4 Braewood Hills | KPFF Consulting Engineers STORMWATER ANALYSIS development means pre-European development. The existing runoff calculations for the site can be found in Appendix 2. Water Quality Facilities, Flood Control and Flow Control There are six vegetated treatment facilities on site. The first is a vegetated swale that will treat the public extension of Randy Lane as well as the lots fronting that roadway. This swale is called Swale 1 – Upper on the attached Basin Map. This swale will be constructed under the PEPI and maintained by the City of Eugene. Swale 1 – Lower is adjacent to the existing, public Randy Lane. Swale 1 – Upper will drain into Swale 1 – Lower, so it will serve to provide additional treatment for the upper basins. Swale 1 – Lower will also provide treatment for the lots fronting the public portion of Randy Lane. Swales 2, 4, 5, and 6 are adjacent to Stonehedge Way. They captures and treats the runoff from the lots that are east of Stonehedge as well as the road itself. This swale moves the runoff laterally across the hill, slowing the water and helping to reduce the peak of the runoff from the site. Swale 3 is located at the low end of the site. It treats Stonehedge Way and Stonehedge Court as well as the lots located in the northwest corner of the site. Swale 3 is located at the bottom of the lower detention ponds, so this stormwater facility is providing treatment as well as detention. All swales act as a cut-off for the hill as well as providing treatment. Runoff from the vegetated facilities will be collected open detention ponds. At the end of the piped system, five tiered detention ponds will hold the water at the 2-year and 10-year storms. The runoff from Swale 1 (Upper and Lower), the runoff from Swale 2, and the runoff from Swale 4 enter the detention system at the upper tier. The runoff from Swales 5 and 6 enters the detention system at the lower tiers and at the beginning of Swale 3. Each tier has an undersized piped draining to the next tier that will act as an orifice. Each pipe only has one orifice, which is sized to control the 2-year storm to the correct release rate. This means that the 5-year and 10-year release rate is well under the pre-development conditions. At the 2-year storm, the pre-development rate is 2.28 cfs, so the detained rate must be equal to or less than that rate. The detention basins have orifices with sizes that vary between 8” and 9.5”. The peak flow from the system is 1.19 cfs. See the Proposed Basin Map in Appendix 1 and the Runoff and Detention Output from Storm and Sanitary Analysis (SSA) in Appendix 4 for the orifice sizing. At the 10-year storm, the pre-development rate is 5.19 cfs, so the detained rate must be equal to or less than that rate. The same orifices will be used for the 10-year storm, but the individual basins will detain much more water. They are sized to detain 2’ of water and still have 1’ of freeboard. The post-development rate is 3.05 cfs. See the Proposed Basin Map in Appendix 1 and the Runoff and Detention Output from Storm and Sanitary Analysis (SSA) in Appendix 4 for the orifice sizing. Operations & Maintenance All the facilities described in this report will be publicly maintained facilities. City of Eugene maintenance staff will maintain the pipes, detention ponds, swales and planters per City requirements. See Appendix 4 for the City’s standard Operations and Maintenance Plan.