EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-7-35. Photograph of physical model, Barbers Point Harbor, HI (Briggs et al. 1994)
(1) A harbor basin open to the sea through an inlet can resonate in a mode referred to as the Helmholtz
or grave mode (Sorensen 1986b). This very long period mode appears to be particularly significant for
harbors responding to tsunami energy and for several harbors on the Great Lakes that respond to long-wave
energy spectra generated by storms (Miles 1974; Sorensen 1986; Sorensen and Seelig 1976).
(2) Water motion characterizing the Helmholtz mode is like that of a Helmholtz resonator in acoustics.
It is analogous to the spring-mass system with one degree of freedom, discussed earlier in this section, where
the spring is similar to the basin water surface and the mass represents water in the inlet channel. During
Helmholtz resonance, the basin water surface uniformly rises and falls while the inlet channel water oscillates
in and out. The period of this mode is greater than the fundamental mode. The resonant period is given by
(Carrier et al. 1971).