Wave-plus-current induced span shoulder migration in three dimensional scour around submarine pipeline

The purpose of this study is to investigate the rate of span shoulder propagation under conditions of waves and current. The study analyzed 117 cases from previous and present investigations, which were divided into three categories: pure wave, wave-plus-current, and pure current. Furthering the framework of Sui et al. (2021) for pure current conditions, the relative current strength was included in the present study to incorporate the effects of the wave component in a general wave-plus-current condition, through a systematic dimensional analysis. For a given excess Shields parameter, the pure current case has the largest migration velocity compared to the wave conditions. Incorporating the wave components into the pure current decreases the rate of the span shoulder propagation. A new model is proposed to predict the rate of span shoulder propagation while considering the dependency of current strength, excess Shields parameter, and embedded depth. The new model has a determination coefficient of 0.8, indicating its ability to accurately predict the rate of the span shoulder propagation under general wave and current conditions. Parametric studies show that increasing the excess Shields parameter increases the migration rate while increasing the embedment depth, ratio of the pipe diameter to the grain diameter decreases it.