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Memorandum Page 1 of 2 800 Willamette Street, Suite 400, Eugene, OR 97401 541.684.4902 FAX 541.684.4909 Eugene, OR Portland, OR DATE: May 13, 2026 PROJECT: 2500261-DevNW Floral Hill SUBJECT: Subdivision Stormwater Analysis TO: City of Eugene FROM: Anna Backus, PE Public Works KPFF Consulting Engineers PHONE: 541.735.9249 EMAIL: anna.backus@kpff.com This memo has been prepared to show that the proposed Floral Hill Subdivision meets the requirements of the 2025 City of Eugene Stormwater Management Manual and Eugene City Code, including stormwater quality, flood control, flow control, oil control and source control. Existing Conditions The site is located on the east side of Floral Hill Drive just north of where it turns to the east. The site is currently undeveloped and vegetated with grass, brush and trees. There are consistent slopes across the site to the east. The soils on site are mainly rated as Hydrologic Soil Group D, with a small area on the south and east side rated as Hydrologic Soil Group C. This means that the soil on site has slow or very slow infiltration rates when wet. There is no stormwater infrastructure on site. Floral Hill Drive has a public ditch on the west side of the road. The ditch is inaccessible to the proposed development site due to the continuous slope to the east from Floral Hill Drive. Instead, the public main serving the site will be the 42-inch storm main in Riverview Street. Proposed Conditions The proposed development includes a public street, two private streets and housing. All impervious area on site will be offset or treated with vegetated facilities. Stormwater runoff will travel through the site as a combination of sheet drainage, gutter flow, overland concentrated drainage and through storm pipes. Treated runoff will be collected from the proposed vegetated facilities. It will then be conveyed through a storm main extension in an easement down the hill to the 42 -inch main in Riverview Street. Stormwater Quality The City of Eugene requires water quality treatment of an equivalent area for all new or replaced impervious surfaces. The 2025 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 sized. Finally, if the site has insufficient room for filtration facilities, a greatly increased SDC fee may be paid. As described above, the on-site soils have very slow infiltration rates. Additionally, some of the stormwater facilities are close to buildings or property lines, so the plumbing code prohibits infiltration. Therefore, only vegetated filtration facilities have been selected for water quality treatment.