Local scour evolution around monopile and pile-group foundations under unsteady flow

Offshore wind energy has developed rapidly, increasing demand for pile foundations in marine environments. This study investigated scour around a monopile and a square 2 × 2 pile group through flume tests under a unidirectional current with linearly increasing and decreasing velocity. 3D bed-topography measurements examined the effects of peak velocity, velocity change rate, and G / D . For the monopile, scour developed rapidly, continued after peak velocity, and then partially backfilled. Among seven surveyed instants, the largest measured scour depth generally occurred at t = 2 T / 3 ; the depth at t = T / 2 reached 78.8%–97.8% of that value. Within fixed-peak-velocity groups, a lower velocity change rate allowed scour to develop more fully, whereas cross-group differences also reflected peak velocity, total duration, duration above critical incipient velocity, and cumulative hydraulic forcing. Relative to steady-current references based on time-averaged velocity, unsteady flow increased the largest measured scour depth by approximately 33%–46%. For the pile group, increasing G / D from 1.5 to 3.5 changed the bed configuration from an integrated scour hole to superposed local scour holes and weakened backfilling. The W ∗ – Z ∗ effective-flow-work method described the largest measured scour state more consistently than the final state, with a preliminary pile-group extension using spacing-dependent correction factors.