presumably from below the neoprene rubber seal along the sides and upstream
end of the traps. Shortly after the facility had been filled, the submerged weight
of the traps was measured and the total weight of the 20 traps was calculated to
be 1,712 kg (relative to 2,888 kg measured with the traps empty and the facility
drained). It was necessary to remove air from beneath the rubber seals. A rod
was used to agitate each trap about the downstream suspension point, which
released air pockets. The traps were reweighed, and the total trap weight was
calculated. The operator then spent a total of 15 min jostling the downstream end
of each trap, allowing a significant amount of air to escape from beneath the
rubber seals. The total weight of the traps was then calculated to be 1,748 kg, an
increase of 36 kg. In other words, about 36 ℓ of air had escaped from below the
seals during the jostling process. The pockets of air below the seals had reduced
the total effective trap weight by about 2 percent. The weight of some individual
traps increased by as much as 6 percent.
If air is not released from below the seals, the air pockets may escape during
an experiment, creating a potentially significant error in the quantity of sand
measured in each trap. Based on the results from this experiment, it has become a
standard procedure to remove as many air pockets as possible from beneath the
rubber seals each time the facility is refilled and prior to conducting an
experiment.
Tests with the facility full of water
The last set of static tests was performed to assess the influence of the
location of the load in the trap. A weight of 96 kg was measured at the upstream,
center, and finally the downstream end of the trap in a similar manner as was
discussed previously with the traps empty and the facility drained. For all three
positions, the maximum variation in the sum of the three load cells did not
exceed 1.0 kg, a maximum error of approximately 1 percent. These errors are
slightly larger than the errors measured with the traps empty and the facility
drained.
Static tests then were conducted to quantify the measurement accuracy of all
20 traps, in a similar manner as previously conducted with the traps empty and
the facility drained. A weight of 96 kg was placed in the center of the first trap,
measured, and then the test was repeated for all 20 traps. The average measured
weight for the 20 traps was 95.81 kg. The maximum and minimum weights
recorded were 98.5 and 91.8 kg, respectively. The standard deviation was 1.5 kg,
or approximately 1.5 percent of the average. These errors are slightly larger than
the errors measured with the facility drained. However, these results are within
the design criteria of developing a sediment trapping system with the ability to
measure the average submerged weight of sand in the 20 traps with an absolute
accuracy of 3 percent.
Dynamic Performance Tests
Dynamic tests were performed to examine the structural integrity of the trap
design while waves and currents were generated in the facility. Three tests were
conducted. The first test was conducted by running all 20 pumps in the LSC
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Chapter 4
Sediment Trapping and Dredging Systems
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