The temporal development of clear-water local scour depth at cylindrical bridge piers in uniform sand beds is considered. New data are presented and used to quantify the influence of flow duration on the depth of local scour. An equilibrium time scale (t*) is defined. The data show that both t* and the equilibrium scour depth (dse) are subject to similar influences of flow and sediment parameters, as might be expected because they are inherently interdependent. A method is given for determination of the time for development of dse for a given pier, sediment, and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole. The results show that the scour depth after 10% of the time to equilibrium is between about 50% and 80% of the equilibrium scour depth, depending on the approach flow velocity.

Local scour around cylindrical bridge piers in uniform, cohesionless sediment was investigated experimentally. Three empirical functions which relate equilibrium scour depth with approach velocity, sediment size and How depth were obtained. The variations of equilibrium scour depth with approach velocity show that the depth of scour decreases at velocities just above the threshold until it reaches a minimum at about twice the threshold velocity. Thereafter, it increases again until a maximum is reached at the transition flat bed condition. At still higher velocity, the equilibrium scour depth decreases due to the formation of antidunes. Both the effects of sediment size and flow depth were found to be independent of the stage of bed particle motion and the approach velocity. Adjustment factors, for the effects of sediment size and flow depth were defined and evaluated.

The objective of this study is to review existing methods and propose alternative or additional devices for scour protection around bridge piers. Engineering methods used in dealing with pier‐scour problems include placing coarse riprap around the foundations. An inverted filter layer is normally placed beneath the riprap to overcome leaching problems. Practical difficulties in laying the filter according to specifications, especially in deep water, often cause failure of the riprap layer. The study examines the effect of using a slot in the pier, or a collar around it, as devices for controlling the depth of scour. Experiments conducted in clear‐water conditions show that the use of a slot alone (with a width equal to one‐fourth of the pier diameter) can reduce equilibrium scour depth by as much as 20%. A combination of slot and collar can further reduce the scour depth. These devices may be useful as additional or alternative devices for scour protection.

Engineering methods to deal with pier-scour problems include placing coarse riprap stones around the bridge pier. This study aims to investigate the stability of a riprap layer around a cylindrical bridge pier. Experiments conducted in a laboratory flume identified three different modes of riprap failure. They can act singly or combine to render a complete failure of the riprap layer. These modes of failure are riprap shear failure, winnowing failure, and edge failure. The study proposes a semiempirical method to size stones for riprap protection. The procedure for this method was illustrated with field data from three different bridge sites. The experimental data show that a thick riprap layer can prevent winnowing in the absence of a filter layer. The thick riprap layer can also sustain a partial breakup of the layer with the capability of rearmoring the scour hole, preventing a total disintegration of the riprap layer. Finally, the study proposes two empirical relationships, which describe the effect o...

The study investigates the threshold condition for the initiation of cohesionless sediment transport on a nonhorizontal streamwise slope. Theoretical study of the stability of a sediment particle lying on a non-horizontal bed slope shows that the critical shear stress is a function of the streamwise bed slope. Experiments conducted with a closed-conduit flow show that the equation derived from force analysis adequately evaluates the critical shear stress of sediments lying on slopes ranging from steep positive to negative (adverse). Tests conducted with a closed-conduit flow avoid the problems associated with those conducted with an open channel flow. In the latter flow condition, uniform flow is often difficult to achieve in a steep channel, and impossible to achieve when the streamwise bed slope is adverse.

The primary objective of this study is to improve understanding of the mechanism causing scour in unidirectional current. Experiments have shown how local scour develops around submarine pipelines in noncohesive sediments. The study shows that piping is the dominant cause of the initiation of scour. Piping and the stagnation eddy combine to undermine the pipeline, and mark the onset of scour. The critical hydraulic gradient associated with the initiation of scour is equal to the flotation gradient of the bed sediment. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the pipe induces this hydraulic gradient. When a pipe is just embedded, the onset of scour does not occur if the ratio of the flow depth to pipe diameter exceeds 3.5. Similarly, the onset of scour does not occur for half‐buried pipes. The reduction in pressure gradient across the pipeline for these flow/pipe combinations accounts for the lack of scour. The onset of scour can be prevented by placing an ...