Memorandum <br />Page 2 of 2 <br />May 8, 2026 <br /> <br /> <br />collected by perforated underdrains and conveyed to the public storm system at an approved discharge <br />location. In areas located at least 10 feet from building foundations and where infiltration testing <br />demonstrates adequate rates, infiltration of treated r unoff may be utilized. Until infiltration testing is <br />completed, zero infiltration is assumed for sizing purposes. Final facility types will be confirmed during the <br />Building Permit phase. <br /> <br />Refer to the attached Basin Map and the Runoff and Water Quality Calculations for additional detail. <br /> <br />Area 1, as shown on the Basin Map, currently assumes no paving. If additional walkways are introduced, <br />runoff will be mitigated using vegetated treatment facilities consistent with SWMM requirements. <br /> <br />The project includes an eco‑roof, which reduces effective impervious area at a 1:1 ratio. Runoff from the <br />eco‑roof will be routed to the on‑site stormwater system and discharged at an approved location. Roof areas <br />not treated by the eco‑roof will be directed to vegetated treatment facilities. <br />Flood Control <br />The destination for the stormwater is a stormwater main located within Alder Alley, which outfalls to the <br />Millrace. This area has not been identified as being subject to flood control restrictions in the Stormwater <br />Basin Master Plan. <br />Flow Control <br />The proposed site does not drain directly into a headwater stream or drain to a pipe that discharges to a <br />headwater stream. The Millrace elevation is controlled by a weir, so the site is not subject to City of Eugene <br />flow control requirements. <br />Source Control and Oil Control <br />Oil control is not required as there are no vehicular areas being added to the site. <br /> <br />The trash enclosure will be covered, hydraulically isolated and will drain to sanitary sewer to meet source <br />control requirements. No other proposed improvements trigger source control standards. <br /> <br /> <br />Attachments: Basin Map <br /> Stormwater Calculations <br /> <br />2600120-kg