To reduce the high expenses of armour layers for the protection of sandy beds in rivers and coastal areas around structures a so called geometrically open armouring consisting of a single layer of large rocks, is often tied nowadays. In most models it is assumed that the mean flow (and shear stress) above the armour layer pentrates in the pores between the armour elements and that this penetrated flow is responsible for the erosion of the sand bed below the armour layer. However, the relatively thin armour layer these models predict does not always suffice for a safe armouring of the sand bed in practice. Measurements of the flow velocities in the proes between armour elements in a flume using the laser-Doppler technique suggest a completely different erosion mechanism Erosion appears not to be caused by the mean pore flow. Instead small-scale locally generated velocity fluctuations in the pores lift the sand particles from the bed; large-scale fluctuations, due to large-scale turbulence in the main flow, transport the sand proticles through the armour layer into the flow above.
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