Re-parameterization of equilibrium scour depths and time scales for monopiles

Empirical formulations for the engineering prediction of equilibrium scour depths, as well as time scales (for both scour and backfilling), are re-visited for monopiles. Following prior work, it is argued on physical grounds that dimensionless time scales ought to scale proportionally to the Shields parameter raised to the −3/2 power, based on scaling common to numerous sediment transport rate formulae. This is in contrast to stronger power dependence found in many existing formulations based on laboratory experiments, which may become unreliable when extrapolated to (storm/strong) field scales. In light of this, existing data sets for scour and backfilling time scales are re-analyzed. Novel parameterizations are developed for their prediction which are in line with the argument above, while maintaining similar coherency with existing data as in previous parameterizations. Re-analysis of existing data sets for equilibrium scour depths is likewise performed. An existing framework valid for slender monopiles is extended to incorporate the so-called large monopile regime (non-negligible diffraction), especially important e.g. for modern wind turbine installations.