suspension point in the center of the downstream end of the trap. At each
suspension point a load cell is attached to the frame, and the trap is hung from the
load cell with a stainless steel cable. A stainless steel threaded rod at the upper
end of each cable provides a mechanism for leveling the traps during installation.
Three suspension points were used, rather than four (one on each corner) so that
the load cells only restrain the traps from vertical motion, and not in the other 5
degrees of freedom. This guarantees that the vertical force on the load cell is not
influenced by, for example, the vertical component of a lateral load induced by
hydrodynamic forces, or from a slight rotation of the trap induced by a non-
uniform distribution of sand in the trap. This type of design provides the most
accurate measurement of the quantity of sand in each trap, and also makes it
possible to calculate the center of mass of the mound of sand that accumulates in
each trap. Although calculating the center of mass is not a priority, it could be
useful for determining such things as whether or not there is a relationship
between the longshore position of the center of mass in a given trap, and the
speed of the longshore current in that flow channel. A prototype sediment trap
was constructed and tested during the design phase to optimize the design prior
to construction of the 20 traps. Several improvements to the design were made
while evaluating the prototype.
Figure 21. Two sediment traps are suspended from a set of support frames via
stainless steel cables and load cells
Load cells and design capacities
Hermetically sealed load cells (shear beams) are used to measure the quantity
of sand in each trap. The load cells are made of stainless steel construction and
are airtight and splash proof. The load cells are bolted to the bottom surface of
the upper horizontal member of the support frame so that the entire sediment
trapping system has a low profile, as shown in Figure 22. This provides
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Chapter 4
Sediment Trapping and Dredging Systems
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