Capital Hill PUD, Tentative (14-020) <br />March 3. 2017 <br />LO INTRODUCTION <br />This Stormwater Management Plan (SWMP) has been prepared for the Capital Hill PUD located east of Capital <br />Drive in Eugene, Oregon. The new development will create over 30 new single family and multi-family lots by <br />widening and extending Capital Drive, and installing a private road to access a handful of the eastern lots. The <br />new improvements will also include extending all necessary utilities to serve the residential hillside <br />development. <br />This report will display conformance to the updated City of Eugene requirements for stormwater management <br />by examining existing site conditions and analyzing the stormwater management methods for treating and <br />meeting flow control requirements before discharging runoff off site. <br />2.0 EXISTING CONDITIONS <br />2.1 TOPOGRAPHY <br />The existing site is located in the south hills of Eugene which consist of ground vegetation and trees, and has <br />elevations varying between 720' and 920'. Most of the new construction and development will be above <br />elevation 800'. The site has several residential structures that are to remain as property lines will be adjusted <br />to produce 33 total lots, leaving the existing buildings on the new lot layout. <br />Slopes on site are generally greater than 8-10% for a majority of the site with many locations exceeding 20% <br />and a fair amount exceeding 1:2 (V:H). <br />2.2 SOILS <br />According to the Natural Conservation Service Website mapping services, the existing soils at the site are <br />Panther silty clay loam, 2 to 12 percent slopes (NRCS Soil #102C), Philomath cobly silty clay, 3 to 12 percent <br />slopes (NRCS Soil 108C), Philomath-Urban land complex 12 to 45 percent slopes (NRCS Soil #109F), and Steiwer <br />loam, 20 to 50 percent slopes (NRCS Soil #125F). <br />These soils are primarily in the Hydrologic. Soil Group D with the eastern edge falling into the Hydrologic Soil <br />Group C. The site soils were determined using the NRCS Web Soil Survey 2.0. A map of the soils at the site can <br />be found in Appendix B. <br />A Geotechnical Assessment was conducted by Branch Engineering, Inc. for the site and can be available upon <br />request <br />2.3 EXISTING DRAINAGE <br />As mentioned previously, the existing site is underdeveloped with only a few existing structures on-site and no <br />stormwater infrastructure within a reasonable distance for connection. Runoff currently sheet flows from peak <br />of the hill located just east of Capital Drive to the east and west offsite. The drainage runoff to the west drains <br />onto and across Capital Drive and onto neighboring properties. The runoff to the east sheet flows into the <br />forested Ribbon Trail Property owned and maintained by the City of Eugene. <br />Because there is a lack of sufficient storm infrastructure adjacent to the site and the development falls into a <br />designated headwaters area, all new/additional runoff will need to detained on site and released at flow rates <br />matching the peak discharge rate during the flow control storm event. The City of Eugene's Stormwater <br />Management Manual defines the flow control storm as the 10-year design storm for a basin area less than 40 <br />Branch Engineering, Inc. <br />1 <br />