3900 BROADWAY BASE FILE 2016-01-01 MF Import

� 4.1 Treatment Control System Water qualiry treatment is not required by the City of Everett. Treatrnent from proposed pollution-generating impervious and pervious surfaces will occur at the sewage treatment plant, To improve the hydrologic rnnditions of the site, including reducing peak Flow rates, increasing evapotranspiration, and providing treatment of pollutants, a portion of the stormwater runoff from impervious surFaces will be routed to bio-retention facilities(rain gardens), which will use compost amended soils to provide treatment and stormwater runoff retention. These facilities are located along the project site frontage. 4.2 Flow Control System Detention pipe is proposed in two separate locations at the projed site. Western Washington Hydraulic Modei v.3 (WWHM3) is a conti�uous simulation model which was used to size the detention facilities for each location. As both locations are tributary to[he same combined sewer main, and are both part of the same development project, the system was modeled wi[h a single point of compliance. When analyzing botti systems together, the deveioped peak Flow retes are identified as lower[han the pre-developerJ; additionaliy the duration requirement is met from 50 percent of the 2-year recurrence intervaf up to the 100-year, which exceeds the City of Everett requirement. Calcutations for the detention pipe can be found in Appendix 8, Exhibit B-1. Table 1: Detention Pipe Summary i Basin i DetenHon Pipe i Required Length of Pipe Basin 1 I 6.0•foot diame[er � 350 linear feet Basin 2 � 6.0-foot diameter ', 450 linear feet i 4.3 Conveyance System Analysis and Design 4.3,1 Onsite Conveyance System The Ciry of Everett requires that all storm and sewer utility tines are separated onsite. ThereFore, the project site conveyance system consistr of 6-inch, 8•inch, and 12-inch diameter closed drainage pipe systems, which conned to a 30-inch sewer pipe in the Memorial Stadium parking � lot. See Appendix A, Exhibit A-4 for the projed site Conveyance Pipe Map. ' The pipe system was checked for capacity and minimum velocity for the 25-year, 24•hour storm. The Rational Method was used in accordance with the City of Everelt Sformwater Managemen[ Manual. For locations downstream of the detention facilities, the 25-year mitigated and the 100- year unmitigated peak Flow rates were checked for pipe capacity. Conveyance calculations can he found in Appendix B, Exhibit B-2. 4.3.2 Offsite Conveyance System The City of Everett has reviewed and accepted preliminary Aow control caiculations for the proposed project, which demonstrate that the project wili not increase peak flow rates for the Stormwater Site Plan Report O Q O Q Eve,ett School Dishict . Communiry Resource Center 4 208387.10