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<br />Determination of the depth of scour at these sites required the definition <br />of the reference plane. The reference plane was selected as the highest thal- <br />weg elevation found at the cross section during the period over which a given <br />site was studied (fig. 12). The depth of scour at any given time is defined as <br />the distance between the reference plane and the lowest point in the cross <br />section at the time of the survey. <br /> <br />The data in table 15 indicate that the mean depth of scour for the sample <br />of 21 sites is about 3.1 ft (0.95 m). The relationship of mean depth of scour <br />to size of bed material (fig. 13) for 21 sites indicates that mean scour depths <br />range from about 4 ft (1.2 m) for sand beds to about 2 ft (0.6 m) for gravel <br />and cobble bed channels. In addition to sampling'errors, this relationship is <br />influenced by many of the factors that affect the amount of bed scour at a <br />site, such as size of bed material, size distribution (which may cause armoring <br />of the surface material), channel curvature, changes in channel size, variation <br />in suspended-sediment concentrations, bedrock or presence of hardpan, the <br />effects of debris and channel bed irregularities, and shear stress. <br /> <br /> <br /> <br />CHANNEL BED <br /> <br /> <br />g <br /> <br />HIGHEST <br />THALWEG <br />ELEVATION = <br />REFERENCE <br />PLANE <br /> <br />......>-- ". <br />/' -- <br />CHANNEL BED AT TIME X <br /> <br />DEPTH OF SCOUR, d., AT TIME X <br /> <br />nGURE 12. Definition sketch of channel bed scour. <br /> <br />on <br />