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City Building Inspection Department <br />September 12, 1979 <br />Page 4 <br />RETENTION/DETENTION SYSTEM #3 at GRAPHIC ARTS BUILDING <br />poet Development Areas <br />8,773 sq.ft. New Roof Area <br />4,085 Existing Roof or Paving C = .95 <br />4,200 Existing Road <br />488 " Existing Sidewalk <br />1,004 New sidewalk C = .7 <br />90o Existing Gravel <br />C .4 <br />4,535 " Grass Effective C .84 <br />23,985 sq.ft. Total <br />Pre Development Areas <br />2,200 sq.ft. Existing Gravel C .7 <br />920 " Existing sidewalk C e ,95 <br />4,205 Existing Roof or Paving <br />4,200 Existing Road 3-C .4 <br />12,460 Grass Effective C .64 <br />23,985 sq.ft. Total <br />Design Storm <br />Pre Development frequency 1 in 10 years <br />Post Development frequency 1 in 25 years <br />Duration 20 minutes <br />Intensity Pre Development 1.3 in./hr. <br />Intensity Post Development 1.6 in./hr. <br />Pre Development Design Runoff: <br />.64(133H 1)23,9135 = .64(866) - 554 CFT <br />Post Development Design Runoff: <br />6 gq( )( 1) 23,985 = .84(1066) = 895 CFT <br />Runoff Differential = 341 CFT <br />Orifice Opening: <br />Diameter = 2-3/4" 23 985 = .q6 CFS <br />Discharge - .64(1.3) �3 36U <br />Head - 4.66 <br />_ 46 __ .46 = —46 = .042 ft. <br />2 <br />A .63 6 .4(4.66) .63(17.32) 10.9 <br />D = 2 `rA = .115(2) = .23- = 2.78" , use. 2-3/4" <br />ff <br />