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MAY u r 1986 <br />CITY OF EVERETT <br />Pubic N'crks Dcp;. <br />Reid, Middleton & Associates, Inc. <br />Engineers • Surveyors • Planners <br />May 4, 1986 Y <br />File No. 02-86-018 <br />STORE^1ATER DETENTION CALCULATIONS FOR , <br />EVERETT AMO MALL {Sv PC C' Ce <br />PIN. SECTION 18, T. 28 N., R. 5 E., W.M. <br />EVERETT, WAMINGTON b �/� ¢ 3 <br />Existing Site Conditions r� <br />Cc � <br />The existing property is partially developed with an asphalt parking lot / <br />and steel frame building as well as a single family wood frame house, <br />detached garage and gravel drive. The undeveloped portion of the site is <br />wooded. <br />Area of property = 4.438 Acres of which 0.713 Acres are impervious. <br />Prorating the existing runoff coefficient: <br />One half the wooded area slopes exceed 58 = 1.862 Ac @ 0.10 <br />Retraining wooded area slopes are less than 58 = 1.863 Ac @ 0.05 <br />(0.713)(0 90) + (1 863)(0 05) + (1.862)(0.10) <br />4.438 Ac = 0.21 <br />Tr = 360/0.45(60)+ 7/5.3(60)+ 109/3.5(60)+ 11/4.7(60)+ 50/1.8(60)+ 10 min. <br />= 24.37 min. <br />Using 10yr. frequency Everett Curve Storm To = 24.4 min. = i @ 1.15 in/hr., <br />Q allowable = ciA = 0.21 (1.15) 4.438 = 1.07 c.f.s. <br />Using the Yrjanianen and Warren Method for simple detention system design <br />the following calculations are to determine the detention volume for this <br />project: <br />Prorating future runoff coefficient <br />Paved areas and buildings = 179,989.92 sq.ft. = 4.132 Ac @ 0.9 <br />Landscaped areas = 13,347.89 sq.ft. = 0.306 Ac @ 0.2 <br />M <br />0.85 = Fut C " <br />m Qo = 1.07 <br />0.85 (4.438) = 0.2836 <br />W F TE ' <br />f��NA( <br />121 5th Avenue North, Suite 200, Edmonds, Washington 98020 (206) 775.3434 <br />