Choice of percentiles and axes to determine grain resistance

Hydraulic and grain-size data were collected from 20 river sites to compare the ability of different percentiles to act as predictors of channel resistance. Grain sizes between the 91st and 100th percentile gave stronger correlations with roughness height than those between the 81st and 90th percentile. Contrary to previous research, it is suggested that a measure of the short axis of the grains gives significantly better estimates of resistance than the long or intermediate axes.