Preliminary Stormwater Report <br />THE ALTON BAKER <br />EUGENE, OREGON <br />1.0 Site Conditions <br />1.1 EXISTING SITE <br />The project site is located at 177 Day Island Road in the City of Eugene, Lane County, Oregon (Tax Lot <br />200, Tax Map 17032934). The property is zoned C-2 (Community Commercial District) with PD (Planned <br />Unit Development) and WR (Water Resources) overlays and is in a flood hazard area. The site is <br />currently developed and consists of an office building, parking lot, and associated infrastructure. <br />The existing property is approximately 1.55 acres (67,610 SF), with roughly 0.60 acres (26,160 SF) of <br />existing impervious area. Impervious area drains to an existing 6-inch storm drain pipe that outfalls to <br />the Q Street Floodway. <br />1.2 PROPOSED SITE <br />The proposed re-development includes a new multi-family mid-rise building with parking lot, landscaped <br />areas, and associated utilities/stormwater infrastructure. <br />Stormwater runoff is proposed to be conveyed to a private infiltration facility that will overflow to the <br />existing 6-inch outfall on Q Street Floodway. Approximately 31,305 SF of total impervious area is <br />proposed to be tributary to the private facility (i.e. net increase of 5,145 SF in impervious area from <br />existing improvements). The remainder of the area tributary to the private facility (14,360 SF) will be <br />comprised of landscaping/pervious areas. <br />The proposed re-development will not alter the current general drainage pattern towards the Q Street <br />Floodway. Impervious areas will drain to the proposed private facility which will be sized to infiltrate the <br />water quality design storm event. Design storm events in excess of the water quality event will overflow <br />via a beehive inlet to the existing 6-inch storm drain pipe that outfalls to the Q Street Floodway. <br />2.0 Eugene Design Standards and Development Code <br />2.1 STORMWATER FLOOD CONTROL <br />This project is exempt from flood control requirements per EC 9.6791(2)(c). The Q Street Floodway is a <br />manmade drainage system designed to accommodate stormwater run-off generated by the stormwater <br />basin area. The net increase of 5,145 SF of impervious area generates a negligible 0.03 CFS of additional <br />flow for the flood control design storm (5-year event). Supporting HydroCAD calculations are provided in <br />Appendix A. <br />2.2 STORMWATER QUALITY <br />A private stormwater infiltration facility will be utilized to comply with the stormwater quality <br />requirements set forth in EC 9.6792. The proposed infiltration facility will be sized to retain the water <br />quality design storm event without overflow for the onsite impervious areas. <br />The Geotechnical Report included in Appendix C suggests that shallow groundwater is potentially <br />present on the site. Although groundwater level was not observed during the investigation, the report <br />indicates that static levels should closely correlate with the water level in the Q Street Floodway. The <br />ordinary high-water mark is roughly at elevation 404.85. Thus, seasonal high groundwater depth in the <br />The Alton Baker May 3, 2019 <br />Preliminary Stormwater Report Page 1 <br />