was undertaken of the site bathymetry (1999) to select the offshore slope that
resulted in the most severe waves at the project site. This occurred near the
southerly limit of the spur where waves propagating from the west-southwest
traveled up a slope of approximately 1:40.
A second consideration in setting the offshore bathymetry is that the physical
model needed to be laid out such that a sufficient depth of water exists at the
wave machine to allow generation of the required storm waves.
During the testing program, concerns were raised that the offshore slope in
the physical model may not be in fact flat enough, thereby allowing larger wave
heights at the test section than would exist in nature. To remedy this problem,
sandbags were placed offshore of the test section to promote depth-limited
breaking. This effectively reduced the nearshore slope to 1:60 from the test
section out to the 33-ft contour (some 500 ft seaward of the test section). By
comparison, the original slope was 1:30 from the test section, which positioned
the 41-ft contour 500 ft seaward of the test section.
Three different offshore bathymetries were modeled over the course of the
test program:
a. The initial bathymetry consisted of a 1:30 nearshore slope extending
seaward to the 57.8-ft contour. From there, a 1:10 slope extended out to
the floor of the flume, which is at the 110.5-ft contour. (The offshore
slope used in the 2-D testing was composed of a 1:30 slope extending
15 m from the test section. This slope extended seaward 980 ft to a depth
of 58 ft). The test section was built on a 6-m-wide flat section at the
viewing windows as shown in Figure A9. This bathymetry was used for
Tests A, B, C, and W.
b. The second bathymetry was a modification of the initial slope to allow
for a sediment bed at the test section. Construction of a 15-cm-deep sand
bed at the test section necessitated a slight steepening of the offshore
region just in front of the test section. This bathymetry was used for
Test D. To alleviate the steepening of the offshore slope sand bags and a
concrete beam were placed offshore to match a 1:60 offshore slope.
c. Once the effect of the sand bed had been evaluated, the sand section was
removed and the bed returned to initial conditions. With the return to the
original water level in the flume, the sandbags were moved further
offshore (but the concrete beams were removed) to try to maintain the
milder 1:60 offshore slope. This bathymetry was used for Tests E, F,
and G.
wave gauges. Tests of these probes show that they exhibit a highly linear water
level to voltage response, with calibration errors of less that 0.5 percent model
scale over the whole calibration range. This represents an accuracy of 0.5 mm in
A19
Appendix A
Stability Analysis of a Submerged Spur, North Jetty, Grays Harbor, WA
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