depth-limited wave height and period conditions at north jetty following van der
Meer and Pilarczyk (1990) and Vidal, Medina, and Martin (2000) suggest that a
stone diameter of at least 7 to 8 ft might be required to achieve stability. Stones
of this size could be impractical should the structure need to be constructed using
split-hull dump barges. However, it was recognized that the north jetty
environment and submerged spur geometry are generally close to or just beyond
the limits of most laboratory investigations of stability and structure
performance. Furthermore, even in the case of conventional rubble-mound
breakwaters, information is lacking for depth-limited breaking waves (Melby and
Kobayashi 1998). Therefore, conclusions based on previous empirical studies
need to be considered with caution. A physical model study, as documented in
the appendix, was deemed necessary to more fully evaluate structure stability.
A series of large-scale 2-D flume tests was undertaken at the Canadian
Hydraulics Centre (CHC) in Ottawa, Canada, to evaluate the cross-sectional
stability of a proposed spur constructed with a relatively small stone size. Tests
were also undertaken for a structure composed of 4-cu yd geotextile bags
(geobags). Conceptual rock armor and geobag cross-sections are shown in
Figures A1 and A2. Structural stability and general performance were evaluated
for the most severe combinations of wave height and period anticipated at the
site. The interaction between the rock-armored structure and the sand seabed was
also investigated. Design waves for the empirical analysis and physical modeling
were determined from an analysis of wave measurements offshore of Grays
Harbor. Results of the large-scale flume tests conducted by CHC are summarized
in this appendix. Davies (2001) gives a more comprehensive presentation of the
test results.
Development of Design Waves at North Jetty,
Grays Harbor
Design wave heights and periods for the analytical and physical model
investigations were developed by applying the following procedure:
a. Review existing studies and available wave data for the Grays Harbor
entrance.
b. Analyze extremal statistics to determine the offshore wave heights for
various return periods.
c. Analyze joint occurrence of significant wave heights and associated peak
periods at the project site. Extend to expected durations of significant
wave heights and associated peak period combinations over an
anticipated 25-year lifespan for a submerged spur.
d. Select the 10 largest wave events measured at Grays Harbor since 1982.
Determine the typical spectral shape for these extreme wave events.
e. Select prototype design significant wave height, peak period and spectral
shape for an offshore location.
f. Develop scaled significant wave height, peak period, and spectrum for
the wave tank. Adjust for shoaling to determine the correct wave
spectrum given the water depth in the wave tank near the wave
generation.
A2
Appendix A
Stability Analysis of a Submerged Spur, North Jetty, Grays Harbor, WA