A multi-scale approach for scour time development at monopiles due to currents

Scour time development is traditionally predicted using a two-parameter model, where the scouring process is schematized with an equilibrium depth and a time scale. In this study, we demonstrate that a four-parameter model to determine scour time development around monopiles will lead to significant improvements in accuracy of the prediction. The physical interpretation of this is that different primary hydrodynamic forcing mechanisms induce scour during different phases of the scouring process. The first, rapid phase of development, is associated with large amplification of the flow around the pile due to obstruction which leads to quick scouring. The second, slow phase of development is associated with a more steady horseshoe vortex and influence of the scour hole on the mean flow field which lead to slower development of the scour depth. In this study, we make use of various different datasets existing in literature to demonstrate that the overall time-evolution of scour fits significantly better when assuming that there are two development phases, each with their own distinct timescales. The results of this study may be particularly interesting for temporary structures, where an accurate prediction of time development in the first rapid phase is required. • This study demonstrates that scour time evolution is well described by considering multiple time scales. • Scour predictions using multiple time scales are more accurate than single time scale predictions. • The findings of this study are particularly useful for temporary structures where detailed information of early-stage scour is needed.