0.6
pre-test
40 min
80
130
180
180 - rebuild
0.4
230
280
330
380
430
430_rebuild
0.2
480
530
630
0.0
-0.2
-0.4
-0.6
-0.8
0
5
10
15
20
Cross-shore location (m)
Figure 71. Evolution of beach profile shape for the plunging-breaker case
Note that the pretest profile had a break-point bar that is approximately 1 m
seaward of, and in deeper water than, the bar on the near-equilibrium profile. A
larger incident wave height was used initially in the plunging-breaker
experiment. However, the location of incipient wave breaking was too close to
the offshore-most flow channels. Therefore, a decision was made to reduce the
incident wave height. The position and shape of the bar evolved fairly rapidly in
response to the change in wave height, reaching the near-equilibrium condition
after only a few hours. The inner portions of the profile changed relatively little,
compared to the dramatic changes observed in the bar region. The profile shape
that was molded initially, for the plunging-breaker experiment, was the
equilibrium profile shape that was produced by the irregular waves that had a
similar incident significant wave height but were characterized by spilling
breakers.
The beach profile shape for the spilling breaker case evolved to its near-
equilibrium condition more slowly, taking approximately 14 hr. The difference
is attributed to the fact that the initial profile for this case was molded to a
theoretical equilibrium profile shape, approximated by a series of planar beach
sections. Evolution to equilibrium required much more movement of sand from
the inner- and midsurf zone to the offshore. Evolution to equilibrium for the
plunging-breaker case primarily involved sand movement in the bar region.
Steadiness and repeatability of measurements
Process measurements were made for 10-min sampling intervals. The
instrumentation bridge was stationary at one alongshore coordinate during each
10-min interval. To obtain measurements at other cross-shore transects, the
bridge was repositioned and measurements were acquired for another 10-min
127
Chapter 10
Longshore Sediment Transport Experiments
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