Wave-induced drift of pumice: transitions from forward to backward transport

Floating pumice rafts generated by eruptions of submarine volcanoes can be transported over large distances, yet the role of surface waves in controlling pumice drift remains poorly constrained. Wave-induced transport is often parameterized indirectly in numerical simulations despite limited experimental observations. Here we investigate the wave-induced drift of floating pumice using laboratory wave-tank experiments under controlled wave forcing. Natural pumice and analog cubes with characteristic sizes of 5–10 cm and a range of densities were subjected to regular waves with frequencies of 1.0–1.6 Hz and varying wave steepness. Pumice motion was quantified using video-based tracking. Pumice with positive freeboard consistently drifted in the direction of wave propagation, with velocities increasing with wave frequency and amplitude. When normalized by theoretical Stokes drift, velocities collapsed into a similar range across pumice densities but showed systematic dependence on wave steepness, indicating the importance of nonlinear wave effects. The corresponding Keulegan–Carpenter numbers (KC ≈ 0.3–3.4) indicate that the wave-induced orbital excursion was comparable to the body dimensions, suggesting that finite-size wave–body interactions may influence the drift. In contrast, pumice approaching neutral buoyancy exhibited persistent drift opposite to the wave direction under certain conditions. Velocity and orientation records reveal a subharmonic response with a dominant period approximately twice that of the incident wave. Step-like modulation of the pumice tilt angle at this doubled period produces alternating large and small forward excursions, skipping large drift events every other cycle. These results demonstrate that wave–pumice interactions evolve with buoyancy and size and can alter both the magnitude and direction of drift. During intermediate stages of pumice raft evolution, when fragment size approaches the wave orbital excursion, such interactions play a critical role in transport and should be explicitly considered in pumice dispersal models following eruptions of submarine volcanoes.