Length scale for evaluating wave-induced pipeline scour

This study proposes a new length scale that measures the size of the wave-induced vortex street on one side of a pipeline. By conducting dimensional analysis, we show that the equilibrium scour depth under a pipeline, when normalized by the vortex street size, is generally dependent on the densimetric Froude number. However, by analyzing the published data, the dependence is found to be insignificant if the current velocity is smaller than the maximum orbital velocity of water particles at the bed. This result suggests that the normalized scour depth can be simply taken to be a constant for both wave-alone and wave-plus-current conditions.