A flume was used to study the effect of granules on the equilibrium of a stream flowing over a movable bed of fine sand. Addition of granules to flows over a dune-covered sand bed resulted in armouring of troughs between dunes, and in a decrease in the friction factor and sediment discharge. It is inferred that the latter two effects resulted from a decrease in amplitude of bed forms due to the armouring. Addition of granules to a flow over an initially flat sand bed caused an increase in both friction factor and sediment discharge. Similar results were obtained by Guy and others (1966, p. 72–73) for both the dune and flat bed configurations.
During some runs a long low hump formed on the bed in a reach where granule concentration was slightly higher than average. By means of this hump, the depth and velocity were adjusted to yield a sediment discharge in the reach of high granule concentration that was equal to sediment discharge elsewhere in the flume.