Experimental and numerical study on the flow characteristics around spur dikes at different length-to-depth ratios

Abstract

A spur dike is an in-stream structure, one end of which is situated on the bank of a channel, while the other extends into the current. Its primary functions include protecting stream banks and enhancing aquatic habitats. The motivation of this study stems from the hypothesis that a wake parameter, defined as the product of the length-to-depth ratio of spur dikes and the bottom friction factor, serves as the key parameter in characterizing the flow pattern around these structures. The objectives of this study are two-fold: (1) to determine the critical value of a wake parameter for the transition from three- to two-dimensional flow fields around spur dikes, and (2) to examine the bed shear stress distributions around spur dikes with varying length-to-depth ratios. Flow visualization experiments and large-eddy simulations (LESs) were conducted with length-to-depth ratios ranging from 1 to 7.5, corresponding to wake parameter values between 0.005 and 0.075. The experimental and numerical results revealed various flow structures around and in the wake of the spur dike. When the length-to-depth ratio was close to 1, the two- and three-dimensional flow patterns coexisted around the spur dike. However, as the length-to-depth ratio increased, the three-dimensional flow structures diminished and the two-dimensional flow structures became more pronounced. For an length-to-depth ratio of 7.5, the flow pattern showed fully two-dimensional structures featured by an upstream and a pair of downstream recirculation regions with negligible secondary flows, a horseshoe vortex, and a free-surface vortex. A horseshoe vortex system contributed to a significant increase in both mean and root mean square bed shear stresses near the tip of spur dikes, especially when the wake parameter values were low. The upstream recirculation zone was associated with very low bed shear stress at high values of the wake parameter. The results of this study corroborated that the flow around the spur dikes becomes fully two-dimensional when a wake parameter exceeds a critical value of approximately 0.04.