THE INFLUENCE OF PRESSURE FLUCTUATIONS ON THE FLOW BETWEEN ARMOUR ELEMENTS

Robert Booij, Wim S.J. Uijttewaal, Patrick van Os, Harry L. Fontijn, Jurjen A. Battjes

Abstract


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 ofa single layer of large rocks, is often tried nowadays. In most models it is assumed that the mean flow (and shear stress) above the armour layer penetrates 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 pores 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 particles through the armour layer into the flow above.

Keywords


armor flow; armor; pressure; pressure fluctuation

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