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ERDC/CRREL TR-02-14
The scour monitor system, developed at CRREL, is based on time-domain
reflectometry (TDR). This technology lends itself to use as a robust instrument
capable of surviving ice and debris accumulations. A scour monitor is installed in
the riverbed and is nonintrusive because it need not be inserted in the flow (or
through the ice) to measure the local bed elevation nor will its presence have a
significant effect on the depth of scour. Being inserted into the bed also reduces
the risk of instrument damage, as it is sheltered from the floating ice and debris.
The TDR technology operates by generating an electromagnetic pulse (or a
fast rise-time step) and coupling the pulse to a transmission line. The pulse
propagates down the transmission line at a fixed and known velocity. When the
pulse encounters a change in boundary conditions, a portion of the pulse's energy
is reflected back to the source from the boundary. The remainder of the pulse's
energy continues to propagate through the boundary until another boundary
condition (or the end of the transmission line probe) causes part or all of that
energy to be reflected back towards the pulse source. The round-trip travel time
of a pulse and the dielectric medium through which the pulse travels enables
calculation of the physical distance from the TDR source to the level of each
dielectric-interface boundary encountered. A simulated reflected TDR signal is
shown on the left side of Figure 33. The first boundary indicated is the electrical
connection to the transmission line. It is followed by a watersediment interface,
with the remaining wave energy reflected from the end of the transmission line.
Functionally the probe has two parts: the upper sensor and the lower anchoring
section. Two parallel pipes, serving as a transmission line, are welded to a lower
anchoring section, as shown in right side of Figure 33. For this study the sensor
section was fabricated using 1.25-in.-diameter pipes at 3.0-in. centers. The
probe's sensing length was 5 ft. A plastic block on the top of the probe was used
to mechanically connect, but electrically isolate, the two parallel pipes. The
plastic block also served as a protective junction box for connecting the coax
cable from the on-shore instrumentation to the top of the probe. Introducing the
electromagnetic pulse at the top of the probe avoids the undesired effect on pulse
propagation time caused by the variations in dielectric constant associated with
various sediment layers surrounding the probe. The top-connection approach also
makes the system suitable for use in conductive cohesive soils. The probe's
sensing length is selected in accordance with the desired measurement resolution.
The instrumentation uses 251 points to digitize the reflected wave and corre-
sponding travel distance.
The lower anchoring section laterally stabilizes the probe when bed sediment
is eroded, exposing the probe to impacts from bed load material and water pres-
sure. The length of the anchoring section is a function of the requisite lateral
resistance of the bed material; it is typically on the order of 7 ft. The coax cable
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