the deeper depth. It is hoped that with the installation of three ponds as proposed in Option 1 that the Type 2 wetlands can be restored to a high quality Type 2 wetland by reestablishing the native sedges, rushes and grasses that once grew undisturbed before being altered some 40 to 80 years ago. Findings that may be considered in an approval recommendations 1.The proposed land alteration of the Type 2 wetlands or "fresh meadow" will restore a higher quality Type 2 wetlands similar to the original wetland that existed 40 to 80 years ago. ..iiiliiflbm * M