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• <br /> /N%/41.:', Anw PUBLIC WORKS <br /> Al 'ETT <br /> MEMORANDUM <br /> TO: Heather Griffin, P.E. <br /> FROM: Erik Emerson, P.E. <br /> DATE: January 14, 2019 <br /> RE: Zerzura Building (PW 1802-030) <br /> 17025 19th Ave SE <br /> Adjustment to Storm Water Standards <br /> CC: Project File <br /> The applicant proposes an alternate flow control structure configuration from those included in the <br /> Ecology Stormwater Management Manual for Western Washington, 2014 edition, and the City's <br /> Design and Construction Standards and Specifications. The proposed flow control riser will be <br /> similar to those included in the standards except that a rock filled"strainer"will be placed below and <br /> around the bottom orifice as shown in the attached plans. The strainer will protect an approximately <br /> 1/4 inch orifice,which is smaller than the %2 inch orifice normally allowed in a standard flow control <br /> riser. The applicant has proposed this configuration as an experimental approach and agreed to more <br /> intensive inspection,maintenance, and documentation requirements than would normally occur. <br /> Adjustment Requested.Allow the proposed flow control riser configuration as equivalent to a <br /> conventional flow control riser for this experimental installation. <br /> Findings of Fact <br /> • The applicant's engineer has indicated that a detention system meeting the flow control <br /> standard is not feasible without major revisions of the site plan. Reduction of depth in order <br /> to meet the flow control standard with a 1/2 inch minimum orifice size requires increasing the <br /> footprint of storage area. The footprint is constrained by the limited space within the drive <br /> aisle between buildings and the need to install other utilities within the same space. See the <br /> attached Plans. I have explored a number of different options for alternate configurations <br /> with the engineer without finding one which can achieve the standard within the space <br /> available. <br /> • The proposed revision is hydraulically consistent with the standard design and easily <br /> modeled using the standard WWHIVI routines. The only deviation from the standard is the <br /> size of the orifice. The minimum orifice size is a maintenance consideration, not a hydraulic <br /> one. The proposed alternative design aims to prevent plugging of the smaller orifice using the <br /> proposed strainer device. <br /> C:\Users\hgriffin\AppData\Local\Microsoft\Windows\INetCache\Content.Outlook\RSNKD6WM\Zerzura-Stormwater Standards <br /> Adjustment.doc <br />