Sand-bed trenching and scour mechanisms under a moving submerged jet: An experimental study

Scour induced by moving submerged jets is central to subsea trenching and burial operations, yet previous studies have mainly focused on transient scour development or limited impinging heights, leaving the coupled control of nozzle moving speed and impinging height on residual trench formation insufficiently understood. In this study, Froude-scaled laboratory experiments were conducted to investigate the trenching process of a vertically moving submerged jet over a sand bed, with particular attention to the combined effects of nozzle translation speed and impinging height ratio H/d. The results show that increasing the nozzle moving speed from 40 to 120 mm/s reduces the transient trench depth by approximately 50%, mainly because the jet residence time and effective momentum delivery to the bed are shortened. More importantly, three distinct impinging-height regimes are identified for moving jet scour: impingement–overspill (H/d < 10), deflection–deposition (10 < H/d < 25), and wall-jet diffusion (H/d > 25). These regimes correspond to different sediment transport pathways and explain the observed non-monotonic variation of residual trench depth with impinging height, including a deposition-dominant regime in which the residual trench may become shallower than the initial bed level. Based on the observed mechanisms and non-dimensional analysis, a semi-theoretical predictive model for residual trench dimensions is developed by incorporating the effect of nozzle moving speed into the erosion parameter. The results clarify the mechanism transition governing moving jet scour and provide a physically based framework for predicting trench morphology in subsea jet trenching and backfilling operations.