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Pamela Sheldon May 22, 1997 Page No. 3 Internal Drainage Although the internal conveyance system (storm sewer and ditches) is usually not part of a PUD proposal, in this case it does need some discussion as part of our review. The key to the sediment ponds working successfully is getting the runoff to the pond. The original plan proposed using the Mn/DOT ditch as a conveyance system, since approximately 5.3 acres of existing area on the site goes to the ditch. It would be feasible to route some of the proposed development to the wetland since a fair amount of area already goes there. The problem is not with the water quantity, it is with the water quality aspect of the drainage. For the Mn/DOT ditch to be used as a conveyance system, a water quality pond would need to be built prior to discharging into the wetland. As proposed with the pond in the east corner, it appears unfeasible to get the runoff to the pond due to existing culvert under Highway 10, and the fact that the wetland is DNR protected. An alternative would be to build a pond next to the proposed office building. If built, It would need to be sized for runoff from both the existing Mn/DOT right-of-way and the site. Drainage of the existing development to the central pond appears matter-of-fact except for the Condo Building. A storm sewer would need to be placed around the building, or a new pond constructed in the southwest corner of the site. A pond on the southwest corner may need to be sized to handle Silver Lake Road, or provisions made to isolate it from off-site drainage.• 5, Conclusions: 1. The Mn/DOT ditch could be used for drainage if a sediment pond is built next to the proposed office building 2. A storm sewer system for the development can be constructed to drain a large portion of the site to the central sediment pond 3. Special considerations for drainage of the front of the Condo Building are not addressed in the this site plan. Water Elevations of the Existing Wetland The Local Water Management Plan (LWMP) assumed that the site would be Commercial Development. As proposed, the site would have less impervious surfaces than assumed in the LWMP. The LWMP also assumed that the pond directly south on the other side of the St. Paul Water Utility Aqueduct would be part of this wetland for storage purposes. The calculated highwater elevation in the LWMP is 906.2 with a proposed minimum building elevation of 908.2 for the wetland. Also, this site was assumed to be a regional pond, therefore on-site detention ponding is not required. The calculations for impacts to the highwater elevation to the wetland submitted by the developer did not include about 26 acres of area which drain from the north. The total area to the wetland is about 50 acres, not 13 as used by the developer's engineer. Therefore, the information submitted • by the developer cannot be used to determine highwater elevations impacts to the wetland. During final plat development of area, a highwater elevation should be determined based on the