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9. FLOW CONTROL <br />The site design for the improvements is for approximately 8,277 sf of new impervious <br />surfaces. However, since this project is directly adjacent to Puget Sound, the collection and <br />conveyance of the site runoff to the base of the bluff within the BNSF R/W which is collected <br />by a rock lined ditch system and culvert that drains directly into Puget Sound, the direct <br />discharge of the runoff meets the City Flow Control Requirements. <br />A. Conveyance Calculations <br />Per the City of Everett Standards the rational method was used to calculate the peak <br />100-year flow rate for the proposed impervious .surfaces directed to the piped <br />conveyance system that outfalls at the base of the bluff to the northwest. <br />The peak Q for the 100-year event was calculated to be 0.59 cfs. Using Manning's <br />equation, a 6" pipe with a minimum slope of 0.64% has the capacity to convey that <br />runoff without any backwater conditions (See Appendix A for '6" HDPE" Manning's <br />spreadsheet). This 6" HDPE pipe will run at a very steep slope down the bluff and <br />will have more than enough capacity to convey site runoff. Therefore, the pipe <br />system northwest of CB #1 will be 6" HDPE pipe to the bottom of.the slope with <br />diffuser tee for energy dissipation. <br />Per DCSS Section 4-2.3(2) private storm drains which receive surface runoff must <br />be 8" min. Roof and foundation drains are exempt from this requirement. Therefore, <br />the mainline conveyance around the south/west side of the house will be 8" PVC. <br />Using Manning's equation, it was calculated that this pipe would have a capacity to <br />convey 1.12 cfs at a minimum 0.5% slope without a backwater condition which is <br />more than enough capacity. (See Appendix A for'8" PVC" Manning's spreadsheet). <br />The other approximately 1/4 half of project runoff (roof.runoff) will enter the system <br />from the northeast side of CB #1 via a 4" PVC downspout connection with a minimum <br />slope of 0.5%. Again, using Manning's equation, it was calculated that this pipe <br />would have a capacity to convey 0.20 cfs without a .backwater condition which is <br />more than % of the total site runoff (See Appendix A for '4" PVC" Manning's <br />spreadsheet). <br />10. WETLAND PROTECTION <br />No downstream wetlands are located between this project and Puget Sound. <br />Mazza SFR <br />November 2019 <br />'Revised. Aprif 2020 <br />19-1014 <br />Page 10 <br />I <br />El <br />n <br />CI! <br />L <br />d <br />1 <br />