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CHAPTER 6. SEDIMENT TRANSPORT MODELS
Besides the operational limitation imposed by wave generators and current generating machinery, movable-bed models have several additional construction and operational considerations. When sand is initially placed in
the model it needs to be saturated and compacted by wave action. This
will also help to wash out some of the very fine dust-like particles, which
then can be drained off with the water. After compaction, additional sand
will probably be needed to bring the bed profile up to its initial elevation.
In 2-d flume studies a reference grid can be drawn on the glass side wall
along with a sketch of the profile. This enables quantitative information on
profile evolution to be obtained from photographs.
In 3-d studies the lateral boundaries of the model present a problem because longshore currents transport sand to the downdrift boundary where
it needs to be removed so it doesn’t accumulate. Likewise, an equal amount
of new material must be introduced at the updrift model boundary to maintain a sediment budget in the model. Of course, specific circumstances must
be considered such as terminal structures that trap sand at the downdrift
end.
It is well known that running waves for extended periods of time in a
wave tank will build up the long-period seiching energy in the tank; however, little is known of how this affects movable-bed model studies. Dally
(1991) compared profile development in a mid-scale wave flume for an irregular wave train that was run as a series of short bursts of 5 minutes
duration and as a series of long bursts lasting 30 minutes each. He reported significant differences in equilibrium profile development with the
longer duration bursts which tended to smooth the profile and shift the
bar shoreward. Therefore, the length of wave run should be considered in
the operation of a movable-bed model. This is less of a problem in basin
studies.
Remolding the bottom topography is difficult and time-consuming, even
for two-dimensional tests in a wave tank. Therefore, the experimenter
should be careful about designing and conducting a testing program in
order to minimize the number of tests in the program and to assure no
tests have to be repeated because critical data were not obtained. An
additional problem occurs if the model sediment has a wide distribution or
a bimodal distribution. Sediment sorting is likely to occur during the test,
and it will be necessary to re-mix the sediment before further tests can be
conducted. And finally, it is important to deny cats access to your model
sand supply!
CHAPTER 6. SEDIMENT TRANSPORT MODELS
Besides the operational limitation imposed by wave generators and current generating machinery, movable-bed models have several additional construction and operational considerations. When sand is initially placed in
the model it needs to be saturated and compacted by wave action. This
will also help to wash out some of the very fine dust-like particles, which
then can be drained off with the water. After compaction, additional sand
will probably be needed to bring the bed profile up to its initial elevation.
In 2-d flume studies a reference grid can be drawn on the glass side wall
along with a sketch of the profile. This enables quantitative information on
profile evolution to be obtained from photographs.
In 3-d studies the lateral boundaries of the model present a problem because longshore currents transport sand to the downdrift boundary where
it needs to be removed so it doesn’t accumulate. Likewise, an equal amount
of new material must be introduced at the updrift model boundary to maintain a sediment budget in the model. Of course, specific circumstances must
be considered such as terminal structures that trap sand at the downdrift
end.
It is well known that running waves for extended periods of time in a
wave tank will build up the long-period seiching energy in the tank; however, little is known of how this affects movable-bed model studies. Dally
(1991) compared profile development in a mid-scale wave flume for an irregular wave train that was run as a series of short bursts of 5 minutes
duration and as a series of long bursts lasting 30 minutes each. He reported significant differences in equilibrium profile development with the
longer duration bursts which tended to smooth the profile and shift the
bar shoreward. Therefore, the length of wave run should be considered in
the operation of a movable-bed model. This is less of a problem in basin
studies.
Remolding the bottom topography is difficult and time-consuming, even
for two-dimensional tests in a wave tank. Therefore, the experimenter
should be careful about designing and conducting a testing program in
order to minimize the number of tests in the program and to assure no
tests have to be repeated because critical data were not obtained. An
additional problem occurs if the model sediment has a wide distribution or
a bimodal distribution. Sediment sorting is likely to occur during the test,
and it will be necessary to re-mix the sediment before further tests can be
conducted. And finally, it is important to deny cats access to your model
sand supply!