Author(s): Robert Booij; Roland Jansens
Linked Author(s): Robert Booij
Keywords: Erosion; Sand bed; Armoring; Geometrically open; Pore flow; Waves; Large-scale turbulence
Abstract: Around structures erected on sandy beds in rivers and coastal areas the bed often has to be protected from scouring by armor layers, consisting of relatively large rocks. Classical design rules lead to conservative and expensive multi-layer armoring, a so-called geometrically closed armoring. To reduce the high expenses of this kind of protection a so-called geometrically open armoring, consisting of a single layer of large rocks, is often preferred nowadays. In most predictive models it is assumed that the mean flow and the shear stress above the armor layer penetrate in the pores between the armor elements and that this penetrated flow is responsible for the erosion of the sand bed below the layer. However, the relatively thin armor layer these models predict, does not always suffice for a protection of the sand bed in practice. Measurements of the flow velocities in the pores between armor elements in a flume using the laser-Doppler technique suggest a completely different driving mechanism for the pore flow and hence a different erosion mechanism. Two flow situations were examined: A situation with a steady ambient main flow above the armor layer and a situation with surface waves and a zero average main flow velocity above the armor layer. In both cases the mean pore flow was found to be driven by the pressure gradients in the armor layer, which are induced by the pressure gradients in the flow above the layer. Pressure gradients associated with a surface slope, with waves, or with large turbulent structures play a dominant role. Below waves the erosion of the sand bed appears to depend on the maximum pore flow and the locally generated small-scale flow structures, which detach the sand grains from the bed. Below steady flow the upward displacement of the detached sand particles through the armor layer is a bottleneck for erosion. Here large-scale velocity fluctuations in the armor layer, due to large-scale turbulence in the main flow, transport the sand particles into the flow above. The knowledge obtained will ultimately lead to different design rules.
Year: 2001