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If only two sets of stage-discharge or velocity-discharge data are <br />available, selection of data pairs for construction of the two-point <br />rating curve should follow the following guidelines: <br />1. The higher discharge measurement should be at least twice as <br />high as the lower discharge measured. <br />2. If the slope of the log-stage versus log-discharge curve is <br />less than 1.4, or greater than 4.0, a third point should be <br />added by adding the estimated point of zero flow. <br />3_ -Hydraulic predictions-should not be-made for flows-which art <br />less than 0.77 times the minimum measured flow, nor for flows <br />higher than 1.3 times the maximum measured flow. <br />If the Manning equation is used, the gradually varied flow pro- <br />cedure is preferred to the assumption of uniform flow in the prediction <br />of the stage-discharge relationship. When using the Manning equation to <br />determine the velocity distribution, it should be calibrated from <br />velocity measurements made across each cross section. The useful range <br />of extrapolations, using the Manning equation with one set of calibra- <br />tion measurements is from 0.4 to 2.5 times the calibration discharge. <br />Table 5 lists the recommended approaches and useful range of flow <br />extrapolations for different numbers of input data points. <br />Table 5. Recommended approaches using alternative numbers of <br />stage-discharge and velocity-discharge measurements. <br />Number or <br />Measurements <br />Aoproach Useful Range of Extra <br />olation <br />1 Manning Equation 0.4 to 2.5 times cali- <br /> bration discharge <br />2 Two-point rating 0.77 times the minimum <br /> system, refined discharge measured, to <br /> data (see text) 1.30 times the maximum <br /> discharge measured. <br />3 Rating Curve 0.4 times the minimum <br /> system discharge measured, to <br /> 2.5 times the maximum. <br /> discharge measured. <br /> SITE. SELECTION CONSIDERATIONS <br />In order for a simulation model to be useful in the planning process, <br />its output must be capable of extrapolation into space and into time. <br />Extensions into time were discussed in the Introduction. The study area <br />41/