INCIPIENT MOTION OF SEDIMENT PARTICLES OVER FIXED BEDS

The paper analyses experimental data on incipient motion of single and grouped sediment particles in flow in flumes with fixed smooth or rough beds with roughness smaller than the particle size. The results are represented as far as possible in dimensionless plots and equations applicable within the experimental range. A comparison with corresponding results for movable sediment beds (of uniform grain size) enables a realistic assessment of the influence of the channel bed condition on the critical values of velocities, shear stresses and particle Froude numbers. The second part of the paper discusses the implications of the conclusions reached in this analysis on sewer design. It shows conclusively that for smaller diameters (less than 500 mm) the traditional method of computing slopes required for self-cleansing velocities gives higher values than required for initiation (and continuation) of movement of inorganic participate matter.