Memorandum <br />Page 3 <br />March 5, 2024 <br /> <br /> <br />Plan will be deficient at the 10-year storm when the basin has been fully developed. Therefore, the whole <br />site is subject to flood control requirements. Although water quality is being addressed by each lot <br />individually, the on-site detention system is sized to accommodate the full development. <br /> <br />Flood control at this site requires that post-development peak flows do not exceed existing peak flows at the <br />10-year storm event. The existing flows calculated only include the areas of private Piculell Way and private <br />Randy Lane. See Exhibit 1 in Appendix 2 for the existing basin areas studied and Appendix 3 for existing runoff <br />rates. The overall existing runoff rate is 5.19 cfs at the 10-year storm. <br /> <br />As noted above, infiltration on site is not possible, so the peak rate of runoff will be controlled through <br />detention instead of retention. Detention will be provided in underground pipes within the private <br />conveyance system so that the release rate for the post-development 10-year storm does not exceed the <br />pre-development runoff. Both water quality facilities will flow into the same detention system. <br /> <br />The peak outflow from the site is equal to the peak outflow from the 2nd detention pipe (Det-Pipe-2) included <br />in Appendix 4. That peak flow is 4.38 cfs, though that number may change slightly in the final design. The <br />final proposed runoff number will not exceed 5.19 cfs. <br />Flow Control <br />Flow control is required when the site discharges directly into a headwater stream or connects to a main that <br />discharges into a headwater stream. Discharge for the project will be via 12-inch mains that will eventually <br />discharge into Videra Creek, which is a headwater stream. <br /> <br />Flow control requirements for this site match flood control requirements, so the 10 -year storm peak runoff <br />rate from the proposed development must be equal to or less than the existing runoff rate at the 10-year <br />storm. See the flood control section for more information. <br />Source Control and Oil Control <br />Source control is required for facilities that are highly likely to generate pollutants that are not addressed <br />through the implementation of vegetated stormwater facilities. None of these higher-risk facilities are <br />proposed as part of this development, so source control is not required. <br /> <br />Oil control standards are triggered with large parking lots, vehicle storage, or a high number of additional <br />daily trips. None of these triggers apply to the proposed development, so oil control is not required. <br /> <br /> <br />Attachments: Appendix 1 – Basin Maps <br /> Appendix 2 – Existing Runoff Calculations <br /> Appendix 3 – Water Quality Runoff Calculations <br /> Appendix 4 – Detention Calculations <br />Appendix 5 – Typical Vegetated Stormwater Facility Section <br />Appendix 6 – Draft Operations & Maintenance Plan <br /> <br /> <br />2100216-kg