NCHRP Synthesis 396 – Monitoring Scour Critical Bridges - TSP2

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scour monitoring data are then transmitted from the master
unit to a central office and/or posted on the Internet.
Sonars
Sonar scour monitors are mounted onto the pier or abutment
face (Figures 3–6) to take streambed measurements, and each
is connected to a data logger (Figure 2). The sonar instrument
measures the distance from the sonar head to the riverbed and
back based on the travel time of a sound wave through water.
The data logger controls the sonar system operation and data
collection functions. The data logger is programmed to take
measurements at prescribed intervals. Sonar sensors normally
take a rapid series of measurements and use an averaging
scheme to determine the distance from the sonar transducer
to the streambed. These instruments can track both the scour
and refill (deposition) processes. The early sonar monitors
used existing fish finders. Currently, new sonar monitors
range from the fish finders to smart sonar transducers, both of
which are commercially available.
Magnetic Sliding Collars
Magnetic sliding collars (Figures 7 and 8) are rods or masts
that are attached to the face of a pier or abutment and driven
or augered into the streambed. A collar with magnetic sensors
FIGURE 4 Schematic of a sonar scour monitoring system
(see Figure 3) (Courtesy: Hardesty & Hanover, LLP).
FIGURE 3 Scour monitoring system mounted on a pier on the
Robert Moses Causeway over Fire Island Inlet, New York (circled)
(Copyright: Raimondo di Egidio 2002).
FIGURE 5 Schematic of sonar scour monitoring system
(Lagasse et al. 2001a).