Flow and scour patterns around a non-submerged spur dike in a mobile-bed channel

Abstract

Flume experiments were conducted under mobile bed conditions to study the three-dimensional flow patterns and bed elevation changes occurring around a non-submerged spur dike. Local scour and bed topography were examined using an ultrasonic distance sensor, and an Acoustic Doppler Velocimeter (ADV) was employed for measuring three-dimensional instantaneous velocities. The equilibrium state of scour was found to be achieved after approximately 28 days, and the maximum scour depth, observed at the tip of the spur dike, was approximately 13 cm. Upstream of the spur dike, the scour slope was steeper than downstream, and wavy bed surfaces were observed downstream, repeating erosion and deposition. This study identified negative vertical velocity upstream of the spur dike indicating the presence of the horseshoe vortex system, with the center of the downward flow moving toward the bed as it approached the spur dike. Downstream of the spur dike, a region with strong streamwise velocity was observed within the scoured zone, and the presence of secondary flow directed towards that area was confirmed. Additionally, changes in flow patterns owing to mobile bed conditions were analyzed by comparing them with a previous study conducted under fixed bed conditions (Jeon et al., 2018). It was concluded that bed deformation has the greatest influence on lateral velocity among the three-dimensional flow velocity components. Differences in the distribution of Reynolds stresses and turbulence energy were also found to be significant. Compared to the rigid bed condition, momentum exchange in the lateral and vertical directions was more active in the mobile bed condition.