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SECTION 2: TRAPEZOIDAL AND CIRCULAR CHANNEL ANALYSIS PROGRAMS <br /> Depth of flow in a circular channel, except in cases in which the Normal Depth <br /> is above about 88% of the channel diameter. In such cases, the program may <br /> complete the maximum 30 iterations without reaching the required accuracy. <br /> 2.4.3.2 Flow Velocity at Normal Depth <br /> After the program computes the Normal Depth, the flow velocity in the channel <br /> is computed as simply the flow rate divided by the cross-sectional area of the <br /> flow. The velocity is assumed to be constant throughout the cross-section. <br /> The flow velocity is an important consideration in many channel design <br /> situations. The allowable flow velocity may be limited by local drainage <br /> criteria. <br /> 2.4.3.3 Froude Number at Normal Depth <br /> The Froude Number is the ratio of the inertial forces to the gravitational <br /> forces in a flowing fluid. It is computed using this formula: <br /> V <br /> Froude Number - (1/2) <br /> (PA/T) <br /> in which: <br /> V - Flow Velocity (fps) <br /> j g - Acceleration due to gravity - 32.2 feet/sec/sec <br /> A - Cross-sectional Area of Flow (square feet) <br /> T - Top Width of Flow (feet) <br /> If the Froude Number is greater than one (1.00) , then flow in the channel is <br /> "super-critical" . A Froude Number less than one is more common, indicating <br /> "sub-critical" flow. Section 2.3 of this manual contains more information on <br /> critical flow and Critical Depth. <br /> 2.4.3.4 Velocity Head at Normal Depth <br /> Water flowing in an open channel contains two major types of energy: potential <br /> energy and kinetic energy. Potential energy is expressed as the elevation of <br /> the water surface. Kinetic energy is expressed as the "velocity head" . The <br /> term "head" can also be stated as "energy level" . <br /> The velocity head is computed using the following formula: <br /> 2 <br /> Velocity Head - V <br /> 2g <br /> in which: <br /> V - Flow Velocity in the channel (fps) <br /> g - Acceleration due to gravity - 32.2 feet/sec/sec <br /> DODSON & ASSOCIATES, INC. THE DODSON HYDRAULICS LIBRARY, PAGE 17 <br />