Description of 2-D Physical Model
The 2-D physical model investigation is described in terms of experimental
setup, test program and an assimilation of the results. More detail concerning the
physical model test results may be found in Davies (2001).
Model scale
Physical model studies of breakwater armor stone at Froude-scales between
1:30 and 1:50 are typical and have been successfully used in the past for physical
model studies of conventional rubble-mound structures (Hughes 1993). However,
for a wholly submerged structure in a high-energy breaking wave environment, a
larger scale model is indicated. The relatively small stone size and rock gradation
indicated by the constructability analysis will result in smaller porosity and
smaller transmissivity to fluid flow than for conventional structures. Therefore,
interstitial viscosity may be significant in contrast with the situation of
conventional rubble-mound structures. A larger scale model permits greater
possibility of ensuring the correct scales of turbulence and Reynolds law scaling.
Also, the presence of large breaking waves at north jetty introduces
significant amounts of breaking-generated turbulence that may be a decisive
factor controlling stone stability. Therefore, because of the structure's
submergence and the potential importance of breaking-generated turbulence and
interstitial flow, a large model scale was required. The Wave Research Flume at
the CHC was chosen for the study because it can handle large-scale models (the
flume is 98 m long and 2.8 m deep, and can generate waves up to 1.5 m high).
The flume also offers viewing windows at the test section to allow visual
assessment of structure performance.
A 1:20 Froude-scaled model of the spur cross section was built in the Wave
Research Flume (Figures A9 to A13).
Scaling of rock weight
The model scale factor of the armor stone must be carefully selected to
ensure that the submerged stability of the stone is reproduced correctly. This is
required in cases where the densities of the model stone and prototype stone
differ, and where fresh water is used in the model to represent sea water.
The Hudson formula (Shore Protection Manual 1984) is a widely accepted
equation for the stable weight of submerged armor material under wave attack.
Scaling armor weights using this formula will yield a model stone or armor unit
having the same stability as the prototype condition.
A13
Appendix A
Stability Analysis of a Submerged Spur, North Jetty, Grays Harbor, WA
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