Laserfiche WebLink
<br />In alluvial rivers. stage-discharge measurements usually do not <br />plot along a single line, but exhibit a wide scatter of points. This <br />scatter does not represent measurement error but rather distinct <br />trends in the behavior of the river. In positioning a line through <br />the scatter of points, the line should generally be positioned high <br />for levee grade flow lines and low for navigation depth requirements. <br />Other requirements might require other positions of the line. <br />Measured data often do not extend high enough or low enough to <br />bracket the full range of discharges, in which case an extrapolation <br />technique is necessar1. One approach is to plot log-stage versus <br />log-discharge and extrapolate the resulting relationship beyond the <br />experienced range since, often times, rating curves will plot as <br />straight lines or as lines which are piecewise-straight. Another <br />technique is to calculate a representative energy slope from the <br />experienced data and use this slope, cross section information, and <br />n-values, in a uniform flow equation such as the Manning equation <br />to calculate points on the rating curve beyond the range of exper- <br />ienced data. This technique depends a great deal on the representa- <br />tive slope to give reliable results. <br /> <br />Section 4.08. Obtaining Field Data <br /> <br />Traditionally, survey parties have gone into the field to obtain <br /> <br /> <br />the needed geometric data. Levels and taped distances were sometimes <br /> <br /> <br />used. but the alidade, plane table and stadia measured distances were <br /> <br /> <br />more popular since the resulting data could be plotted while being <br /> <br />4.16 <br />