| Abstract: | Bagnold's theory of bedload stresses and bedforms requires that the tangential stress exerted at the base of a moving bedload dispersion in high stage plane beds is carried entirely by the grain stress T. Derivation of T from experimental data in flow channnels shows, however, that the fluid contribution τ′ is still dominant (≈ 1 to 2 × T). Particle concentrations in the experimental bedload dispersions approached 10 per cent and it is inferred that no appreciable concentration gradient exists. Grain collisions are thought to be most probable whenever more than 1 former stationary grain layer is in forward motion. High stage plane beds may be unstable when grain collisions cease. Current ripples evolve from the amplification and migration of chance defects due to unsteady fluid impulses generated by injection, or burst, phenomena in the viscous sublayer and not through any inherent static shear resistance deficit exerted by stationary bed grains. |
