Standard Deviation of Bed Elevation as a Measure of Hydraulic Roughness for Gravel-bed Streams.

Hydraulic roughness height is a key parameter for velocity prediction. There are not yet firmconclusions about whether characteristic particle size (D84, D90...) or the standarddeviation of bed elevation is better indicator for the hydraulic roughness of gravel-bedstreams. A data set of 1713 flume experiments and 515 field measurements wasused to test whether or the standard deviation was a better predictor of the streamvelocity, and to explore possible reasons for the difference between the two roughnessmeasures. Tests were performed using 4 well-known flow resistance equations,based on either flow depth or a dimensionless discharge as independent variables.Results showed that the standard deviation of bed elevation was consistently a betterpredictor of the velocity than D84 in terms of the root-mean square error. In addition,Ferguson equation with the standard deviation as hydraulic roughness shows bestoverall performance in velocity prediction. In particular, when using dimensionlessdischarge as the input variable, logarithmic matching equation with the standarddeviation as hydraulic roughness is recommended for its explicit form. The data set wasseparated into two sub-sets with or without typical bed forms or large wood debris.The results showed that the existence of bed forms and large wood debris could bethe reason why the standard deviation outperformed as a measure of hydraulicroughness for velocity predictions. Characteristic size measures the grain roughnessonly, whilst variations in bed elevation combine the effect of surface roughness andthat of some large flow resistance elements such as bed forms and wood debris. [ABSTRACT FROM AUTHOR]