Scour hole depth prediction around pile groups: review, comparison of existing methods, and proposition of a new approach

A dataset of 365 laboratory tests for scour hole depth (SHD) around pile groups (PGs) under unidirectional aligned flow is compiled, and the performances of the existing equations are comparatively evaluated on the dataset using several statistical indices. A formulation based on a correction of HEC-18 equation provides the best estimate with a correlation factor of 0.58. The test durations of the considered data ranged between 4 and 389 h. A time factor (K-t) is proposed to take into account the temporal variation of the SHD around different PGs. Among the datasets, 51 long-duration experiments are scrutinized to show the temporal variation of scour depth toward equilibrium state. The time duration for these tests is up to 16 days. The proposed K-t factor for PGs has a superior performance compared to existing single-pier time factors. Subsequently, the equilibrium scour depths are calculated by extrapolation of scour depths reported at the end of the experiments using the K-t equation. The results showed that only 27-93% of the equilibrium scour depths were obtained at the end of the experimental measurements. Finally, a new equation for prediction of equilibrium SHD around PGs is proposed, which has 10% less prediction error than the existing equations. This comprehensive comparative study is a significant step forward in the correct estimation of current-induced SHD around PG foundations of hydraulic and coastal structures.