Mobile‐Bed Friction at High Shear Stress

Analysis demonstrates, and experiments confirm that the frictional behavior of mobile beds at high shear stress is inherently different from either of the two classical cases—smooth wall and rough wall. For those two cases, the characteristic lengths for friction are based on viscosity and grain size, respectively. For the new mobile-bed friction relation this length varies with the thickness of the shear layer occupied by the moving bed-load particles. This thickness is itself directly related to shear stress, giving rise to the prediction that at high shear stress (Shields ordinate greater than unity) the effective roughness ratio (effective roughness/particle diameter) should be approximately five times the Shields ordinate. The high-shear-stress experimental results show a variation in accord with the prediction.