Time Development of Scour around a Cylinder in Simulated Tidal Currents

A laboratory flume experiment was performed to investigate the time development of scour around a vertical cylinder acting as a scaled model of an offshore wind turbine monopile in tidal currents. The tidal current was simulated by resolving each half-cycle into three time steps, between which flow velocity and depth were varied. Flow direction was reversed between half-cycles, which were otherwise identical. Between them, the three time steps exhibited clear water, transitional, and live-bed conditions. The experiment was run over two full-simulated tidal cycles. The scour hole formed tended to a symmetrical shape after two half-cycles and was both shallower and slower developing than the scour hole in a unidirectional current test carried out in the same flume. This was due mainly to the variable rates of scour caused by the variable flow conditions within each half-cycle, and to a lesser extent to the infilling of the scour hole when the current direction reversed. The lower scour depth recorded in tidal conditions implies that the amount of scour protection required may be less than previous studies suggest.