Effects of Streamwise Abutment Length on Scour at Riprap Apron-Protected Setback Abutments in Compound Channels
Publication: Journal of Hydraulic Engineering
Volume 147, Issue 3
Abstract
Scour at bridge abutments usually consists of local and contraction scour that correlate because of the flow field that develops both scours. Flume experiments were conducted using simulated compound channels to investigate the effect of a setback vertical-wall abutment’s streamwise contraction length on scour depth and pattern. The adequacy of the current design method for riprap aprons was also examined. The prescour flow measurements showed that an abutment with small streamwise length may divert the highly turbulent zone toward the main channel and downstream that beyond the protection of the apron. By contrast, the turbulent zone of a long abutment tends to occur near the abutment face and within the apron area. The erodible bed experiments showed that the abutment length may significantly affect scour depth, morphology, and temporal development for apron-protected abutments but only has a minor influence on unprotected abutments. For apron-protected abutments, an increased abutment length reduces the magnitude of the deepest scour and moves the scour hole closer to the contracted section and the abutment face, which is consistent with the prescour flow field. Depending on the extent of setback, scour may also extend into the main channel. It was found that the existing abutment scour predictors should consider streamwise abutment length in order to reduce underestimation for short contractions and overestimation for long contractions. The design of riprap aprons should also be improved accordingly and integrated with the predictors.
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Data Availability Statement
All data obtained in this study are available from the corresponding author upon reasonable request.
Acknowledgments
This study was undertaken in parallel with National Cooperative Highway Research Program (NCHRP) Project 24-37, for which the third author was a key researcher. The authors wish to acknowledge Professor Terry Sturm, principal investigator (PI) for Project 24-37, for providing leadership and for giving generous help in understanding scour processes at bridge foundations, which indirectly improved the research reported herein.
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Journal of Hydraulic Engineering
Volume 147 • Issue 3 • March 2021
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© 2021 American Society of Civil Engineers.
History
Received: Jun 12, 2020
Accepted: Oct 12, 2020
Published online: Jan 9, 2021
Published in print: Mar 1, 2021
Discussion open until: Jun 9, 2021
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