Abstract. An experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0.3 m wide tilting flume. The concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport. The experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter. A revised Shields diagram relating critical stress, grain Reynolds number, and depth to grain size ratio has been derived. This has implications for the calculation of sediment transport rates, design of stable alluvial channels, and physical hydraulic modeling. Re, = --, One of the most obvious sources of the experimental data scatter and differences among the various studies is the subjectivity in identifying the threshold of sediment motion. The subjectivity arises because the entrainment of sediment is not a sudden event but a gradual process of bed mobilization characterized by increasing frequency of particle displacement as flow strength rises. Various researchers have used different definitions of threshold (or critical) conditions. Shields [1936] determined critical shear stress corresponding to "zero sediment transport" by extrapolating backward measured transport rating curves. As has been demonstrated by Paintal [1971

Hydraulic data defining the dune:antidune transition in fine gravel are compared with potential flow theory, and information is drawn from published experiments and field-based studies. Attention is given to both transitional bedforms and the development of downstream-migrating antidunes. In the latter case, most data pertain to sand beds and not to gravel. Empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower Froude numbers at greater relative depths. Although a critical Froude number of 0AE84 may reasonably be applied for the beginning of the dune to antidune transformation, lag effects (and a possible depth limitation) ensure that transitional bedforms may persist across a broad range of Froude numbers from 0AE5 to 1AE8. This latter observation has great relevance for palaeohydraulic estimates derived from outcrop data. Whereas the application of theoretical bedform existence fields, based upon potential flow theory, to fine gravel was previously purely speculative, the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems. For the first time, laboratory data pertaining to downstream-migrating gravel antidunes are compared with theory. These bedforms have been reported from certain experimental near-critical flows above sand or gravel beds, but have been observed infrequently in natural streams. However, there are no detailed studies from natural rivers and only a few contentious identifications from outcrops. Nevertheless, the limited hydraulic data conform to theoretical expectations.

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Results of an experimental study of the initial motion of coarse uniform sediments are presented in this paper. The experiments were conducted in an 8 m long, 0·3 m wide by 0·3 m deep tilting flume. Natural sediments were used and they ranged in size from 1 mm to 14 mm. The purpose of the experiments was to determine flow conditions associated with the initiation of bed sediment motion. To eliminate the subjectivity in defining the beginning of streambed movement, the concept of critical-bed state is linked to the intensity of sediment motion. The experiments reveal that critical-bed shear-stress for sediment motion depends not only on the grain size, but also on the bed slope. The latter fact is explained by the effect of relative depth (depth to grain-size ratio) on overall flow resistance. It is also shown that the value of critical dimensionless bed shear-stress is not constant for rough turbulent flow, as is usually assumed. The same conclusion follows from the measurements of turbulence characteristics near the surface of the bed. A revised Shields diagram relating critical stress, grain Reynolds number and bed slope is derived for different intensities of sediment motion. Finally, a diagram relating critical flow depth, bed slope and grain size is presented in a form suitable for use by design engineers.