Capital Hill PUD, Tentative (14-020) <br />June 19, 2017 <br />1.0INTRODUCTION <br />This Stormwater Management Plan (SWMP) has been prepared for the Capital Hill PUD located east <br />of Capital Drive in Eugene, Oregon. The new development will create over 30 new single family <br />and multi-family lots by widening and extending Capital Drive, and installing a private road to <br />access a handful of the eastern lots. The new improvements will also include extending all <br />necessary utilities to serve the residential hillside development. <br />This report will display conformance to the updated City of Eugene requirements for stormwater <br />management by examining existing site conditions and analyzing the stormwater management <br />methods for treating and 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 <br />trees, and has elevations varying between 720' and 920'. Most of the new construction and <br />development will be above elevation 800'. The site has several residential structures that are to <br />remain as property lines will be adjusted to produce 33 total lots, leaving the existing buildings <br />on the new lot layout. <br />Slopes on site are generally greater than 8-10% for a majority of the site with many locations <br />exceeding 20% 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 <br />site are Panther silty clay loam, 2 to 12 percent slopes (NRCS Soil #102C), Philomath cobly silty <br />clay, 3 to 12 percent slopes (NRCS Soil 108C), Philomath-Urban land complex 12 to 45 percent <br />slopes (NRCS Soil #109F), and Steiwer 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 <br />Hydrologic Soil Group C. The site soils were determined using the NRCS Web Soil Survey 2.0. A <br />map of the soils at the site can be found in Appendix B. <br />A Geotechnical Assessment was conducted by Branch Engineering, Inc. for the site and can be <br />available upon request. <br />2.3 EXISTING DRAINAGE <br />As mentioned previously, the existing site is underdeveloped with only a few existing structures <br />on-site and no stormwater infrastructure within a reasonable distance for connection. Runoff <br />currently sheet flows from peak of the hill located just east of Capital Drive to the east and west <br />offsite. The drainage runoff to the west drains onto and across Capital Drive and onto neighboring <br />properties. The runoff to the east sheet flows into the forested Ribbon Trail Property owned and <br />maintained by the City of Eugene. <br />Because there is a lack of sufficient storm infrastructure adjacent to the site and the development <br />falls into a designated headwaters area, all new/additional runoff will need to detained on site and <br />released at flow rates matching the peak discharge rate during the flow control storm event. The <br />Branch Engineering, Inc. <br />