Analysis of Resistance in Fluvial Channel

The variation of friction factor in fluvial channels in terms of the governing principles is examined and related to the bed forms. Laboratory measurements show that local surface drag on a bed covered by bed forms varies from zero at the reattachment region of the surface of discontinuity formed at a crest, to a maximum at the next crest downstream. On exceeding a critical temporal mean value of the shear velocity, the friction factor increases rapidly as ripples form and reaches a peak value where the steepness of the bed forms is a maximum. As the velocity increases, more dunes of decreasing steepness are formed which give place to transition flat bed, for which the friction factor has approximately the initial flat bed value. With the formation of anti-dunes, the friction factor increases again. Plotted as a function of mean velocity or shear velocity the numerical values of the friction factor, f, are different for any given set of flow and sediment parameters. Definite trends appear which indicate a simple presentation of resistance data consistent with physical reasoning.