Physical modelling of local scour at twin piles under combined waves and current

Under the actions of ocean waves and current, severe local scour can be induced around pile groups, significantly compromising the safety of marine structures. A series of flume tests were conducted for physical modelling the local scouring process around twin piles in the cohesionless soils under combined waves and currents, and compared with the pure current case. The effects of non-dimensional pile spacing (G/D) and flow skew angle (alpha) on the scour depth and time scale of scour around twin piles are intensively examined. The experimental observations indicate that the influence of pile spacing on the scour depth development is much more significant for the side-by-side (alpha = 90 degrees) arrangement than that for the tandem arrangement (alpha = 0 degrees). With the increase of flow skew angle alpha, the maximum scour depth is remarkably enhanced within the examined range 0 <= G/D <= 3.0. When G/D = 3.0, the pile group effect on the time scale is generally negligible for pure current cases, whereas a prominent pile-group effect can still be observed for combined wave-current cases, especially for the side-by-side arrangement. A parameter of "equivalent pile diameter" is then introduced for evaluating the maximum scour depth at the twin piles with the previous formulas for the single pile. Based on the existing and present experimental data, the empirical formula of dimensionless equivalent pile diameter as the function of G/D and alpha is established to predict the maximum scour depth at the twin piles