rotated so that waves are normally incident to a fictitious entrance gap. The
fictitious gap is centered on the actual gap but with width reduced to the projec-
tion of the actual gap onto a line perpendicular to the incident wave direction.
This approach is inconvenient for automation because much of the diffraction
diagram would need to be accessible to accommodate the variety of incident
waves in a typhoon time-history.
A more convenient approach for oblique waves is to take an appropriately
reduced wave height and apply it as a normally incident wave. Then diffraction
diagrams can be applied directly and the dock area always falls in a very
localized part of the diagrams. The reduction factor used for oblique waves is the
square of the cosine of the angle between the actual wave direction and normal
incidence. For incident wave directions of importance, this approach results in
diffraction coefficients similar to those from the manual analysis previously
discussed.
A Fortran program HARBOR was written to calculate the time-history of
significant wave height at the dock due to diffraction through the harbor
entrance. A diffraction coefficient look up table was compiled for various gap
widths and locations along the dock. Diffraction diagrams for sea conditions
was used to give a diffraction coefficient representative of the actual relative gap
width for each case.
Local wave generation inside Apra Harbor
Waves generated inside Apra Harbor by typhoon winds were calculated with
the relationship (Headquarters, U.S. Army Corps of Engineers 1989)
H sloc = 0.0177U c F 0.5
1.23
(18)
where
Hsloc = significant height due to local wind wave generation, in feet
Uc = local wind speed, in miles per hour
F = local fetch, in miles
This relationship was included in the Fortran program HARBOR. Local fetch
distances from the dock area were included for 10-deg increments in wind
approach direction. Since the time-history of local wind speed is available from
PBL wind model runs discussed previously, a time-history of locally generated
significant wave height at the dock can be calculated.
26
Chapter 3
Modeling Approach
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