5
Harbor Oscillations
To evaluate harbor resonance characteristics, the CGWAVE numerical model
was run for the existing harbor. Incident longwave periods ranged from 25 sec to
1,000 sec in very fine increments, as discussed in Chapter 3. These evaluations
were included because oscillations can be an important part of interpreting the
harbor wave response and identifying suitable alternative harbor sites. Amplifi-
cation factor results are presented in the following section. Discussion of the
results relative to operational performance criteria is given in the final section of
this chapter.
Amplification Factors
Background
Amplification factors for the long waves involved in harbor oscillation
behave differently than those for wind waves and swell. Long waves, because of
their length relative to harbor dimensions and their reflectivity from harbor
boundaries, form standing wave patterns in the harbor. Standing wave behavior
in a simple closed basin of uniform depth is illustrated in Figure 17. In the
fundamental mode of oscillation, antinodes occur at both basin walls and a node
midway between walls. The distance between walls is equal to one-half of the
oscillation wavelength. Second and third modes of oscillation are also
illustrated. Antinodes always occur at the walls. Additional antinodes and nodes
occur at regular intervals between walls, with the number of antinodes and nodes
dependent on the mode of oscillation.
The water surface in a standing wave has its greatest vertical motion at
antinodes. There is no vertical movement at an ideal node, but horizontal
velocities reach a maximum there. In terms of amplification factors, Aamp,l , this
behavior gives large values of Aamp,l at antinodes and small values around nodes.
Contrary to wind waves and swell, small values of Aamp,l are not necessarily
indicative of a tranquil harbor area.
Phases in a standing wave also behave differently than for typical wind
waves and swell. For example, the water surface in the fundamental mode of
oscillation in Figure 17 simultaneously reaches a maximum at every point to the
left of the node. These points are all in phase. At the same time, every point to
the right of the node reaches a minimum value. These points are also in phase
29
Chapter 5 Harbor Oscillations
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