ABSTRACT
As soon as liquid starts flowing, hydrodynamic forces are exerted upon the solid particles of the bed. When the flow velocity over a bed of noncohesive material has increased sufficiently, individual grains begin to move in an intermittent and random fashion. Bed instability results from the interaction between two stochastic variables. At first, every grain on the bed surface can be assumed to be potentially susceptible to an instantaneous critical bed shear stress. The grain becomes unstable if the instantaneous bed shear stress exceeds the critical one. Due to the random shape, weight and placement of the individual grains, these critical shear stresses will have a probability distribution. The other random variable is the instantaneous bed shear stress generated by the flow. The probability that the instantaneous bed shear stress is greater than a characteristic critical shear stress is a measure of the transport of sediment. Classical methods of Brahms, Hjulström and Shields, and the influence of turbulence on the stability of bed particles are discussed in an introductory way (Section 2).
