Experimental study on the effect of Froude number on scour initiation and morphology in low KC regime

A clear understanding of scour around monopile foundations is increasingly important as offshore wind expands. This study investigated local scour under irregular waves, focusing on the low Keulegan-Carpenter number regime ( KC < 6), where scour is typically seen as negligible. Hydraulic model experiments showed that significant scour can still start in this regime, mainly driven by the sediment Froude number ( Fr ), which increases inertial forces relative to gravitational forces, thereby lowering the threshold for sediment incipient motion. Scour was classified as near field, transitional field, or far field based on where the maximum scour occurred. Deep Near Field scour mainly occurred in the post-shedding regime ( KC > 6), with active vortex shedding, while the pre-shedding regime ( KC < 6) generally showed Far Field or minor scour due to weaker vortex development. These results show that Fr determines chiefly when and where scour begins in the low KC regime, underscoring the need for new criteria that account for inertial effects. Future research should examine the effect of the Froude number under wave-current conditions to determine whether these findings apply to complex marine environments.