<br />3 <br /> <br />Braewood Hills | KPFF Consulting Engineers <br />STORMWATER ANALYSIS <br />The downstream streams are considered floodwater streams, which trigger flow control requirements. <br />However, the SLOPES V requirements of limiting the post-development runoff to pre-European development <br />runoff at the 2-year and 10-year storms are more stringent. <br />Proposed Conditions <br />The proposed conditions will include single-family houses on individual lots, private streets and shared <br />driveways. The private streets and shared driveways are assumed to be 100 percent impervious. They will be <br />treated by vegetated facilities. <br /> <br />For the detention calculations, the proposed lots are assumed to be 20% impervious with most of the large, <br />steep lots left as wooded and vegetated. The detention system is sized to detain the proposed runoff at the <br />2-year and 10-year storm event, assuming that the private streets are 100% impervious and that the lots are <br />20% impervious. If lots are developed with more impervious area than this assumption, they must provide <br />detention for all runoff beyond the assumed 20% impervious area. <br /> <br />For the water quality calculations, the proposed lots are assumed to be 50% impervious. Water quality will <br />be provided by swales located along the private streets and at the low side of the lot along the existing Randy <br />Lane and the west property line. These vegetated swales will slow the water significantly and create a longer <br />path of travel that more closely mimics how water travels through the wetlands. These assumptions are <br />conservative. They are intended to be higher than the likely impervious percentages, given the amount of <br />protected wetlands that will remain on site. <br /> <br />Some of the 15.50-acre site is outside of the study area because of unimpacted Goal 5 areas and unimpacted <br />areas downhill of the storm system. Additionally, Lot 39 at the northeast corner of the site is not currently <br />planned for development. When it is developed, it will be required to provide water quality and detention <br />within the lot boundaries. <br /> <br />A total of 12.39 acres are assumed to be impacted and included in the study. For the limits of the study, see <br />Exhibit 2 in Appendix 1. The runoff from the development to the east has not been considered in water <br />quality or detention, but the site piping is sized to accommodate peak flows from the existing outfall. <br />Proposed Stormwater System <br />Stormwater Requirements <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 />considered. Finally, if the site has insufficient room for filtration facilities, a greatly increased SDC fee may be <br />assessed. <br /> <br />The on-site soils allow for limited infiltration and there is a concern about slope stability if infiltration is <br />attempted. Therefore, filtration facilities have been selected to meet the requirements of vegetated <br />treatment. <br />