Coupling Effects of Flow Regimes and Pulsation Frequencies on the Spatio-Temporal Evolution of Monopile Scour Through Experimental Study

Scour around monopile foundations is a pivotal challenge in nearshore engineering, as it undermines sediment support and threatens structural stability. This study systematically investigates the dynamic evolution of scour under four distinct flow regimes—steady, sinusoidal, pulsatile, and irregular—coupled with varying pulsation frequencies (39, 69, and 100 Hz). Utilizing a laboratory flume and underwater high-resolution imaging, near-pile flow velocities and morphological development were monitored in real time. Results indicate that the pulsation frequency, acting as the primary energy input, dictates the ultimate scour scale and acceleration. Three distinct evolutionary modes are identified: “gradual advancement” at 39 Hz, “ Rapid development phase” at 69 Hz, and “instantaneous stabilization” at 100 Hz. Higher frequencies concentrate energy release into the incipient stage, drastically shortening the duration to reach equilibrium. Morphological analysis reveals that equilibrium scour shapes are highly regime-dependent, manifesting as teardrop (steady), elliptical (sinusoidal), pronouncedly elliptical (pulsatile), and semi-circular (irregular) configurations. While scour dimensions generally scale with frequency, their sensitivity is governed by the flow regime; Constant Current Flow exhibits the highest volumetric vulnerability, whereas pulsatile flow demonstrates the greatest morphological stability. These findings provide a theoretical framework for predicting scour geometry in complex marine environments and optimizing foundation protection strategies.