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<br />\ <br />-I, <br />L <br />_,~c~ <br />----~ <br /> <br />: <br /> <br />through a conversion of potential energy. <br />The ratio of inertial to gravitational forces is an important <br />measure of the state of flow, as shown by the dimensionless number <br />known as the Froude number: <br /> <br />F = V <br />{9D <br /> <br />(3-3) <br /> <br />where: <br /> <br />" <br />" <br /> <br />V = velocity <br />D = characteristic length term defined as cross section area <br />divided by water surface width <br />g = acceleration of gravity <br />When flow is at critical depth, the inertial and gravitational <br />forces are equal and F = 1. If F is less than 1. the water depth <br />is above critical and the st~te of flow is subcritica1. The influence <br />of gravity forces dominates the inertial forces; flow has a low velocity <br />and it is often described as tranquil flow. If F is greater than 1, <br />the water depth is below critical, inertial forces dominate the gravity <br />forces, and the flow is described as rapid or shooting. The state <br />of flow is supercritical. Therefore, "state of flow" also refers <br />to the relationship between the flow velocity and a critical velocity. <br />The characteristic length used in computing the Froude number <br />is an average depth for the cross section. This average value. then. <br />defines an average Froude number. It is possible to have point <br />velocities which exceed the average critical velocity and still have <br />flow that is essentially subcritical. This is illustrated when surface <br />waves, generated by tossing a stone into a flowing stream. are swept <br /> <br />3.05 <br />