Abstract
Most studies on local scouring at grade control structures have principally focused on the analysis of the primary flow field, predicting the equilibrium scour depth. Despite the numerous studies on scouring processes, secondary currents were not often considered. Based on comprehensive measurements of flow velocities in clear water scours downstream of a grade control structure in a channel with non-cohesive sediments, in this study, we attempted to investigate the generation and turbulence properties of secondary currents across a scour hole at equilibrium condition. The flow velocity distributions through the cross-sectional planes at the downstream location of the maximum equilibrium scour depth clearly show the development of secondary current cells. The secondary currents form a sort of helical-like motion, occurring in both halves of the cross-section in an axisymmetric fashion. A detailed analysis of the turbulence intensities and Reynolds shear stresses was carried out and compared with previous studies. The results highlight considerable spatial heterogeneities of flow turbulence. The anisotropy term of normal stresses dominates the secondary shear stress, giving the impression of its crucial role in generating secondary flow motion across the scour hole. The anisotropy term shows maximum values near both the scour mouth and the scour bed, caused, respectively, by the grade control structure and the sediment ridge formation, which play fundamental roles in maintaining and enhancing the secondary flow motion.
Highlights
The presence of natural or man-made structures on riverbeds plays an important role in the evolution of river morphology and sediment entrainment
Most studies on local scouring at grade control structures have principally focused on the analysis of the primary flow field, predicting the equilibrium scour depth
We essentially focus on the hydrodynamic characteristics of flow across scour holes developed downstream of a grade control structures (GCSs) in sand riverbeds
Summary
The presence of natural or man-made structures on riverbeds plays an important role in the evolution of river morphology and sediment entrainment. Owing to the high velocity of the entering jet flow, a large amount of sediment erosion locally occurs downstream of the GCS, forming the scour hole. Secondary current cells often generate a sort of undulating bottom shear stress distribution in the transverse direction, affecting the whole depth-flow field and the free-surface flow pattern [20,21]. The increase of flow velocity due to tributary junction generates a zone of maximum scour located near the center of the confluence. This zone is characterized by dominant flow convergence and a consistent pattern of secondary circulation. This paper aims to: (i) check the development of secondary currents in scour hole downstream of a GCS, (ii) analyze the evolution of the turbulence structure in the scour hole at the equilibrium condition, and (iii) try to understand the physical origin of secondary current cells across the equilibrium scour hole
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