UO North Campus June 15, 2018 <br />Conditional Use Permit Stormwater, Sanitary Sewer, and Water Analysis Report <br /> <br /> <br />differing conditions. Specifically, deposits of fill are present in multiple locations throughout the site. <br />Fill deposits The historical borings indicate <br />fill material may have a high silt content and may have a significantly lower infiltration rates than the <br />native sand and gravel soils. <br /> <br />Groundwater: Monitoring well data presented in the 1990 ESA indicates seasonal high groundwater <br />- <br />data indicates a similar depth to groundwater within the study area. <br /> <br />Future Geotechnical Investigation: Future development projects will include geotechnical <br />investigation, infiltration testing, groundwater measurement, and engineering studies to determine if <br />and where infiltration is feasible. <br /> <br />Stormwater Management Approach for Future Development <br />Development within the site will adhere to the applicable stormwater management requirements at <br />the time of the development. The City of Eugene stormwater management requirements are set <br />forth by Eugene Code (EC) Section 9.6790-9.6796 and the Eugene Stormwater Management <br />Manual (SWMM). The 2014 SWMM is the current edition at the time of this report. Additional <br />regulatory requirements may apply to projects impacting waters of the state. The following is a <br />summary of the applicable stormwater management standards and the approach for stormwater <br />management for development and redevelopment at the site. <br />Stormwater Destination / Flood Control (EC 9.6791): EC 9.6791 and SWMM Section 1.4.3 require <br />projects to demonstrate that the receiving public storm drain systems have adequate hydraulic <br />capacity to handle increase flows from proposed development or redevelopment. The site is located <br />entirely within the Willamette River Basin, which has not historically experienced flooding problems. <br />As such, prescriptive flow control or detention requirements do not apply. However, detention may <br />be appropriate for certain localized areas where the receiving storm drain system does not have the <br />capacity to accept additional flows. The following provides a summary of the potential destination <br />points within the site and applicable flood control considerations. <br /> <br />Onsite Infiltration: Onsite infiltration will be utilized to the maximum extent feasible, in accordance <br /> <br />with EC 9.6792 (see Water Quality, below). Piped stormwater conveyance systems will likely be <br />required to accommodate overflow from infiltration systems, or provide primary conveyance in <br />areas where infiltration is not feasible due to soil conditions, environmental concerns, or other <br />factors. Piped conveyance systems will discharge to existing public or private storm drainage <br />infrastructure or waterways at the one of the destination points listed below. <br /> <br />South Millrace (Basin A / Destination Point A): Destination Point A is the Millrace located south <br /> <br />of the railroad tracks and upstream from the Millrace control structure, and is the destination <br />point for the Basin A. The Millrace and downstream conveyance system is a public stormwater <br />system. <br /> <br />The water level within the Millrace is governed by a control structure located at the west end of <br />the site. The control structure consists of an adjustable weir gate that can be manually operated <br />(lowered) during extreme storm events. In 2008, the City of Eugene modified the Millrace control <br />structure to lower the water level and address localized flooding issues observed during the 2006 <br />flood. Based on discussions with City staff, the control structure has adequately addressed <br /> <br /> <br /> <br /> <br /> |4 <br /> <br />