Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory¯ows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively ®ner grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional¯ows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are`unworked' at the start of the experiments.The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly de®ned. The pivoting angles of the beds used here are measured using a new technique designed speci®cally for comparison with the threshold data. The measured angles repeat the ®nding that the coarse grains are more mobile than the ®ner fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form F aD c (D i /D 50 ) b (after Li & Komar, 1986), where a, c and b are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship.The entrainment of grains is found to be easier beneath unidirectional¯ows than combined¯ows, in accordance with previous authors' ®ndings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave¯ows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive ef®ciency of the overlying current¯ow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near-threshold¯ows.

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