Roughness Calibration to Improve Flow Predictions in Coarse‐Bed Streams

Logarithmic and variable-power equations that use the bed D84 grain size as a roughness metric reproduce general trend of flow resistance in streams with coarse beds, but they are unreliable for predictions individual reaches. For site-specific application these equations, I propose an effective height can be calibrated by making single measurement. test this idea using published velocity-depth data eight coarse-bed reaches varied character. In 52 trials (8 × 2 3 or 4 alternative calibration measurements), single-measurement reduced root-mean-square error predicting velocity at all depths up to 79% (median 66%) compared D84. This approach may useful when prescribing environmental flows, estimating bankfull discharge, bedload transport channels which Manning's n is likely vary considerably discharge.