bathymetry and steep beach slopes so additional features (such as an ebb shoal) could easily be
added. In addition, a fine sand (placed over the concrete bottom in a thick veneer) served as both a
tracer and as a fully mobile bed. A 1:50 undistorted scale was assumed to determine reasonable inlet
dimensions to model; however, other scales can be assumed to accommodate study of specific
processes with the simplified bathymetry. For this study a fully movable-bed was placed along the
entrance channel, from a region just inside the jetties bayward.
Modeled Conditions. The approach was to run initially with a wave-only condition, followed by
waves with a steady-state flood current, and then waves with a tide (scaled at 1:50). The wave
generator was oriented perpendicular to the channel centerline. Initially, the rock jetties were
permeable, but were made impermeable with thin aluminum sheeting placed vertically along the
length of the jetty centerline. This eliminated sand from the beaches passing through the permeable
structure and reaching the channel and changing channel bathymetry. For the wave-only and wave-
plus-steady-state current runs, the water level was +1.5 m relative to mean low water (mlw). The
semidiurnal tide level varied between 0.0 and +1.5 m, mlw, with a period of 105.4 min (Froude time
scale = 1:7.07, model to prototype). The initial condition for the movable-bed portion of the
bathymetry was composed of two different side-slopes for the inner-bank on each side of the
channel.
Inner-Bank Erosion Experiment. The first wave condition was chosen as 2.7-m height, 10-sec
period (scaled to prototype), and no currents were reproduced. The model was run until equilibrium
plan view and vertical profile shape were reached for the given wave and water level, and then the
bathymetry was measured. For this wave-only condition, the lateral run-up produced by the water
level variation as wave crests, then troughs, moved past the terminal end of the structure, initiated the
erosion at the jetty terminus. The run-up and run-down motion at the tip of the landward terminus of
the jetty created a notch in the sand, which gradually deepened, then widened, permitting increasing
wave diffraction to carve out the embayment. The erosion region developed from a straight bank
line into the crenulate shaped embayments as seen in Figure 5. The eroded region reached an
equilibrium shape after about 20 hours model time. The addition of currents with the waves
increased the size of the embayment, but the initiating process was observed to be the same as the
no-current experiment. Sediment eroded from the embayment moved along the edge of the inlet
channel. With the no-current condition, sediment built out into the center of the entrance channel
and reduced the rate of removal from the eroded region. The flood current increased the angle of
wave attack and movement of sediment out of the embayment. With a tide plus waves condition, a
similar shape embayment formed as for previous conditions, larger than the no current condition, but
smaller than the wave plus steady-state flood current. The variation in tidal elevation permitted a
deeper cut in the embayment. With currents (steady-state or tidal) present, the eroded volumes
increased in the range of 8 to 45 percent.
Inner-Bank Erosion Prevention Methods. Follow-up experiments examined potential solutions to
minimize erosion by placing hardpoints to create smaller embayments or altering jetty structure slope
and lateral extension into the channel. A diamond shape (in plan view) was built on the end of the
right jetty and functioned to reduce wave energy at the end of the jetty. Because of its extension into
the beach, it also prevented flanking, as shown in Figure 6, top photo, right side. On the left side
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