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<br />"" <br /> <br />Memorandum <br /> <br />To: <br /> <br />Mark Statz, Jeff <br />McDowell <br /> <br />Date: 9/25/2007 <br /> <br />Project: Hidden Spring Park <br /> <br />From Earth Evans <br /> <br />Client: City of Centerville <br /> <br />Re: <br /> <br />Rain Garden Stormwater Analysis <br /> <br />File No: 616-07-155 <br /> <br />Remarks: <br /> <br />This memo summarizes our stormwater analysis of the Hidden Spring Park project. A chain of <br />treatment measures were constructed for the project including a small rain garden, swale, <br />depression storage and shoreland protection. Post-construction issues with slow drawdown in <br />the rain garden and draintile re-routing led to additional analysis of the stormwater system. <br /> <br />Rain Garden Sizing <br />Runoff from the LaMotte Drive storm sewer system discharges into a small rock fore bay then <br />the rain garden. The rain garden functions as the primary water quality treatment feature. It <br />was sized based on Rice Creek Watershed District (RCWD) proposed new rules that require <br />infiltration/biofiltration volume adequate to contain the 0.8-inch event (which exceeds their <br />existing requirement of the runoff from the 0.34-inch event). The rain garden was surveyed to <br />determine what volume is actually available based on constructed elevations. The following <br />table compares the as-built volume and RCWD requirement. <br /> <br />Scenario Volume (cf) <br />RCWD Existing Standard (0.34-inch event) 392 <br />RCWD Proposed Rule (0.8-inch event) 1.3 50 <br />As-built Volume (893 contour to the bottom) 1,674 <br /> <br />As shown in the table, the volume provided in the rain garden is adequate to meet the RCWD <br />proposed rule if the 893 contour is closed around the entirety of the rain garden. The minimum <br />elevation for the outlet structure is 892.9 to still provide adequate volume to meet the <br />proposed rule. Technically RCWD cannot require conformance to the new rules until they are <br />adopted (likely December 2007), but the site was reviewed based on compliance with the new <br />rules. <br /> <br />Overflow Elevation <br />The rain garden was designed with three outlets - draintile, 12-inch outlet structure and <br />overflow to the swale. Due to the clayey soils, draintile was utilized to allow for complete <br />drawdown of the rain garden. The 12-inch outlet structure is the primary outlet and along with <br />the grass overflow is routed to the downstream swale. The purpose of this design is to: <br /> <br />. restrict the discharge rates for small storm events via the 12-inch outlet structure <br /> <br />. Bonestroo <br /> <br />233S Highway 36 W <br />51. Paul, MN 55113 <br /> <br />Tel 651-636-4600 <br />Fax 651-636-1311 <br /> <br />www.bonestroo.com <br />