A semi-empirical prediction model for scour depth around riprap-protected monopiles under steady currents

Local scour around monopiles remains a major concern for offshore foundations, and riprap protection is widely used to mitigate scour. However, residual deformation and scour within riprap protection layers may still occur, and their prediction remains less established than conventional riprap sizing. In this study, a combined experimental and numerical framework was developed to estimate the maximum scour depth within an intact, sufficiently thick riprap layer around monopiles under steady currents. The numerical model was used to determine the maximum local bed shear stress around the pile, while flume tests provided measured riprap scour depths under varying flow, water depth, pile diameter, bed sediment and riprap size conditions. A local excess shear stress parameter was introduced to represent the hydrodynamic loading exceeding the critical stability threshold of riprap particles, and a semi-empirical equation was proposed by incorporating dimensionless sediment and riprap particle sizes. The proposed model provides an approach for estimating residual scour depth around riprap-protected monopiles.