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Verizon Communications-Stormwater Site Plan Everett,WA <br /> Vwq = 28,629 CF <br /> Presettling Volume = 30% x VWq = 0.30 x 28, 629 CF = 8,589 CF <br /> The following calculation is per the Everett Stormwater Management Manual, Chapter 3 <br /> section 3-3.6.1, Wetvault/Tank sizing. <br /> V=1676.5 x (8,589 CF/ 86,400 Seconds) x 8' Depth = 1,000 CF <br /> 1,000/6' Depth = 167 SF <br /> 167/ 10' Wide = 16.7' Length <br /> A 10`x20'x6' pre-settling vault will provide 1,200 CF of storage and will be more than <br /> adequate to provide sufficient presettling for the proposed 6-month, 24-hour stormwater <br /> flows. A dividing wall will run lengthwise down the middle of the vault in order to <br /> lengthen the flow path and satisfy the requirement for a minimum flowpath length of 3:1. <br /> As an alternative to the proposed presettling vault, the presettling requirement is also <br /> allowed to be met through the use of any other device approved by the Washington State <br /> Department of Ecology with a General Use Level Designation. <br /> A bypass structure will be installed upstream of the pre-settling vault per the City of Everett <br /> Stormwater Management Manual, Chapter 3 section 3.6.1, "Flows greater than the peak <br /> flow for the water quality design storm (the 6-month, 24-hour event) should bypass the <br /> facility." The structure shall be placed upstream of the proposed pre-settling chamber. A <br /> down-turned elbow with an orifice of 4" will allow the 6-month, 24-hour event to be <br /> conveyed to the water quality facilities, and a higher outlet elevation will provide a bypass <br /> route for the larger start events. <br /> Conveyance <br /> An on-site conveyance analysis will be performed when construction documents are <br /> prepared for the design of the site infrastructure. <br /> 100-Year Flood/Overflow Condition <br /> The stormwater system for this project has been designed to address all storm events, up to <br /> the 100-year, 24-hour event, in accordance with the City of Everett design criteria <br /> described in previous sections of this report. In the event of a larger storm, the primary <br /> system may fail. In that event, stormwater would initially directly discharge to the <br /> downstream conveyance system through the bypass structure. <br /> As flows increase, we anticipate that an overflow of the on-site stormwater system <br /> conveyance would allow surface water to discharge directly to the drainage system within <br /> West Casino Road. <br /> PACLAND Job#50183001 Page 24 <br />