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07/09/97 12:24 FAX 612 490 2150 SEH ST. PAUL 2002/003 • MEMORANDUM ■ST.PAUL,MN 0 MINNEAPOLIS, MN 0 ST.CLOUD, MN 0 CHIPPEWA FALLS,WI 0 MADISON,WI 0 LAKE COUNTY,IN TO: Pamela Sheldon,Community De lopment Director FROM: Rocky J. Keehn, P.E. DATE: July 9, 1997 RE: #487-97-Review of Silver Lake Commons SEH File Number,A,MOUND9701.00(PF 80.12) Based on our review of the grading plan (dated 6-27-97), pond layouts (dated 6-27-97), and calculations (dated 6-26-97)for the above referenced project,SEH offers the following comments for your consideration: 1. The pond volume proposed is adeq\ate to treat the storm water,however,the inlet and outlet • pipes to the pond are too close together. This will cause"short circuiting"of the pond and thus greatly reduce the effectiveness of its sediment removing capabilities. Recommendation: Relocate the inlets to the pond so to discharge on the north side of the pond or relocate the pond outlet to the southeast corner of the pond. 2. There is a low point catch basin and the beginning of a storm sewer system near the entrance to the site that will capture runoff from the central drive area and convey the runoff to the proposed pond. In order for the pond to reduce the flow rate to post-developed conditions,the 100-year runoff to the low point m'st be conveyed to the pond. One way is to size the storm sewer system to handle a 100-year storm. Since there are no storm sewer calculations with the information submitted for our review, we cannot determine if the storm sewer will handle the 100-year event. The other option is to use the east drive to convey the excess flow from the low point to the pond. Based on the elevations shown on the grading plan, the height of the curb is about the same as the overflow elevation to the east drive. Hence, it appears that if the storm sewer system cannot handle the 100-year flow and water will go into the MnDOT ditch system at a higher rate than the post-developed discharge rate. Recommendation: Submit calculations that show the storm sewer was sized to handle the 100-year flow\regrade the entrance to the site to assure excess flow will go east to the pond, .