Experimental Study of Local Scour around a Vertical Cylinder under Wave-Only and Combined Wave-Current Conditions in a Large-Scale Flume

A series of physical experiments under wave-only and combined wave-current conditions are conducted in a large-scale wave flume. It is found that waves with strong nonlinearity may lead to downstream movement of the maximum scour point under wave-only conditions, which is attributed to the unbalanced effects of the wave crest and wave trough. Based on the surveyed topographies of scour holes and measured temporal variation of scour depth in different locations in the hole, the weighting ratio Ucw is adopted as a criterion to distinguish the wave-dominated and current-dominated combined flow. This classification criterion is found to have a characteristic value of 0.44, at which the nondimensional characteristic timescale coincidentally gains a turning point at lower wave Shields numbers. The equilibrium timescale is found to be represented linearly by the characteristic timescale. Empirical formulas established based on small-scale flume data yield reasonable results of scour depth for wave-only conditions and for a relatively larger Keulegan-Carpenter (KC) number in combined wave-current conditions but underestimate the results for the lower KC number conditions in combined flow.