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Clear-water scour at comparatively large cylindrical piers

AbstractNew long-duration clear-water scour data were collected at single cylindrical piers with the objective of investigating the effect of sediment coarseness, Dp/D50 (Dp = pier diameter; D50 = median grain size) on the equilibrium scour depth and improving the scour depth time evolution modeling by making use of the exponential function suggested in the literature. Experiments were carried out for the flow intensity close to the threshold condition of initiation of sediment motion, imposing wide changes of sediment coarseness and flow shallowness, d/Dp (d = approach flow depth). The effect of sediment coarseness on the equilibrium scour depth was identified; existing predictors were modified to incorporate this effect for U/Uc≈1.0; Dp/D50>≈60 and d/Dp≥0.5; the complete characterization of a known scour depth time evolution model was achieved for U/Uc≈1.0, 60

Clear-water scour at pile groups

AbstractGroups of piles are frequently used as bridge foundations. Different group configurations, characterized by different pile spacing, skew-angle, number, and arrangement of pile group columns interact differently with the flow field and lead to different scour patterns and equilibrium scour depth. There have been a number of past studies on the characterization of scouring at pile groups, but most of them report short duration scour experiments. A priori, such short durations may be postulated to inherently carry important uncertainties into existing scour predictors. In this study, 75 long-duration laboratory tests were run under steady, clear-water flow close to the threshold for initiation of sediment motion, to address the effect of time, pile spacing, skew-angle and number of pile group columns on the equilibrium scour depth. Pile groups consisted of matrical arrangements of one, two, or three columns of four rows, with spacings of 1, 2, 3, 4.5, and 6 pile diameters; the tested skew-angles were...

Effect of Viscosity on the Equilibrium Scour Depth at Single Cylindrical Piers

AbstractMost studies on scouring seem to have overlooked the effect of viscosity, the assumption being that the flow is free of viscous effects inside the scour hole because of the presence of highly turbulent flow structures such as down-flow, horseshoe vortex, and wake vortices irrespective of the approach flow regime. The present work reports the results of experiments conceived to revisit the effect of viscosity on scouring. The experiments were performed by changing the Reynolds-like numbers for two flow shallowness values and keeping constant the remaining dimensionless parameters that control the scour process. The values of the Reynolds number corresponded to transitional approach flows. The experimental results indicate that scouring does depend on viscosity in usual laboratory conditions, the viscous effect leading to scour depths in the safety side.