United States Department of Transportation
i
Real time measurement of scour depth around bridge piers and abutments : final report.
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2015-01-01
[PDF-3.73 MB]
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Edition:Final report, January 2011 to January 2015
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Abstract:Scour is one of the most significant threats to bridge infrastructure and is the leading cause of failure within the
United States. Scour monitoring is an approved countermeasure as reported by the Federal Highway Administration. As
the monitoring techniques available range from simple devices that rest on or in the channel bed to advanced scanning
systems that provide a bed contour profile, a concise study of the state of the art in real time scour measurement
capabilities is required.
The available techniques for monitoring scour are reviewed to highlight the governing physics, to evaluate field
performance, and to identify the effect of environmental factors on accuracy and reliability. From this assessment, two
devices are selected for further study: a sonar fathometer and a Time Domain Reflectometry (TDR) device. A novel
device, called a Vibration-based Turbulent Pressure (VTP) sensor, is proposed which exploits the turbulence in open
channels to locate the bed level. This sensor vibrates at significantly higher amplitude when in the channel flow relative
to an identical sensor located in the sediment.
The vibration-based method, time domain reflectometry, and sonar devices are evaluated against simulated field
conditions in order to determine their relative sensitivities to environmental conditions. These tests reveal that sonar and
time domain reflectometry devices can be influenced by channel salinity and temperature. In addition, the sonar device is
shown to be sensitive to the suspended sediment concentration in the channel, its height relative to the bed, and bed
topography within the sonar beam. The vibration-based method is shown to be the least sensitive to environmental
factors in the channel. In addition, the VTP device can provide reliable results in highly misaligned flows. Finally, the
performance of TDR, sonar, and the vibration-based technique is evaluated under field conditions.
The field tests reveal that all instruments perform at their accuracy level. The sonar must be deployed close to the
bed to guarantee that the beam will be contained within the scour hole and would not have large width. In addition, the
site should not have high suspended load concentration. The TDR is insensitive to suspended sediment; however, salinity
greater than 0.5 parts per thousand renders the instrument inoperable. The VTP is proven to be insensitive to all the
environmental factors. However, field deployment reveals that debris accumulation may cause the instrument to report
false bed location.
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