Abstract
Scour evolution and propagation around a cylinder in natural cohesive sediment was uniquely investigated under multi-flow event varying sequentially by velocity magnitudes. This flume study differs from others that only used test sediment with commercially available clays for single flow. The objective of this study was to explore the potential differences in scour hole development in natural riverbed sediments subjected to varying flow velocity scenarios, advancing our understanding from existing studies on scour. The study consisted of 18 experimental runs based on: velocity, flow duration, and soil bulk density. Scour hole development progressed initially along the cylinder sides, and maximum depths also occurred at these lateral locations. Scour hole depths were less for higher soil bulk densities (≥1.81 g/cm3) compared with lower densities, and erosion rates were slower. It was observed with all flow sequences that scour depths were similar at the end of each experimental run. However, scour initiation was observed to be time dependent for soils with higher bulk density (1.81–2.04 g/cm3) regardless of flow velocity sequences. The observed time dependency suggests a process feedback with the scour hole development initiated at the cylinder sides, which influence local 3D hydraulics as the scour hole depth progresses.
Highlights
Improving our understanding of local scour around bridge piers is needed to better manage infrastructure integrity
A design manual from the US Federal Highway Administration (FWHA) [4] provides predictive equations for scour at bridge piers and abutments, and generally these equations are well accepted for non-cohesive riverbed sediments
Scour rates in cohesive soils are slow and represent a fatigue failure behavior compared to non-cohesive sediments, where attaining equilibrium scour depth may take several days to years under multiple flow events [6,16]
Summary
Improving our understanding of local scour around bridge piers is needed to better manage infrastructure integrity. A design manual from the US Federal Highway Administration (FWHA) [4] provides predictive equations for scour at bridge piers and abutments, and generally these equations are well accepted for non-cohesive riverbed sediments. This is not the case for cohesive bed sediment and riverbank soils. Few studies have been conducted on cohesive soil scour due to the complex erosion processes of cohesive sediments itself [5,6,7,8,9,10,11,12,13]. Further research is needed to advance our understanding of the multifaceted interactions between initial scour hole development in cohesive sediments around bridge piers and the hydraulics that promote hole shape and depths over time
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