Memorandum <br />Page 2 <br />March 5, 2024 <br /> <br /> <br />Stormwater Quality <br />The City of Eugene requires water quality treatment of an equivalent area for all new or replaced impervious <br />surfaces. <br /> <br />The 2014 City of Eugene Stormwater Management Manual (SWMM) implements a hierarchical system in <br />which infiltration facilities must be considered first. If infiltration is not possible, filtration facilities must be <br />sized. If the site has insufficient room for filtration facilities, a greatly increased SDC fee may be paid. <br /> <br />As described previously, the on-site soils have very slow infiltration rates. Additionally, all of the stormwater <br />facilities are close to property lines, so the plumbing code prohibits infiltration. Therefore, only vegetated <br />filtration facilities have been selected for water quality treatment. <br /> <br />To meet the stormwater quality requirements, three drainage basins will be treated by three vegetated <br />facilities (see Exh-2 in Appendix 1). <br /> <br />Basin 1 consists of most of the private drive Stonehedge Way, Stonehedge Court and shared driveways. It is <br />treated by a swale located on the north end of the private drive Stonehedge Court. Stonehedge Way also has <br />a shed section, a curb and a gutter but most of the drainage will be collected by catch basins and piped to <br />the swale. <br /> <br />Basin 2 consists of private Randy Lane and shared driveways and is treated by a stepped rain garden at the <br />south corner of the intersection of Randy Lane and Blacktail Drive. Randy Lane has a shed section with a curb <br />and gutter on the south edge, so the runoff will enter the rain garden through a curb cut at the low en d of <br />the drive. <br /> <br />Basin 2A includes the south quarter of Stonehedge Way and shared driveways. The runoff will be collected <br />by a catch basin and piped to a rain garden at the south corner of the intersection of Randy Lane and Blacktail <br />Drive, adjacent to the rain garden for Basin 1. <br /> <br />Basin 3, consisting of a shared driveway for lots 1, 2, 6 and 7, is treated by a filter strip west of the driveway. <br />The driveway is 143 feet long and 14 feet wide totaling 2000 SF. 2000 SF x 0.2 = 400 SF required for filter strip <br />treatment. The filter strip will extend 5 feet west of the driveway and will run the length of the driveway, <br />totaling approximately 700 SF. <br /> <br />These facilities have been sized using the SLOPES V requirements, which have a water quality storm that <br />slightly exceeds the City of Eugene water quality storm event. Therefore, they meet the City's requirements <br />for water quality. See Appendix 3 for the runoff and water quality calculations. <br />Please note that each lot will address water quality individually when they are developed. <br />Flood Control <br />All stormwater runoff from the site will be treated and piped to Hawkins Lane to an existing public 12-inch <br />pipe that crosses Hawkins from east to west. <br /> <br />The public 12-inch main has been flagged as being subject to flood control requirements in the 2002 <br />Stormwater Basin Master Plan. However, a downstream 54-inch CSP main noted as AM27B in the Master