Pier Scour in Clear-Water Conditions With Non-Uniform Bed Materials
CHAPTER 6. CONCLUSIONS
After critical review of pier scour under clear-water conditions with non-uniform sediment mixtures, a scour mechanism for understanding pier scour processes and a scour depth equation for design were proposed. In general, pier scour results from flow-structure-sediment interactions, and equilibrium scour depth is determined by the flow-structure and flow-sediment interactions. The following conclusions are presented:
The flow-structure interaction sets up a favorable pressure gradient along the pier perimeter and in the wake flow region. The perimeter pressure gradient is explained by Prandtl’s boundary layer theory and the wake pressure gradient by the motion equation of vorticity. The flow-structure interaction also results in a vertical stagnation flow, generating horseshoe vortices at the foot of the pier and playing an important role in the formation of maximum equilibrium scour depth.
The flow-sediment interaction results from the flow-structure interaction, where the pressure gradient determines sediment motion. That is, the perimeter pressure gradient and the horseshoe vortices dislodge sediments to the wake of pier, and the wake pressure gradient due to vortex motion further moves sediments downstream to a low pressure zone where large vortices are broken into small eddies.
The sediment-structure interaction occurs in the flanks of the pier. That is, sediment size affects scour processes but has nothing to do with the equilibrium scour depth at the leading edge.
Equilibrium scour depth is hypothesized to increase with pier blocking area and Hager number, but it decreases with sediment non-uniformity. This hypothesis was confirmed by the laboratory and field data under various flow-structure-sediment conditions.
A new clear-water scour design equation was proposed. This equation is conservative, as is appropriate, but it reduces the extent of over-design compared with the CSU and Sheppard-Melville equations and has a higher reliability index than the CSU equation.
This research is based on a limited database, indicating that extensive experimental research and further analyses are needed for complete understanding of pier scour in clear-water with non-uniform sediment mixtures.
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Federal Highway Administration