recirculation system at approximately half of the maximum discharge rate. The
physical response of the traps was observed, and data were collected and
evaluated. The traps remained essentially motionless during forcing with
currents alone and did not seem to affect the variability of the measurements.
The range of fluctuations was similar to that observed during the static tests with
the facility drained.
A second and third test were conducted by including regular and irregular
wave forcing to the currents, respectively. The wave and current conditions were
the same as those previously used during the fixed-bed hydrodynamic
experiments. The downdrift wave guide, constructed of marine plywood, was
removed for these two tests to assess the behavior of the traps in a "worst-case"
scenario, since the wave guide significantly reduces the amount of wave action
within the flow channels. There was no significant movement of the traps during
any of these tests, although they did vibrate minimally during the second and
third test if waves were generated.
Figure 26 illustrates the range of variability in the measured time series
during the second and third test. The standard deviation for each of the 60 load
cells is plotted for both the regular and irregular wave cases. Load Cell No.1 is
located at the onshore end and load cell No. 60 is located at the offshore end. As
anticipated, the standard deviations decreased significantly close to the shoreline
because wave heights are much smaller in that region. In general, the two time
series appear to be very similar with average values of the standard deviation of
approximately 3.0 kg further offshore. These variations are significantly larger
than in the static test cases; however, it is possible to numerically filter the data
using a filtering routine during postprocessing of the data (Chapter 8). In
addition, these fluctuations were reduced considerably after the downdrift wave-
guide was reinstalled, which will be the case for the majority of the LST
experiments.
Prior to these tests, there was concern that for regular wave forcing, the traps
would be more severely agitated because the hydrodynamic loading would occur
at a constant frequency. However, this was not the case, as shown in Figure 26.
During the regular wave case two load cells experienced a significantly high
standard deviation, but this was not attributed to the regular wave forcing,
because this increase only occurred on 2 of the 60 load cells.
Summary
This chapter documents the design, operation, and performance of the
sediment trapping and dredging system developed for the LSTF. The mechanical
design of the sediment trapping system has proven to be structurally robust. The
dredging system has proven to work effectively and can be operated by one
person. Results from the static performance tests with the facility both drained
and filled with water showed that the 20 traps have an accuracy of well within
the design criteria for the system. The dynamic performance tests showed that
pumping a longshore current in the absence of wave forcing has an insignificant
effect on the variability of the measured load cell data. However, combined
wave and current forcing does increase the variability of the output signal from
the load cells.
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Chapter 4
Sediment Trapping and Dredging Systems
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