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<br />The average standard error of estimate of equation 10 Is 28 percent <br /> <br />and ranges from -24 to +32 percent for the data in table 1. Equation 10 <br /> <br />data in table I. <br /> <br />in terms <br /> <br /> <br />was used to predict n for the sites, and the percent deviation of com- <br /> <br />puted from observed values is also shown in table I. The algebraic mean <br /> <br />of percentage differences was 5.8 and ranged from -44 to 123 percent, <br /> <br />indicating that equation 10 tends to sl ightly overestimate n. The <br /> <br />standard deviation of the percentage differences was 31 percent. A <br /> <br />~& <br />) <br />--, <br /> <br />scatter diagram of predicted and observed Manning roughness coefficients <br /> <br />is shown in figure 6. The values having the greatest scatter are typi- <br /> <br />cally low-flow measurements. The validity of. the Manning equation at <br /> <br />low flows may be subject to question. However, the scatter increases <br /> <br />when the ratio of <br /> <br />R to d50 (relative smoothness) is less than 7 <br />~..... e~,,~...+- <br />Roughness-prediction equationsAwere deveToped <br /> <br />of relative smoothness. However, the standard error of estimate for <br /> <br />fo r the <br /> <br />equation 4 was considerably higher and did not fit the data as well as <br /> <br />equation 10. <br /> <br />Data from Barnes (1967) and Limerinos (1970) were used to determine <br /> <br /> <br />if equation 10 produced reasonable results and to det~mine the equa- <br /> <br />tion's range of applicabil ity. These data are based on 59 observations <br /> <br />of n in which slopes are greater than 0.002 and the hydraulic radii are <br /> <br />less than 7 ft (2.1 meters). The algebraic mean of the percentage <br /> <br />differences in the results was -7.8. The standard deviation of the <br /> <br />percentage differences was 23 percent and ranged from -44 to +50 percent. <br /> <br />3S- <br /> <br />