Flow Resistance in Coarse Gravel Bed Rivers

Flow resistance relations are deduced from data on coarse gravel bed reaches of New Zealand rivers. Measurements of hydraulic variables were made during reasonably uniform subcritical flows at cross sections regular in confirmation. Channel bed and bank roughness are not distinguished, and surface bedmaterial is characterized by the median size. When the channel boundary is rigid the Darcy-Weisbach friction factor is only a statistically significant function of the ratio of hydraulic radius to median size of surface bedmaterial. A model of this form explained 60 percent of the friction factor variance inherent in the field data set. With active bedload transport conditions and changing bed topography, the Froude Number based on median surface bedmaterial size was statistically significant, and 40 percent of friction factor variance was explained by this latter model.