Estimation of scour depth at a single non-submerged vertical spur dike under unidirectional currents

Accurately estimating equilibrium scour depths at spur dikes remains challenging due to the complexity of scour around these structures. Scour critically threatens spur dikes built in river estuaries for enhancing navigability and preventing bank erosion. This study conducted 41 experiments to determine the approach flow velocity corresponding to scour initiation at a non-submerged spur dike, which enables prediction of equilibrium scour depth. The results showed that scour incipient velocity depended on spur geometry, approach flow characteristics, and sediment properties, rather than being a fixed value. A new formula for equilibrium scour depth prediction was proposed, based on the scour incipient velocity and the excess abutment Froude number, yielding a determination coefficient of 0.93. The formula demonstrated broader applicability and greater accuracy than previously reported formulas, with 98% of data within a ±25% error margin. The dimensionless sediment size was introduced to capture the effects of sediment size on scour, providing an alternative to the sediment coarseness ratio, which is challenging to replicate in laboratory settings. These findings offer valuable guidance for engineering design and protection of spur dikes under unidirectional flow.