Experimental and numerical investigation of wave loads on a vertical cylinder over complex topography

Complex nearshore topography affects wave propagation, alters wave–structure interactions, and poses challenges to the safety of coastal infrastructure. Clarifying these effects is therefore essential for reliable load prediction and resilient coastal infrastructure design. This study combines physical experiments and three-dimensional Computational Fluid Dynamics (CFD) simulations to investigate regular wave evolution over both a simplified slope–platform bed and a realistic irregular bed, evaluating their impacts on vertical cylinder loads. A Topography Influence Factor (TIF) is introduced for quantitative assessment, revealing a dual-modulation effect of the topography on the structural loads. On the simplified bed slope, premature wave breaking dissipates energy, thereby reducing the peak horizontal forces by 31%–39.5% relative to a flatbed. Conversely, the sudden topographic transition at the simplified bed platform edge induces wave steepening and transient impacts. Consequently, horizontal loads on the cylinder at the simplified bed platform surge by approximately 70.0%–160.0% compared to those observed at the simplified bed slope. Furthermore, the realistic irregular topography functions as a hydrodynamic buffer zone, where natural bed irregularities stimulate lateral wave energy scattering and extend transient impact durations. This disruption of spatiotemporal continuity reduces extreme slamming loads compared to the simplified topography. These quantitative findings provide a theoretical foundation for optimizing infrastructure siting and guiding disaster mitigation designs in complex coastal environments.