A unifying framework for particle entrainment

Abstract

A new framework is presented that describes the hydrodynamic entrainment of sediments. The framework is derived from the general equations of particle stability and fluid motion and covers a wide range of scales, from that of the individual particle to en masse entrainment at the scale of the stream reach. Particle entrainment is explicitly shown to be driven by the combination of instantaneous fluid stress and pressure gradients, particle surface contact forces, particle submerged weight, and the weight and motion of particle-associated fluid. Of these factors, bed shear stresses, across-particle differences in pressures and fluxes of momentum, and sediment bed characteristics are found to be key factors in particle entrainment. The conventional Shields entrainment parameter is shown to be applicable to averaged en masse entrainment by steady uniform 2-D flows. This nondimensionalized bed shear stress at threshold is further demonstrated to be a function of interparticle forces and bed normal hydrodynamic forces, and thereby sediment size and other fluid, sediment and bed characteristics. Observed Shields parameter dependencies are discussed. The presented framework, which includes all relevant particle and fluid characteristics and flow dynamics, may aid understanding of entrainment mechanics, the design and analysis of further studies of entrainment, the design of parameterizations that lead to the solution of entrainment problems, and numerical modeling of combined fluid and sediment dynamics.