approach can be considered to reduce runup and overtopping. For overtopping,
VJ recommend a short-crested wave reduction factor of
γβ = 1 - 0.0033β
(17)
where
β = angle between wave approach direction and shore normal, in deg
Harbor Side Analysis
Flooding in the proposed project area can be affected by processes on the
south side of Cabras Island, along the commercial docks, as well as by processes
along the ocean side. Harbor side considerations are somewhat different than
those along the ocean side. Key concerns are waves at the dock face combined
with elevated swl during typhoons, which can potentially cause water to rise up
onto the dock apron. Typhoon effects on water level for the dock area are
available from ADCIRC modeling, since the model grid includes a detailed
representation of the commercial harbor.
Waves arriving at the dock are principally from two sources. Waves
generated by typhoon winds in deep offshore areas can propagate through the
entrance gap between Glass Breakwater and Orote Peninsula, and continue
across the harbor to the dock area. The second source is waves generated inside
the harbor by local typhoon winds. Although harbor fetches are short, they are
sufficiently long that winds during the intense part of a nearby typhoon can
generate significant wave heights of up to 1.5 m (5 ft) or more. The approach for
estimating waves from these sources is discussed in the following subsections.
Wave propagation through harbor entrance
Wave propagation through the harbor entrance is estimated using informa-
tion from Figures II-7-14 through II-7-17 in Headquarters, U.S. Army Corps of
Engineers (2001). The figures give diffraction coefficients for normally incident
waves propagating through a breakwater gap of various widths. Gap width is
expressed as a multiple of the incident wavelength. The figures are based on
tests with directionally spread irregular waves. Considering exposure of the dock
area to waves coming through the entrance, the relatively uniform water depth in
the harbor along the path between dock and entrance, and the configuration of
the entrance gap, these diffraction figures are judged to be a reasonable
approximation.
To automate diffraction analysis for application to the time-history of
typhoon conditions, several adaptations were necessary. First, the figures repre-
sent only waves normally incident to the breakwater gap. In a typical manual
analysis of oblique wave approach, the diffraction diagram would effectively be
25
Chapter 3
Modeling Approach
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