Scour around monopile foundations for off-shore wind turbine in presence of steady and tidal currents

This note describes the scour induced by tidal currents around piles under live-bed conditions, on the basis of physical model tests carried out at Aalborg University (DK). Fine sand was placed in a flume equipped with a reversible pump. Scour was leveled by an automatic laser profiler. Experimental time evolution of scour is satisfactorily given by a simple rule, function of the volume occupied by the pile, the test duration and the sediment mobility. Such rule allows for a quantitative description of erosion in equilibrium conditions, although with some uncertainty. The scour depth S is S/D≈1.3 (D is the pile diameter). For unidirectional currents, eroded volume V at the pile is V/D3≈13, the hole having an assymmetrical shape. For tidal currents, two cases are distinguished, when tidal period is much longer or much shorter than time scale of erosion: in the first case the hole is asymmetric and keeps “drifting with the tide” with volume V/D3≈16; in the second case the hole is symmetric and is stable, with volume V/D^3≈13.