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� .� PC Exhibit E <br /> . - <br /> wsB . <br /> &Assoc- Infrastructure ■ Engineering ■ Planning ■ Construction 701 Xenia Avenue South <br /> Suite 300 <br /> Minneapolis,MN 55416 <br /> Tei: 763-541-4800 <br /> Fax: 763-541-1700 <br /> Memorandum � <br /> To: City of Orono <br /> From: Pe Willenbring, A'SB &Associates,Inc. <br /> Date: September 3, 2009 <br /> Re: Stormwater Management Plan for Anderson Property on Lot 1, <br /> Block 2 Town Site of Langdon Park <br /> This memorandum outlines the details of a stormwater plan for the above-referenced property. <br /> The plan,which incorporates both filtration and infiltration BMP's,has been designed to <br /> virtually eliminate downstream runoff volumes and pollutant loadings from over one-half of the <br /> lot area, and should address City requirements for managing runoff from this parcel. Elements <br /> of the plan are described below: <br /> 1. Runoff from a 11,300 sq ft area,that includes road right of way(see attached figure)will <br /> be directed through a pretreatment/infiltration pond at the south end of the lot. For <br /> significant rainfall events,when runoff volume is sufficient,water will fill up the pond <br /> and be allowed to overflow to an existing 6-inch HDPE storm sewer that will carry this <br /> water into a proposed new filtration/infiltration system that will be constructed under a <br /> proposed sidewalk on the north end of the parcel. <br /> 2. The filtration/infiltration area will consist of 2 cells, each having a width of 10 feet, a <br /> depth of two feet and a length of 20 feet. Each cell will be filled with 15 cubic yards of <br /> rock, and based on 40%voids,will provide 160 cubic feet of stormwater storage within <br /> the voids of the rock prior to letting any of this water overflow. <br /> 3. Water will fill up the first cell, and if enough runoff is available,will then overflow to the <br /> second. For events having rainfall significantly greater than 2 inches, an overflow will be <br /> constructed to allow runoff to overflow the second cell and be directed into a surface <br /> swale that will filter the water across this swale area prior to discharge across Crystal Bay <br /> Road. The total volume of water that can be stored within the 2 cells will be 320 cubic <br /> feet. Based on a conservative infiltration rate of 0.2 inches per hour, even if the cells are <br /> filled to the overflow elevation,they will be dry within 12 hours. <br /> ��Qi�l��� <br /> SEP p 3 2009 <br /> CITY OF ORONO Minneapolis.St.Cloud <br /> � Equal Opportunity Employer <br />