Incipient motion of streambeds
Results of an experimental study of the incipient motion of streambeds are reported. The experiments were conducted in an 8 m long, and 0.30 m wide by 0.30 m deep glass-walled tilting flume and an 18 m long, 0.80-1.10 m wide by 0.15 m deep trapezoidal concrete channel. The purpose of the experiments was to determine flow conditions associated with the initiation of bed sediment motion and to investigate near-to-threshold bedload transport. Uniform and graded natural sands and gravels were used as bed material. A new approach to the description of critical state of the bed based on the intensity of sediment motion, or transport intensity, is proposed. This approach describes the state of the streambed mobility in terms of the fraction of bed particles mobilized in unit time, which eliminates the subjectivity in defining threshold conditions and provides a probabilistic description of the process of sediment entrainment. On the basis of the flume data obtained for uniform sediment, a relationship between the intensity of particle motion and sediment transport rate is established. This relationship allows any measured transport rate to be expressed in terms of the probability description of the process of sediment entrainment. The experiments reveal that critical bed shear stress for incipient motion of uniform sediment depends not only on the grain size, but also on the bed slope. This 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 coarser gravel. The same conclusion follows from the measurements of turbulence characteristics near the bed. A revised Shields diagram relating critical stress, grain (or alternatively, different probabilities of sediment entrainment). The experimental results are formalized to provide a generalized method for calculating the bedload transport rate and critical Shields stress of coarse uniform sediments.