Predicting Equilibrium Scour Depth around Monopiles for Waves and Combined Waves and Currents

Abstract In this study, a new predictive framework is introduced to calculate equilibrium scour depth for waves, currents, and combined wave current conditions. A common way of modeling the impact of waves on scour depth is to assume that wave-induced scour converges to a steady-current scour depth associated with similar forcing conditions. This convergence is dependent on the resemblance of the flow structure around the foundation to a fully developed flow structure associated with a steady current. The present paper introduces a new parameter to express in the form of a modified Keulegan-Carpenter (KC) number that combines (1) the wave velocity plus current velocity and (2) the classic KC number. It is shown that the aforementioned modified KC number may be obtained via the Navier-Stokes equations. Furthermore, instead of assuming a constant value for the steady-current related scour depth, as is commonly done, this scour depth is explicitly calculated. The newly introduced predictive equations show good performance on a large set of experimental data for scour due to only waves, combined waves and currents and steady currents, outperforming commonly used other methods. By eliminating the assumption of a constant value for steady-current scour, the model especially yields better performance in predicting deepest scour depths compared to predictive equations that do assume a constant value. No residual trend with the most important nondimensional parameters was observed.