Drag-dominated local wave loading on a slender vertical cylinder in the vicinity of the free-surface

Accurate prediction of wave loading on offshore structures, particularly offshore wind systems, requires reliable representation of local forces in the crest-to-trough zone where existing depth-averaged Morison-type approaches remain uncertain. This study investigates this region using local force measurements on a drag-dominated vertical cylinder subjected to deterministic focused wave groups, with emphasis on steep non-breaking and breaking waves. The aim is to improve guidance on coefficient selection by clarifying the physical mechanisms governing near-surface loading. Predictions using fully-nonlinear kinematics show that the remaining discrepancies cannot be explained by incident-flow modelling alone, but reflect limitations of depth-averaged loading coefficients. The inferred elevation-dependent drag coefficients reveal distinct behaviours for breaking and non-breaking waves. In non-breaking cases, C d decreases towards the free surface as horizontal momentum is diverted into run-up. In breaking cases, C d first increases at intermediate elevations, consistent with downstream free-surface depression and enhanced pressure difference, before decreasing rapidly near the surface. In this enhanced loading region, the measured drag approaches, but does not exceed, a reference estimated based on the incident momentum-flux. The results provide a physically informed basis for elevation-dependent drag coefficients parameterised by a local Froude number, offering practical guidance for near-surface force prediction beyond empirical re-calibration.