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<br />RCWD Alternative Compliance Sequencing Report <br />City of St. Anthony Village PAGE 21 <br />WSB Project No. 1626-53 <br /> <br />B. Inventory of Existing Water Quality BMPs Constructed Prior to 2008 <br />The following stormwater BMPs were constructed by the City of St. Anthony <br />Village prior to the RCWD’s volume reduction requirements that were initiated in <br />2008. Since the credit system was not initiated by the RCWD at that time, these <br />BMPs have not been formally recognized by RCWD at this time. However, these <br />BMPs provide significant treatment and should be accounted for when considering <br />the level of treatment needed to meet the RCWD’s Rules. A brief description of <br />each BMP along with the anticipated stormwater volume credit provided by the <br />BMP is listed below. Refer to Figure 4 for the location of the existing BMPs. <br /> <br />1. Shamrock Drive/Silver Lane Biofiltration Basin (RC Ditch 3 Watershed) <br />The City constructed a biofiltration basin off of Shamrock Drive and Silver <br />Lane in 2007. This basin provides treatment to approximately 2.9 acres, of <br />which 0.85 acres (29%) is impervious. This basin currently provides an <br />equivalent annual volume reduction of approximately 46,660 cubic feet. <br /> <br />2. Mirror Pond (RC Ditch 3 Watershed) <br />Mirror pond was constructed in 1973 by the City of St. Anthony Village. <br />The stormwater pond receives runoff from 217 acres. The existing <br />stormwater pond reduces total phosphorus loading by approximately 58% or <br />78 pounds annually. Using the concentration of 0.97 pounds of total <br />phosphorus per acre foot of runoff from the Silver Lake TMDL, the <br />equivalent annual volume reduction is 3,502,763 cubic feet. <br /> <br />Table 5 below provides a summary of equivalent annual runoff decreases that are <br />achieved by existing stormwater BMPs controlled by the City of St. Anthony <br />Village. <br /> <br />Table 5: Equivalent Annual Volume Reduction from Existing BMPs <br />Existing BMP <br />Equivalent Annual <br />Volume Reduction <br />(ft3)* <br />St. Anthony Reuse Facility 683,156 <br />Foss Road Biofiltration Swale 222,732 <br />Filtration BMPs – 2011 Street Reconstruction 31,095 <br />Shamrock Drive/Silver Lane Biofiltration Basin 46,660 <br />Mirror Pond 3,502,763 <br />TOTAL 4,486,406 <br /> <br />*Equivalent annual volume reduction calculated using 0.97 lbs of total phosphorus/acre-foot of runoff consistent with TMDL <br />study for Silver Lake