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MEMORANDUM • <br /> ■ST.PAUL,MN C M/NNEiPOU$ MN C ST.CLOUD, AIN O CHIPPEWA FALLS.WI a MADISON,WI o LIKE COUNTY.IN <br /> TO: Pamela Sheldon, Community De lopment Director <br /> FROM: Rocky J.Keehn,P.E. <br /> DATE4 July 9, 1997 <br /> RE: #487-97-Review of Silver Lake Commons <br /> SEH File Number AMOUND9701.00(PF 80.12) <br /> Based on our review of the grading plan (dated 6-27-97), pond layouts (dated 6-27-97), and <br /> calculations (dated 6-26-97)for the above referenced project.,SEH offers the following comments for <br /> your consideration: <br /> 1. The pond volume proposed is adeq\rate to treat the storm water,however, the inlet and outlet <br /> • <br /> pipes to the pond are too close together. This will cause"short circuiting"of the pond and thus <br /> greatly reduce the effectiveness of its sediment removing capabilities. <br /> Recommendation: Relocate the inlets to the pond so to discharge on the north side of the <br /> pond or relocate the pond outlet to the southeast corner of the pond. <br /> 2. There is a low point catch basin and the beginning of a storm sewer system near the entrance <br /> to the site that will capture runoff from the central drive area and convey the runoff to the <br /> proposed pond. In order for the pond to reduce the flow rate to post-developed conditions,the <br /> 100 year runoff to the low point mist be conveyed to the pond. One way is to size the storm <br /> sewer system to handle a 100-year stow'. Since there are no storm sewer calculations with the <br /> information submitted for our review,we cannot determine if the storm sewer will handle the <br /> 100-year event. The other option is to use the east drive to convey the excess flow from the <br /> low point to the pond. Based on the elevations shown on the grading plan, the height of the <br /> curb is about the same as the overflow elevation to the east drive. Hence, it appears that if the <br /> storm sewer system cannot handle the 100-year flow and water will go into the MnDOT ditch <br /> system at a higher rate than the post-developed discharge rate. <br /> Recommendation: Submit calculations that show the storm sewer was sized tc. wx : . <br /> 100-year flower regrade the entrance to the site to assure excess flow will <br /> go east to the pond, <br />