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Design Criteria: <br /> Our Understanding is that detention is not an issuc for thc west sub-basin due to thc <br /> existence of the West Detention Pond and the Powder Mill Gulch Storm Detention <br /> � � Facilities. A copy of the srorm water detention calculations for the Wes[ Detention Pond <br /> have been included in this report as Appendix A. <br /> � All storm water conveyance systems are to be designed at a 25 year design stonn using the <br /> City of Everett rainfall intensity duration curve. <br /> Storm watcr runoff in excess of the 2 year design storm e��ent are ro bypass directly ro thc <br /> West Detention Pond. <br /> Storm a�ater runoff up to the 2 year developed design sronn are to be routecl to thc <br /> exisUng storm drainage channel adjacent to the northe.ist corner of the proposeA p�irking <br /> lot. <br /> Design Methodology/Procedure: <br /> Since detention of the smrm aater from this Nonh Parking Lot 6rpansion is not an issue, <br /> only piping system conveyance and capacity tables will be generated. Specifically a 2 �ear <br /> and 25 year storm system analysis N�ill be prepared and the orifice calculatioas fcr the 2 <br /> year storm b�Pass ro the existing drainage channel ro the V1'est Detention Pond. <br /> See Tables 1, 2, and 3 for pipe conveyance system analysis. <br /> 2 Year Uesien Storm Bvoass Calculations to West Detention Pond <br /> T'he 2 yr. storm, developed conditions, peak runoff rate = (i.38 c.Lti. (Sec Table 1) <br /> The 25 �T. stomi, de��eloped conditions, peak runoff rate = 13.35 c.Ls.(Sce Table 2) <br /> Using the orifice equation Q = cdA 2gh Assuming H = 3' soh�e for A <br /> 0.62 (A) 64.4(3) = 6.38 c.f.s. A = 0.7409 sy.ft. <br /> I 1`'"" ori5ce = 0.7341 sq.h.; 11"'" orifice = 0.7530 sy.ft. <br /> Use I1°�"' orifice <br /> Check Oow in 18" pipe to n1anhole 3. If d = I.11 frct, ;issuming that the 2 ��e,ir <br /> bypass pipe is set 3 feet oelow the inven of �he 25 year bypass. Using the Critical <br /> Depth Chart prepared for the USDA SCS. (See page With d/D = 0.67, Q/D`n = <br /> 2.45 therefore Q = 2.45(1.SsR) = 6J51 c.f.s. < Q:, bypass = 6.97 c.fs. There`ore <br /> d/D needs to be 0.6R ahich equates to 1.112' of rritiral depth. <br /> 3 <br />