Roughness Calibration to Improve Flow Predictions in Coarse-Bed Streams

Abstract

Logarithmic 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.