3429 15TH ST 2018-02-23 - Laserfiche WebLink

generally impervious with small amounts of landscaping and will reduce net impervious areas from pre-developed conditions. While runoff within Basins "A-14" through "A-16" will not be captured and treated, runoff from Basin "B", which lies outside of project limits, will be intercepted and routed through the detention pond as mitigation. The area within Basin "B" is equal to or greater than the untreated portions of the project site resulting in equivalent treatment of project runoff. The majority of Basin "A" will be diverted into the pretreatment vault and detention pond. Runoff from Basin "A-14" will flow into the existing ditch along the east side of the project within BNSF property. This area will be developed from a primarily impervious condition into a landscaped area, reducing post-construction runoff into the existing ditch from Basin "A-14". Runoff within Basin "A-15" currently flows into the existing ditch to the northwest of the site. Runoff from this basin, post-construction, will be captured and diverted into this existing ditch that lies within BNSF property. This basin is not being routed through the ` pre-treatment vault or pond to avoid a basin diversion. Basin "A-16" consists of a portion of 15th Street within the project limits that currently drains into the existing public conveyance system. Post-construction, this basin will continue to drain into the public system. No additional runoff will be produced by new construction within Basins "A-15" and "A-16" as the impervious surface area within these basins will not increase post-construction. Runoff from the detention pond will be directed into the ditch east of the site. Through a series of ditches and pipes across the property runoff eventually flows into the Snohomish River. See the Post-Construction Drainage Basin Map in Appendix B for a pervious and impervious area summary. Pre-Treatment and Detention Pond Design Per the City of Everett Storm Water Management Manual(2010) and the Storm Water Management Manual of Western Washington the water quality flow capacity of the pre- treatment vault was determined by using the 2012 Western Washington Hydrology Model( WWHM). A CDS Stormceptor unit was selected as the pretreatment device. The WWHM2012 printout for the pretreatment device is included in Appendix "D". WWHM2012 was used in the design of the detention pond. The pre-developed scenario was modeled as pervious to simulate undeveloped land. Post-developed conditions were modeled as pervious and impervious as designed. March 05, 2015 ' Hydrology Report — BNSF Everett Yard Office Page 7 of 8