3 Braewood Hills | KPFF Consulting Engineers STORMWATER ANALYSIS Proposed Conditions The proposed conditions will include single-family houses on individual lots, private streets and shared driveways. The private streets and shared driveways are assumed to be 100 percent impervious. They will be treated by vegetated facilities. For the detention calculations, the proposed lots are assumed to be 20% impervious with most of the large, steep lots left as wooded and vegetated. The detention system is sized to detain the proposed runoff at the 2-year and 10-year storm event, assuming that the private streets are 100% impervious and that the lots are 20% impervious. If lots are developed with more impervious area than this assumption, they must provide detention for all runoff beyond the assumed 20% impervious area. For the detention calculations, the proposed lots are assumed to be 50% impervious. Water quality will be provided by swales located along the private streets and at the low side of the lot along the existing Randy Lane and the west property line. These vegetated swales will slow the water significantly and create a longer path of travel that more closely mimics how water travels through the wetlands. These assumptions are conservative and intended to be higher than the likely impervious percentages given the amount of protected wetlands that will remain on site. Some of the 15.50-acre site is outside of the study area because of unimpacted Goal 5 areas and unimpacted areas downhill of the storm system. Additionally, Lot 39 at the northeast corner of the site is not currently planned for development. When it is developed, it will be required to provide water quality and detention within the lot boundaries. A total of 12.39 acres are assumed to be impacted and included in the study. For the limits of the study, see Exhibit 2 in Appendix 1. The runoff from the development to the east has not been considered in water quality or detention, but the site piping is sized to accommodate peak flows from the existing outfall. Proposed Stormwater System Stormwater Requirements The City of Eugene requires water quality treatment of an equivalent area for all new or replaced impervious surfaces. The 2014 City of Eugene Stormwater Management Manual (SWMM) implements a hierarchical system in which infiltration facilities must be considered first. If infiltration is not possible, filtration facilities must be considered. Finally, if the site has insufficient room for filtration facilities, a greatly increased SDC fee may be assessed. The on-site soils allow for limited infiltration and there is a concern about slope stability if infiltration is attempted. Therefore, filtration facilities have been selected to meet the requirements of vegetated treatment. SLOPES V requirements are applied to this project as part of the Joint Permit Application (JPA) that will be obtained for improvements proposed within the wetlands delineated on the site. SLOPES V requires that the pre-development flow is equal to or greater than the post-development flow for this site. In this case, pre-