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<br />';. <br /> <br />Rock Rapids, Iowa <br />!nternal Drainage Evaluation <br /> <br />Location:Area to west of the fairgrounds bounded by S. 2nd Avenue to the North, the Rail road track to the West, S. <br />8th Avenue to the South, and Tama Street and the levee near alignment #1 to the East Used 1971 topognphic <br />contour map with 5 foot contour intervals of Rock Rapids, Iowa, for basin delineation. <br /> <br />Methodology: Rational Method <br /> <br />Q=CiA <br />where: Q = Discharge (cubic feet per second. cfs) <br />C = Runoff Coefficient (unitless) <br />i = Rainfall Intensity (inIhr) for Time ofConcentratioo (tJ <br />A = Drainage area (acres) <br /> <br />Time of Concentration (tJ determined with SCS Average Velocity charts <br />1 L <br />(,=-:L- <br />60 V <br />where: l" = Time of Concentration (minutes) <br />L = Length of longest water course in feet ( L = 2900 feet). <br />V = Average velocity in feet per second (fps) oflR 55 for various surfaces given a <br />particular slope (S = 0.5%, paved surface). Velocity ranges from 0 to 8.5 fps given a <br />slope from 0 - 3%. (Use table 5.7.1 in Aoolietl HvdrDlof)/_ Chow, 1988). Straight line <br />inteIpOlation for slope of 0.5% yields a velocity of approximately 1.4 fps. <br /> <br />I 2900 <br />(, = - . - = 345 minutes <br />60 1.4 <br /> <br />Runoff Coefficient C = 0.47 <br />Use runoff coefficient table 15.1.1 in Aoolied HvdrD/~ (Chow, 1988) <br />100 year event <br />Poor condition (grass cover less than 50% of the area) <br />Developed <br />Flat (0 . 2% slope) <br /> <br />Rainfall Intensity: i = 4.8 inIhr <br />Use RainfaIIlntensity-Duration-Frequency Curve for Sioux City, Iowa. <br />100 year return period <br />Time of Concentration = 34.5 minutes <br /> <br />Drainage Area: A = 40 acres <br />Use Topographic contour map of Rock Rapids, Iowa. May, 1971 <br /> <br />Q = CiA = (0.47)(4.8)(40) = 90 cfs <br />