Experiments on the pickup flux of sand at high flow velocities

Abstract

Two 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 between 1 and 2. In order to validate this pickup function for dike breaching and jetting of sand, during which the flow velocity ranges within 5–50 m=s, erosion experiments were executed at higher flow velocities. These experiments were executed in an adapted closed flume of the slurry circuit of the Dredging Research Laboratory at the Delft University of Technology (DUT) at flow velocities of 2–6 m=s (Shields parameter is between 50 and 1,000). The results of these experiments are consistent with the theory of dilatancy-reduced pickup. The porosity of the sand bed influences the erosion process, especially at flow velocities of more than 4 m=s bulk property.