Experimental investigation on scour characteristics and predictive model of monopile foundations under breaking waves

Breaking waves in nearshore regions induce intense sediment transport, leading to significant scour around offshore wind turbine monopile foundations constructed in shallow water areas, thereby threatening structure stability. Due to the complex flow behavior and soil response involved in monopile scour under breaking waves, the scour characteristics and predictive models have not been adequately investigated. This study systematically analyzes the effects of breaking wave parameters, monopile location, and water depth on scour depth and morphology through wave flume experiments. A three-dimensional laser scanning technique was employed to show the evolution of seabed topography. The results indicate that breaking waves cause extensive seabed morphodynamic changes characterized by forming a bar-trough system, which superimpose onto the local scour process around the monopile. Additionally, the pile location parameter α, defined as the ratio of the distance between the monopile and the breaking point to the wavelength, significantly influences the primary scour mechanism, determining the scour depth and morphology. Specifically, when α ranges from 0.35 to 0.55, the monopile is situated in the region of fully developed turbulence, and the maximum scour depth found was approximately 0.5D. As the breaker type transitions from collapsing to plunging, it can prompt the formation of the bar-trough system. Conversely, increasing water depth generates a water cushion effect, suppressing the formation of the bar-trough system and reducing the monopile scour depth. Finally, an empirical formula of monopile scour depth based on KC number, modified Ursell parameter, and monopile location parameter is proposed, which can effectively predict the scour depth in breaking waves.