8115 BROADWAY 2016-01-01 MF Import

Hill Murray-Ihaina@c Smdy DRAINAGE PROPOSAL Thc proposal includes construction of a twrnstory IQ294 squarc ioot offic�e building and associated parking on the 0.427acre pazcel. This site is subject to re-0evelopment standards. Thcrcforc, for thc purposcs of dctcntion calculations, thc existing conditions wrre assumed to be second growth forest and the existing impervious areas were not taken into account. � Ncw impervious arcas will indude thc building rooftop (5,420 SF), walkways (1,070 SF) and pa��cd parking arca (SJ 10 SF) totaling I 1,600 SF or 0.2G6 acres. Per City of Everett standards, the detcntion system will be dcsibnod to detain thc 2-year, 10-year, and 100- year (24-Hour) devclopcd storm volumcs and relcase at '/� of thc pre-existing 2-ycar, 10- year, and 100-year frcqucncy storm event rates. As required by the City of EverMt, a. � volumc correction factor for streambank erosion control has been applicd to the dcterttion volumc. Approximatcly 62% of this site will bc covered with impe�-vious area, relating to a 36% incrcase in detcntion volume per Table I-3.1 of the Evcrett Sturmwater Management Manual. The additional volumc was achieved by lengthening the 48" detention pipe without adjusting the control strucmre. The detention system will have a � total volume of approximately 1,822 cf. A flow restrictor/oil pollutiun control device � (FROP) will control the discharge flows out of the pipe. Less than 10,000 ft2 of parking and driving surfaccs are proposed for the site. Thcrefore, per City of Evcrett standards, a wetvault and open water ccll have been (, designed to providc water quality for runoff from the paved parking arca. The wet vault portion of the watcr qualiry trcatmr,nt system will wnsist of 20-linear feet of 36" Diamctcr ADS-N 12 and two 54" manholes. Thc wct tank is designed to hold the volume of the 6-month dcvcloped stortn, which is calculated to be 332 CF, within 3 fcet minimum of dcad storage. An additional Open Watcr Ccll will bc provided per Ciry of � Evcrett Requiremcnt ro polish the asphalt runofi prior to discharge to the I-5 drainag•e system. Dctention for the proposal will bc provided in two parallcl 48" ADS-N12HC pipes 70 fect long. Only runotT from thc parking lot will be routed through the wetvault. The � wetvaelt will dischargc to the detention pipcs. Rooftop runoff will be convcyed directly !o the detcntion system. Flows from the detention system will be conveycd to a dispersion trench ncar the northcast comer of thc site, which is the natural drainage path for this sitc. C WESTERN fNGIN[ERS, INC PROIECT:01-21I-A f,