2 Braewood Hills | KPFF Consulting Engineers STORMWATER ANALYSIS Project Overview and Description Drainage Report This storm drainage report has been prepared to show that the proposed Braewood Hills Third Addition PUD meets or exceeds the stormwater regulations of the City of Eugene and SLOPES V (Standard Local Operating Procedures for Endangered Species, authorized or carried out by the US Army Corps of Engineers in Oregon). This report has been prepared for the Final PUD review, Site Development Permit review, and PEPI permit review. The report will discuss existing and proposed storm drainage and how the proposed facilities will meet the following requirements: vegetated treatment for water quality, flood control, flow control and source control. Site Constraints The project will be constructed on a site that has a 15 to 20 percent average slope and poorly infiltrating soils. For these reasons, infiltration is not an option. Instead, detention will be used to provide flood control for the site. This will be discussed further in the Proposed Stormwater System section. Project Narrative Existing Conditions The site is surrounded by low-density residential or undeveloped lots. Hawkins Drive provides the western border of the project, Randy Lane runs along a portion of the southern edge and existing single-family residences border the east edge of the site. The existing wetlands have several sources. Some of them are from Goal 5 season streams that cut through the site, but at least one of the wetland areas was likely caused by a storm system outfall that was installed as part of the housing development to the east. The outfall itself is just outside of the project boundary. See Exhibit 1 in Appendix 1 for existing conditions, including the existing wetland extents. The site is within the City of Eugene Amazon Basin, per the 2002 Stormwater Master Plan and the receiving water body for this basin is Amazon Creek. Currently, there is no stormwater infrastructure on site. Amazon Creek is not TMDL and/or 303(d) listed. There are no nearby active NPDES Permits. The site and receiving water are not in a groundwater management area or EPA-designated sole source aquifer. Before European Development, the site would have been a wooded hillside. According to the USDA Web Soil Survey (WSS), the site soils consist of a combination of Dixonville silty clay loam, Panther silty clay loam and Philomath cobbly silty clay. All of those soils have a hydrologic soil group rating of D. The resulting runoff curve number for the pre-development condition of this site is 76. This was used to calculate the pre-development runoff rates (see Appendix 2). The downstream streams are considered floodwater streams, which trigger flow control requirements. However, the SLOPES V requirements of limiting the post-development runoff to pre-European development runoff at the 2-year and 10-year storms are more stringent.