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10. Even though internal agreement may be obtained within the <br />computations, the computed water surface elevations may not <br />agree with those measured in the field. In this case, the value <br />of Manning's n is changed, and the process repeated until the <br />energy-balanced water surface elevations "calibrate" with the <br />observed water surface elevations. <br />11. Once calibration is achieved, Manning's n is assumed constant, <br />and the flow profile computed for other discharges of interest. <br />If a range of flows very different from the field measured flows are <br />of interest, the- stage--discharge= approach discussed in the following <br />section is most appropriate. <br />DIRECT DETERMINATION OF STAGE-OISCHARGE RELATIONSHIP <br />The most accurate method of obtaining a relationship between stage <br />and discharge is to measure the discharge at various stages and to develop <br />an equation relating discharge to stage. <br />A stage-discharge relationship is influenced by a number of channel <br />factors such as cross-sectional area, shape, slope, and roughness. The <br />interaction of these factors "control" the stage-discharge relationship. <br />If the stage-discharge relationship does not change with time, the control <br />is stable and can be used without adjustment for changes over time. <br />J <br />The stage-discharge equation is of the form: <br />where, <br />Q = a (s-ZF) b (10) <br />Q = discharge <br />s = stage <br />ZF = point of zero flow <br />a and b = constants derived from measured values of discharge <br />and stage. <br /> <br />lZq