upstream and side wall sheets by trimming the overlapping upstream sheet and
binding the two sheets with a special cement and stainless steel rivets. Silicone
caulking also was used to aid in the sealing process.
Dredging System
During the course of an experiment that has been in operation for a
significant amount of time, the sand that has accumulated in the traps is dredged
from the traps and discharged onto the updrift end of the beach. Depending on
of sand that accumulates in the traps prior to dredging can range from 1,000 to
2,000 kg, or even greater. The decision to dredge the traps is dictated by one or
both of the following criteria:
a. The updrift end of the beach has eroded significantly, causing a
perturbation in the bathymetry and, hence, the nearshore processes in that
region, because sand is not fed onto the beach during the experiment.
b. Sand has accumulated at the updrift end of the sediment traps, causing a
potential obstruction to the flow of current into the downdrift flow
channels.
An 11-hp trash pump is used to dredge and transport a sand slurry through a
series of reinforced flexible hoses to the upstream end of the beach. It is
common practice in the construction industry to always select the diameter of the
suction hose to be larger than the diameter of the discharge hose and to minimize
the length of the suction hose to avoid the possibility of cavitation inside the
pump chamber. For this pump application, however, it was not possible to follow
this rule of thumb. The standard size suction hose to fit this pump has a diameter
of 100 mm. However, it proved to be far too cumbersome for one person to
manually move this large-size hose from trap to trap when filled with a sand
slurry during the dredging operation. Therefore, after some trial and error
testing, the optimum solution was to use a 50-mm suction hose and a 100-mm
discharge hose. All of the hose is designed with wire braided noncollapsible
walls so that the hose does not collapse when draped over the top of the walls of
the flow channels and exterior walls of the basin.
The sand slurry is discharged directly onto the upstream end of the beach. A
diffuser pipe approximately 3.0 m in length is used to distribute the sand slurry
onto the beach to minimize erosion of the beach. The diffuser pipe can be moved
to different locations across the width of the surf zone so that the sand slurry fills
regions where the greatest amount of erosion has occurred. This system has
proved to work quite effectively.
Static Performance Tests
Tests with traps empty and the facility drained
After the traps had been fully constructed and sealed with neoprene rubber,
several tests were conducted to assess the mechanical performance and
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Chapter 4
Sediment Trapping and Dredging Systems
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