13 Vibration-Based Scour Monitoring: Prototype Design, Laboratory Experiments and Field Deployment 141 Fig. 13.5 VTP MSA response of prototype VTPs versus flume flow rates. The slight drop in the mean square acceleration can be attributed to the 10 s measurement time, which may not be sufficiently long to capture the large eddies in the flow. Hence, as the flow rate increases, one should increase the measurement duration 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 Flow Rate [cms] VTP MSA Response [m2/s4] VTP 8 VTP 5 Fig. 13.6 Installed instruments at the field site Figure 13.8 compares the bed elevation history collected by the VTP system against the water surface elevations recorded by the USGS gauge station to confirm the consistency and the sensitivity of our instrument in recording the scour resulting from flood events. It can be seen in Fig. 13.8 that an increase in water surface level leads to a decrease in the VTP identified bed level. Next, the bed elevation histories of TDR and VTP for the Eighteen Mile Creek bridge site are compared in Fig. 13.9. Bed elevations from each instrument are measured with respect to the distance from a common datum (the zero elevation in the plots), which is taken as the top of the concrete pad of the pier (recall Fig. 13.6). Figure 13.9 shows a flood event that occurred around April 7–9, 2014, during which the water level increased up to 305 cm above the datum. The TDR and VTP are both responding to the change in bed level due to the flood event. However, there are differences in the recorded bed elevation partly because the instruments are placed about 46 cm apart resulting in different bed elevations being recorded by TDR and VTP. Both instruments can only record up to their probe length. In our application, the length of TDR probe
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