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Page 3 Date: May 24, 2007 To: City of Arden Hills Planning Commission and City Council % Mr. James Lehnhoff, City Planner Re: Celestica Parking Lot Addition - 4300 Round Lake Road West, Arden Hills, MN made up of natural and fill clay materials that are not conducive to infiltration/filtration methods for the treatment of the first flush storm water events. With the possibility of infiltration/filtration methods ruled out our next objective was to maximize treatment in the existing storm water retention/ detention basins located on the west property of the Celestica site. In our current analysis of the original construction of the retention/detention basins, it is our judgment that the retention/detention basins were never constructed per the original plans or in such a fashion that achievable rate reduction could have been attained. By correcting the two retention/detention ponds located on the west side of the Celestica property the proposed storm water design for the developed site has a lower runoff rate for the 2- year, 10-year and 1 DO-year events than the existing site conditions for each storm event. The retention/detention ponds and the sub-catchments are described in greater detail in the attached stormwater management report. Due to the increase in impervious area, additional retention/detention pond volume will be achieved through the reshaping of the two existing detention/ retention basins. In addition, the retention detention basins will be excavated to maximize sediment volume within the constraints of the site. It should also be noted that the existing wood weir structures are proposed to be removed because of their ineffectiveness and their tendency to be degraded due to freeze thaw cycles causing the structures to heave and break down. The proposed replacement skimmers will be achieved through the use of inverted storm sewer pipes and manholes. These corrections are required to achieve a rate control equal to or less than the storm water runoff from the pre developed site. In addition, filtration components will be added to the existing pond basins that will further enhance the water quality features of the on-site ponding system. The overall net result of the proposed pond modifications is that the runoff reductions for the 2- Year, 10- Year and 1 DO-Year rainfall events are 87%, 34% and 27% (respectfully) of the current runoff volumes. VARIANCE REQUEST Here is PDA's response to the hardship criteria for the variance requested for Celestica Parking Lot Addition: 1. The property in question cannot be put to a reasonable use if used under conditions allowed by the official controls. Response: The highest and best use of the properly In the Gateway Business District is for the type of use with its high skl/l, well paying jobs that currently exist on the site. /n order to retain the existing use in Its current /ocatlon, additional parking needs to be created on site because of the reduction of available parking on the adjacent properly within the Gateway Business Center. 2. The plight of the landowner is due to circumstances unique to the property not created by the landowner. 7200 Hudson Blvd. Suite 110 OAKDALE, MINNESOTA 55128 PHONE (651) 739-8131 FAX (651) 739-0846 www.proterradesign.com