Wave-Induced Scour at Cylindrical Piles: Estimating Equilibrium Scour Depth in a Transition Zone

The dynamics of wave-induced scour at the base of vertical piles are not well defined for some realistic combinations of wave-induced flow and pile diameter. Scour development at marine structures depends strongly on the Keulegan–Carpenter (KC) number, a ratio of flow to structure length scale. Observations and descriptions of scour development in the range 1 < KC < 6 are mostly absent from existing literature. Even though there is little to no vorticity for KC < 6 at a cylindrical pile, a continuum of sediment transport initiated by steady streaming of fluid must exist. Analysis of wave-induced flow and pile characteristics representative of field conditions reveal that sediment mobilization—also referred to as live bed conditions—occurs in the range 1 < KC < 6, even though sediment mobilization is not evident in most laboratory experiments. A comprehensive evaluation of published field and laboratory data for equilibrium wave scour depth at the base of cylindrical piles revealed a marked discontinuity in scour development over the range 1 < KC < 6 that might have been the result of laboratory scaling or capabilities. A new empirical model for estimating wave-induced scour at the base of cylindrical piles was derived by using available laboratory data in such a way as to minimize error over a broad range of flow and pile characteristics. The new equation reduces error by 31% compared with an existing predictive equation and, more important, estimates the equilibrium wave scour depth over a transitional range of KC values not considered previously.