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<br />1)3\}19l <br /> <br />20 <br /> <br />FLOW CHARACTER <br /> <br />The slope of the Arkansas River above Canon City averages about one per- <br />cent, and under typical artificial channel conditions this would create super- <br />critical flow conditions, that is, flow having a Froude number of greater than <br />1.0. Nature, however, nearly always limits natural stream flow to critical <br />velocities or less by creating compensating channel roughness which may consist <br />of nu~rou~ boulders, riffles and pools, a meandering channel, and other features. <br />The Arkansas River above Canon City has all of these. During low flow the most <br />significant is the general series of riffles and pools. Fifty percent distri- <br />bution between riffles and pools is commOn in mountain streams, and the Arkansas <br />River is no exception, as indicated by a study of aerial photographs and ground <br />observations. <br /> <br />An increasing river stage associated with an increasing discharge begins <br />to drown out the pools, and when the pools are drowned, the actual overall <br />channel roughness comes into playas the primary natural feature limiting <br />velocity. However, reaches of rapids and slower water sti 11 generally exist. <br />In essence, the effect of the riffles and pools is dampened as the flow begins <br />to assume a more uniform velocity. <br /> <br />A useful index for open channel hydraulics is the Froude number, F, <br />defined as <br /> <br />F V <br />=~ <br /> <br />where V is the mean velocity of flow in feet per second <br />g is the acceleration of gravity in ft. per second squared, and <br />L is a characteristic length. <br /> <br />In case of river hydraulics the l is generally <br />which is equal to the cross-sectional area, A, <br />direction of flow divided by the top width ,T. <br /> <br />F V <br />=Y;T <br />A <br /> <br />taken as th~ hydraulic <br />of the water normal to <br />Specifically for river <br /> <br />depth D, <br />the <br />channels <br /> <br />The Froude number represents the ratio of inertial forces to gravity forces <br />on the flow of water. As inertial forces become more important, the Froude num- <br />ber approaches 1.0. When inertial becomes the overriding force in moving the <br />water, the Froude number becomes larger than 1.0 and the flow becomes super- <br />critical. <br /> <br />Because natural river flow rarely exceeds a Froude number of 1.0, rough <br />estimates of velocity can often be made by the field observer using the formula <br /> <br />Vc = 5.67 '"VT/A <br /> <br />where V = the critical flow velocity where F = 1.0. <br />c <br />