Basin boundaries

Closed

Open

External forcing

Free

Forced

Dimensionality

2-dimensional

3-dimensional

Basin planform

Simple

Complex

(4) Characteristics are defined as follows:

(a) Closed basin - basin is completely enclosed.

(b) Open basin - basin is semi-enclosed, but open to a larger water body along at least part of one side.

(c) Free oscillations - oscillations that occur without external forcing (although some external forcing

was applied earlier to initiate the oscillations).

(d) Forced oscillations - oscillations in response to external forcing.

(e) 2-dimensional - oscillations are independent of one horizontal dimension.

(f) 3-dimensional - oscillations vary in both horizontal dimensions.

(g) Simple - basin planform is a simple geometrical shape, such as a square, rectangle, or circle.

(h) Complex - basin planform is an irregular shape.

(5) A harbor basin generally has several modes of oscillation with corresponding natural resonant

frequencies (or periods) and harmonics. Figure II-7-26 illustrates the fundamental, second, and third

harmonic modes of oscillation in idealized, perfectly reflecting, closed and open two-dimensional basins.

(6) Following this introduction, the process of resonance is discussed in terms of a more intuitive, but

analogous, mechanical system. Closed basins are covered next, mainly in terms of free oscillations and

simple shapes. Although they are not closed basins, harbors or parts of harbors can behave much like closed

basins under some conditions. The presentation is also applicable to enclosed water bodies such as lakes and

reservoirs.

(7) The last parts of the section are devoted to open basins. Open basins are susceptible to oscillations

forced across the open boundary. Because of the limited size of harbors, other types of forcing, such as

meteorological forcing in the harbor, are generally not considered. Both simple and complex shapes are

presented. The final part describes Helmholtz resonance, a very long-period, non-standing wave phenomenon

that causes water levels over the entire harbor to oscillate up and down in unison. Practical consequences of

harbor oscillation, such as vessel motions, mooring line forces, and fender forces, are not presented in this

section. Motions of small boats moored in resonant conditions and possible mitigation measures have been

investigated by Raichlen (1968). Comprehensive reviews of harbor oscillations are given by Raichlen and

Lee (1992) and Wilson (1972).

II-7-32

Harbor Hydrodynamics

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