<br />5 <br /> <br />Braewood Hills | KPFF Consulting Engineers <br />STORMWATER ANALYSIS <br />Runoff from the Adjacent Site <br />All the facilities are sized to provide water quality and detention for the proposed development area, as <br />described in detail above. However, four lots sitting on the west side of Suncrest Avenue drain into a private <br />storm pipe that outfalls at the eastern property line of the development area. This project is not required to <br />provide water quality and detention for the previous development, but infrastructure is proposed that <br />captures the runoff from the adjacent lots and routes it through the proposed detention system. This runoff <br />is already passing through the site, being captured by one of two existing inlets along Hawkins Lane and then <br />being routed to the open creek on the west side of Hawkins. This project does not impact the quantity or <br />destination of that water. <br /> <br />The design team has viewed this water as “pass through,” meaning it currently flows through the site and <br />will continue to flow through the site as the same rate. We do have a version of the stormwater model that <br />addresses this flow. At the 2-year storm, the peak flow from the adjacent lots is 0.67 cfs. Because this passes <br />through the detention system, this peak flow is slightly mitigated and only increases the peak flow in the <br />system from 2.43 to 2.78. <br /> <br />At the 10-year storm, the peak flow from the adjacent lots is 1.15 cfs. This is not mitigated by the detention <br />system; it is simply passed through at 1.15 cfs to the downstream pipe. <br /> <br />See Appendix 6 for modelled results at the 2-year and 10-year storms that include the adjacent lots. <br />Operations & Maintenance <br />All the facilities described in this report will be publicly maintained facilities. City of Eugene maintenance staff <br />will maintain the pipes, detention ponds, swales and planters per City requirements. See Appendix 5 for the <br />City’s Standard Operations and Maintenance Plan. <br />Analysis <br />The water quality facilities have been sized using the Eugene Stormwater Facility Calculator. The calculator <br />determines runoff using the Santa Barbara Urban Hydrograph (SBUH) Method and NRCS 24-Hour Type 1A <br />Hyetograph as outlined in Appendix M of the City of Eugene’s Stormwater Management Manual. The facility <br />capacity is based on the requirements and facility details found in the Eugene Stormwater Management <br />Manual (see Appendix 3 for the Stormwater Facility Calculator Results). <br /> <br />Detention has been calculated using Storm and Sanitary Analysis (SSA). SSA modeled runoff from the <br />developed lots, time of concentration, pipe capacity and detention volumes. <br /> <br />For analysis purposes, each basin of the property required the following information as input data for the <br />computer model: <br />• Impervious Area (Ai), in acres <br />• Curve Number (CNi), impervious <br />• Travel Time (Tc) <br />• Total Precipitation <br /> <br />