Figure 54 shows the instantaneous submerged weights measured with Sediment
Trap No. 2 during the first 2 hr of test segment Test1H_Case2, which had a total
duration of 3.33 hr. Because of a brief delay at the beginning of this test
segment, the load cells were sampled for several minutes prior to starting the
wave generators. The instantaneous submerged weight measurements contain
considerable fluctuation as a result of the vibrations of the sediment traps caused
by wave and current forcing. The fluctuations tend to be greatest on the
downdrift load cell, where the trap is supported midway across the end of the
trap, rather than at both downdrift corners; as is the case for the updrift end of the
trap. The reason for this design was discussed previously in Chapter 4. The
summation of the three time series also is shown, which represents the total
submerged weight of sand in Sediment Trap No. 2. Although the overall trend
in the data can be accurately quantified, identification of the influence of large
individual waves or groups of waves is not possible because of the noise level in
the measured time series.
140
Downdrift Load Cell
130
Updrift Load Cell #1
120
Updrift Load Cell #2
110
100
Total Weight of Trap #2
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40 50 60 70 80 90 100 110 120
Elapsed Time (minutes)
Figure 54. Instantaneous submerged weight of Sediment Trap No. 2 during the
first 2 hr of test segment Test1H_Case2
There is one other potential error that must be addressed to obtain high-
quality longshore sediment transport data sets. During some experiments, a low
profile submerged shoal tends to accumulate immediately upstream of the
downstream wave guide. This is attributed to imperfections in the downstream
lateral boundary, which are essentially unavoidable when conducting moveable-
bed experiments in the laboratory. More specifically, the cause of this shoal
formation is attributed to a decrease in wave energy immediately upstream of the
downstream wave guide, resulting from a loss of wave energy under the wave
guide. This localized wave height gradient causes a small portion of the
suspended sediment to fall out of suspension and slowly accumulate in the form
98
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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