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RCWD Alternative Compliance Sequencing Report City of St. Anthony Village PAGE 26 WSB Project No. 1626-53 6. Central Park Biofiltration System (RC Ditch 3 Watershed) Project Description: The project consists of a biofiltration system which will be constructed in an existing low area at Central Park that collects untreated runoff from the majority of the 42 acre drainage area. The biofiltration system will consist of draintile pipe encased in a rock trench and located beneath a sand/compost filtration medium. Native vegetation will be planted throughout the raingarden to provide evapotranspiration of stormwater and minimize standing water following rainfall events. Project Benefit: The proposed biofiltration system at Central Park is anticipated to remove approximately 37.5% of total phosphorus or 15 pounds annually. Using the anticipated loading rate from the Silver Lake TMDL (0.97 lbs of TP/AF of runoff), an equivalent annual volume reduction of 673,608 cubic feet is estimated. A portion of this project may be paid for under RCWD grant funding. If grant funding is used for this project, RCWD has indicated that a portion of the water quality credits may not be used to benefit linear project improvements. Table 6 below summarizes the equivalent annual volume reduction resulting from total phosphorus removals for possible improvement projects in the City of St. Anthony Village. Table 6: Equivalent Annual Volume Reduction from Proposed Improvement Projects Project Equivalent Annual Volume Reduction (ft3)* Salo Ponds Alum Treatment System 943,051 Mirror Pond Improvement Project 169,012 Mirror Pond Alum Treatment System 1,605,622 37th Avenue In-line Treatment System 421,560 Stinson Boulevard In-line Treatment System 134,722 Central Park Biofiltration System 673,608 TOTAL 3,947,575 *Equivalent annual volume reduction calculated using 0.97 lbs of total phosphorus/acre-foot of runoff consistent with TMDL study for Silver Lake