Flow structures and hydrodynamic force during sediment entrainment

An experimental investigation of the role of coherent structures and their effect on hydrodynamic forces responsible for inception of sediment motion has been carried out using a laboratory flume. Two types of experiments, namely, movable and fixed ball, were conducted using spherical roughness element beds with particle image velocimetry used to measure the instantaneous flow velocity field. Movable ball experiments revealed the predominance of large sweep structures at the instant of entrainment for both shielded and exposed particles. Fixed ball experiments involving simultaneous measurements of hydrodynamic forces and velocity on the particle at entrainment conditions revealed the significance of impulse in initiating sediment entrainment such that an optimum combination of force and duration is required to produce the threshold impulse. Short-duration lift forces of magnitudes greater than the submerged weight are thought to be responsible for partial lifting of the completely shielded particle. Quadrant analysis and probability distribution function plots of the dominant hydrodynamic force reveal the higher probability of occurrence of high-magnitude force induced by sweep (Q(4)) events.