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8. RUN-OFF TREATMENT <br /> As noted before this project proposes to install less than 5,000 sf of New PGHS, and <br /> increase the total amount of PGHS on-site to approximately 1,010 sf. <br /> Per 2019 DOE SWMMWW"Minimum Requirements 6: Runoff Treatment— Requirements" <br /> a project is required to provide runoff treatment if: <br /> (a) Projects in which the total effective, new and/or replaced pollution-generating <br /> impervious surface (PGIS) is 5,000 square feet or more in a threshold discharge area of <br /> the project; or <br /> (b) Projects in which the total of pollution-generating pervious surface (PGPS) is three- <br /> quarters of an acre or more in a threshold discharge area, and from which there is a <br /> surface discharge into a natural or man-made conveyance system from the site. <br /> Therefore, this project is exempt from this requirement. <br /> 9. FLOW CONTROL <br /> The site design for the new improvements is for 2,702 sf of new/replaced impervious <br /> surfaces. However, since this project is directly adjacent to Puget Sound, the collection <br /> and conveyance of the site runoff to the base of the bluff within the BNSF R/W which is <br /> collected by a rock lined ditch system and a concrete culvert that drains directly into Puget <br /> Sound, the direct discharge of the runoff meets the City Flow Control Requirements. <br /> a. Conveyance Calculations <br /> Under the design review for the adjacent SP the City of Everett Standards were <br /> followed and the rational method was used to calculate the peak 100-year flow rate <br /> for all of the proposed impervious surfaces directed to the piped conveyance <br /> system the outfalls at the base of the bluff to the north. This included the proposed <br /> short plat along with this lot and the lot to the southwest. The original design <br /> accommodated a total of 0.97 ac of hard surfaces. As noted in Section 7 of this <br /> report the conveyance system currently collects runoff from 0.48 ac of impervious <br /> surfaces including this parcel the existing access to the south and the parcel <br /> adjacent to the west. <br /> b. New Improvements Under This Permit <br /> For the new garage and additional driveway this permit application is proposing <br /> 0.06 ac of additional impervious surfaces to be directed to the existing conveyance <br /> system. That would increase the total for the conveyance system to 0.54 ac. This <br /> will leave 0.43 ac of impervious surface capacity in the existing system as it was <br /> as-built and approved by the city. <br /> Langus Garage 15-0403-L <br /> March,2022 Page 10 <br />