Bed‐Surface Size Changes in Gravel‐Bed Channel

Equilibrium sediment transport experiments were made in two flumes, one with a 6‐m long by 0.15‐m wide channel and the other with an 11‐m long and either 0.74‐ or 0.53‐m wide channel. The sediment (0.1 to 32 mm, $D_{50}=3.03\mathrm{mm}$) was fed at the upstream end of the channel and was trapped in the sump at the downstream end. For the runs of this study the bed‐surface grain size became finer and the sediment transport rate increased as the total bed shear stress decreased slightly and then increased. This decrease in bed shear stress was probably caused by the varying coarseness of the bed surface. In the runs with higher transport rates the bed surface was finer, so bed shear stresses were lower. The difference in friction factors (determined from the Moody diagram for rough turbulent flow) for $D_{65}$ of the bed surface sediment and $D_{65}$ of the original sediment mix (or of the sediment in transport) was used to represent the extra friction factor due to roughness changes. Subtracting this quantity from the bed friction factor allowed the calculation of an effective shear stress available for sediment transport. The relation of sediment transport rate and bed‐surface grain size to shear stress was improved by using the effective shear stress rather than the total shear stress.