Abstract
AbstractLogarithmic and variable‐power equations that use the bed D84 grain size as a roughness metric reproduce the general trend of flow resistance in streams with coarse beds, but they are unreliable for predictions in individual reaches. For site‐specific application of these equations, I propose that an effective roughness height can be calibrated by making a single flow measurement. I test this idea using published velocity‐depth data for eight coarse‐bed reaches of varied character. In 52 trials (8 reaches × 2 equations × 3 or 4 alternative calibration measurements), single‐measurement calibration reduced the root‐mean‐square error in predicting velocity at all depths by up to 79% (median 66%) compared to using D84. This approach may be useful when prescribing environmental flows, estimating bankfull discharge, or predicting bedload transport in coarse‐bed channels in which Manning's n is likely to vary considerably with discharge.
Highlights
Predictions of how velocity and discharge vary with flow depth in a stream or river are required for a wide variety of scientific and practical purposes
Bedload predictions may be improved by knowing the total flow resistance as well as the flow depth (Schneider, Rickenmann, Turowski, Bunte, & Kirchner, 2015)
In which Cf and f are the nondimensional Chézy and Darcy-Weisbach resistance coefficients, g is the gravity acceleration, k is a bed roughness height, R/k is the relative submergence of characteristic roughness elements, and n ∝ k1/6
Summary
Predictions of how velocity and discharge vary with flow depth in a stream or river are required for a wide variety of scientific and practical purposes. In which Cf and f are the nondimensional Chézy and Darcy-Weisbach resistance coefficients, g is the gravity acceleration, k is a bed roughness height, R/k is the relative submergence of characteristic roughness elements, and n ∝ k1/6. This 1/6-power relation is suitable for many rivers, but in small coarse-bed streams it nearly always gives a poorer fit to measurements than is obtained using either of two other relative-submergence equations: logarithmic and variable-power. The sand-roughness experiments of Nikuradse (1933) suggest that for relatively deep flow over a plane bed of uniform sediment the roughness height k is equal to the grain diameter D, but in coarse-bed rivers the median grain diameter (D50)
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