Graham Place Subdivision Stormwater Report July 2026 <br />PAGE 5 OF 6 <br /> <br />The following table lists the peak flow rates for runoff from the existing site and the post-construction peak flow <br />discharging from the development areas: <br />Table 5: Pre- and Post-Construction Peak Flow Rates. <br />Storm Event Existing Peak Flow <br />Rate (cfs) <br />Proposed Peak <br />Flow Rate (cfs) <br />10-Year 0.62 0.52 <br /> <br />As shown in the table above, the post-construction peak flow rate for the 10-year storm is less than the pre- <br />construction peak flow rate of the existing condition, meeting detention requirements outlined in the ESMM. <br />All calculation details can be found in the HydroCAD report as part of this report’s Appendix. <br /> <br />Pollution Control <br />The stormwater from the water-quality storm event will be completely treated prior to discharging into the <br />existing piped storm system routing to the public main, as shown in Table 4 above. The proposed rain garden’s <br />HGL during the water-quality storm is lower than the lowest overflow orifice meaning all runoff will filtrate <br />through the growing medium layer and leave through the underdrain prior to exiting the facility. The open <br />storage bottom of the facility will be planted to rain garden standards to provide adequate treatment. The <br />planting plan will be finalized during the building permit process. <br />Conclusion <br />The proposed stormwater system satisfies flood control and detention requirements by containing runoff <br />through the 10-year storm event to pre-construction flowrates, as shown in Table 5 above. Calculations show <br />the rain garden’s hydraulic grade lines are below the rim of the facility during the flood control design storm <br />meaning it is not expected to flood into existing surrounding infrastructure or residences. Proposed stormwater <br />infrastructure is designed to protect public safety through appropriate sizing and treatment of runoff for water- <br />quality storm events. Pipe inlets into the rain garden will include splash pads that follow the Eugene Stormwater <br />Management Manual parameters to mitigate risks of erosion. See this report’s Appendix for more information <br />on splash pad sizing. <br />Runoff is proposed to be treated through plantings and a growing medium layer in accordance with ESMM <br />treatment requirements. A vegetation plan meeting City of Eugene standards will be finalized during the building <br />permit process. As shown in Table 4 above, all runoff is treated during the water-quality storm event. <br />The lowest proposed overflow grate within the stormwater facility is set at 6” above the flowline, meeting the <br />12” maximum ponding depth requirements for a rain garden as stated in Section 2.3.9 of the Eugene <br />Stormwater Management Manual (2025). A freeboard depth of 12” is proposed, meeting the required <br />2”minimum. <br />The proposed storm system will sufficiently collect and route stormwater from the site such that it is not <br />expected to cause any damage to the proposed development or surrounding infrastructure.