C. van Rhee

Sediment Entrainment at High Flow Velocity

In dredging practice sand is eroded at very high flow velocities using water jets. Breaching of dikes or dams is another process where sediments are eroded under the influence of high flow velocities. The existing pick-up functions were developed for relative low values of bed shear stress and hence overestimate erosion. Conventional pick-up functions can be made suitable for dealing with high-velocity erosion by taking the permeability of the sediment into account. This can be achieved by modifying the critical Shields parameter. The procedure is demonstrated using the van Rijn pick-up function. The theory is compared with experiment, and agreement is found to be good.

Advection-diffusion sediment models in a two-phase flow perspective

ABSTRACT Sediment profiles in open channels are usually predicted by advection-diffusion models. Most basic forms consider the terminal settling velocity of a single particle in still clear water. Alternative forms account for hindered settling at higher concentrations. It is not known, however, how these modifications relate to mass and momentum conservation of each phase. For dilute flow, it is known that the original form can be derived from a two-phase analysis, assuming a dilute suspension, neglect of inertial effects in the momentum balance and using a linear drag force formulation. Here we study how and if it is possible to understand the hindered-settling modifications for the non-dilute case, and formulate a relation between advection-diffusion models and parameters involved in the turbulent drag force. This note verifies that the transient two-phase flow solutions converge to steady state, and compares the results to experimental data.

Experiments on the Pickup Flux of Sand at High Flow Velocities

AbstractTwo regimes can be distinguished for the pickup flux of sand. At a Shields parameter of less than about 0.5 (corresponding with flow velocities of 0.5–1  m/s), the erosion process is dominated by the size and density of the grains (grain by grain pickup). At higher flow velocities, the bulk properties of the sand bed start to influence the erosion process. Dilative behavior results in the inflow of water to the sand bed, which reduces the pickup flux (dilatancy-reduced pickup) because of the shearing of layers of sand. A pickup function was recently developed for this regime, incorporating the effect of bulk properties, such as permeability and porosity, on the pickup. This function agrees well with data of previous erosion experiments in which the permeability and porosity of the sand bed were varied. However, these experiments just met the condition for dilatancy-reduced pickup. The flow velocity during these previous experiments was between 1 and 1.5  m/s, while the Shields parameter varied bet...