<br />4 <br /> <br />Braewood Hills | KPFF Consulting Engineers <br />STORMWATER ANALYSIS <br />SLOPES V requirements are applied to this project as part of the Joint Permit Application (JPA) that will be <br />obtained for improvements proposed within the wetlands delineated on the site. SLOPES V requires that the <br />pre-development flow is equal to or greater than the post-development flow for this site. In this case, pre- <br />development means pre-European development. The Existing Runoff Calculations for the site can be found <br />in Appendix 2. <br />Water Quality Facilities, Flood Control and Flow Control <br />There are six vegetated treatment facilities on site. The first is a vegetated swale that will treat the public <br />extension of Randy Lane as well as the lots fronting that roadway. This swale is called Swale 1 – Upper on the <br />attached Basin Map. This swale will be constructed under the PEPI and maintained by the City of Eugene. <br /> <br />Swale 1 – Lower is adjacent to the existing, public Randy Lane. Swale 1 – Upper will drain into Swale 1 – <br />Lower, so it will serve to provide additional treatment for the upper basins. Swale 1 – Lower will also provide <br />treatment for the lots fronting the public portion of Randy Lane. <br /> <br />Swales 2, 4, 5 and 6 are adjacent to Stonehedge Way. They capture and treat the runoff from the lots that <br />are east of Stonehedge as well as the road itself. This swale moves the runoff laterally across the hill, slowing <br />the water and helping to reduce the peak of the runoff from the site. <br /> <br />Swale 3 is located at the low end of the site. It treats Stonehedge Way and Stonehedge Court as well as the <br />lots located in the northwest corner of the site. Swale 3 is located at the bottom of the lower detention <br />ponds, so this stormwater facility provides treatment as well as detention. <br /> <br />All swales act as a cut-off for the hill as well as providing treatment. <br /> <br />Runoff from the vegetated facilities will be collected in open detention ponds. At the end of the piped system, <br />five-tiered detention ponds will hold the water at the 2-year and 10-year storms. The runoff from Swale 1 <br />(Upper and Lower), the runoff from Swale 2 and the runoff from Swale 4 enter the detention system at the <br />upper tier. The runoff from Swales 5 and 6 enters the detention system at the lower tiers and at the beginning <br />of Swale 3. Each tier has an undersized piped draining to the next tier that will act as an orifice. Each tier only <br />has one orifice, which is sized to control the 2-year storm to the correct release rate. <br /> <br />At the 2-year storm, the pre-development rate is 2.50 cfs, so the detained rate must be equal to or less than <br />that rate. The detention basins have orifices with sizes that vary between 7 and 8 inches. The peak flow from <br />the system is 2.43 cfs. See the Proposed Basin Map in Appendix 1 and the Runoff and Detention Output from <br />Storm and Sanitary Analysis (SSA) in Appendix 4 for the orifice sizing. <br /> <br />At the 10-year storm, the flow will be controlled by the orifices and by a weir installed above the orifice, <br />which will allow the water to spill from one tier to the next. The pre-development rate is 5.19 cfs, so the <br />detained rate must be equal to or less than that rate. The post-development rate is 4.96 cfs. See the Proposed <br />Basin Map in Appendix 1 and the Runoff and Detention Output from Storm and Sanitary Analysis (SSA) in <br />Appendix 4 for the orifice sizing. <br /> <br />