138 S. Atamturktur and A. Khan the vibration levels of VTPs. Because these VTPs are distributed throughout the depth of the bridge pier, changes in the vibration levels can be straightforwardly correlated to the changes in the bed level, i.e., scour formation as well as refill. The concept behind this approach for scour detection is fundamentally insensitive to many of the environmental or flow conditions that are reported to hinder the abilities of existing scour monitoring devices [3]. VTP sensors are excited by the turbidity and hence yield even more reliable outcomes as turbidity increases, a condition which would hinder the performance of sonar fathometers. Also, VTP method detects the water/sediment interface in a manner that is not affected by debris (which tends to cause false echoes in sonar fathometers) in the channel. Salinity and temperature, which negatively affect the riverbed measurements of TDR method, has practically no influence on the VTP mechanism. In this paper, we discuss the development of the VTP sensors, present the experimental data obtained in the laboratory conditions and demonstrate preliminary results from the field deployment. 13.2 Sensor Development: General Concepts The sensor system is constructed with eight neoprene VTPs of 0.02 mm radius mounted on a 0.10 m diameter steel pipe. The VTPs are spaced approximately 0.10 m apart, on center (Fig. 13.3). The sensor is assembled using a compression pipe coupling mounted on the steel pipe with a toroid disk sandwiched in between. The flexible plate is fixed to the toroid disk, as shown in Fig. 13.1. Schematic view of different components of VTP device is shown in Fig. 13.2. The laboratory experiments are conducted in a 1.2 1.2 18 m flume available at the Clemson Hydraulic Laboratory. The pipe is buried in the sediment keeping a portion of sensors exposed to the flow. The pipe is supported as shown in Fig. 13.3. The riverbed is represented in the flume with quartz sand, which practically represents a worst case evaluation regarding the performance of the VTP method, as sand is expected to provide less stiff support compared to clay, and lead in higher vibration levels in the sediment. The flow rates in the flume are controlled to vary in between 0.028 and 0.14 cubic meters per second. Commercially available uniaxial accelerometers (B&K 4507 B 006) are used to measure the acceleration at the center of each flexible plate. These accelerometers are connected to a data acquisition system (B&K LAN-XI 3050A-060) through cables. During the acquisition of the time domain acceleration measurements, sampling frequency is set to 25.6 kHz. Fig. 13.1 Prototype VTPs configuration. Components include: (1) the compression fitting, (2) the vibrating membrane, (3) a washer, (4) toroid disk, (5) accelerometer, and (6) the support pipe (Reproduced with permission from[3]) 1 2 5 4 3 6
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