ERDC/CHL CETN-IV-26
June 2000
Inlet Modifications. Effects to channel depth brought about by jetty modification
and extension, construction of additional structures, and navigation channel
realignment can be simulated. However, regions of deposition will require a source
of infilling sediment.
Modeling Caveats: Two caveats apply to movable-bed modeling of the situations described
above. First, this modeling technology pertains only to the equilibrium condition where the
movable bed no longer evolves. Sediment transport rates and the time required to reach
equilibrium are not correctly simulated in the movable-bed model. In other words, the model
will provide reasonable predictions of the final equilibrium bed configuration, but no information
is gained regarding how long this evolution will take in the real world. This is actually beneficial
because it precludes having to simulate tidal cycles in the model. At real inlets, the final
equilibrium condition is maintained for the most part by the peak ebb or flood flow. During the
rest of the tide stage, currents are reduced and sediment motion is greatly reduced. Therefore, in
the movable-bed model we only need to simulate the maximum flow condition long enough to
achieve an equilibrium.
The second caution applies to deposition areas in the movable-bed model. Sediment deposition
will occur in areas where the local flow velocity falls below the sediment incipient motion
threshold. Sediment moved into this region is deposited. The scaling premise of the movable-
bed model scaling relationship covers this situation, but there must be available sediment to have
deposition. At real inlets, sediment sometimes comes from regions of the inlet that are actively
scouring because of structural modification or change in flow condition. However, most
sediment moving through an inlet is being continually introduced via the longshore drift. Some
sediment swept into the inlet deposits on the flood shoal, some deposits in the dredged
navigation channel, and some moves out of the inlet to the ebb shoal. Therefore, if a sediment
deposition problem such as channel infilling is to be studied in a movable-bed model, it is
necessary to introduce the correct quantity of sediment into the model at the appropriate
locations.
The best modeling practice is to validate the movable-bed model by reproducing the existing
condition before exploring design alternatives. This can only be done for regions of inlets that
are close to equilibrium. Provided the movable-bed model bed evolution is in reasonable
agreement with the prototype, then the effects of inlet modifications can be studied with greater
confidence that the model results are good predictors of the expected behavior of the real inlet.
Hughes and Schwichtenberg (1998, 1999) successfully validated a movable-bed model of scour
that occurred on the lee side of the Ventura Harbor breakwater. Once validated, the model was
used to design suitable scour protection for the breakwater toe.
Example Application: Determine the parameters for a movable-bed model of the entrance
channel at Shinnecock Inlet, New York. The size of the model facility requires the prototype-to-
model scale ratio to be NL = 75, and the model sediment is quartz sand with a median grain size
of (de)m = 0.13 mm.
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