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

摘要

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 has a strong dependence 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. While 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 mobilization of sediment, or live bed conditions, occur in the range 1 < KC < 6 even though it 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 reveals a marked discontinuity in scour development over the range 1 < KC < 6 that may be the result of laboratory scaling or capabilities. A new empirical model for estimating waveinduced scour at the base of cylindrical piles is derived 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 to an existing predictive equation but, more importantly, provides estimates of equilibrium wave scour depth over a transitional range of KC values not previously considered.