setup) , due to the tractive force of stormwinds blowing over the water 

 surface in one direction, and the consequent release of the water when 

 the wind shifted in direction or was reduced in speed over a short period 

 of time. However, common usage of seiche now applies to both free and 

 forced oscillations of enclosed and semienclosed bodies of water. Seiches 

 in harbor basins are forced oscillations with periods that are generally 

 determined by the periods of the incident waves which enter the basins 

 from the ocean area surrounding the harbor, rather than the natural or 

 free oscillation periods of the basin. A resonant oscillation occurs 

 when the period of the forcing oscillation is equal to the period of the 

 fundamental (gravest mode) or a harmonic of the natural period of oscil- 

 lation of the basin waters. Water oscillations with periods greater than 

 those of the third harmonic of the fundamental are believed to be rare, 

 except for harbor basins that are somewhat rectangular in shape, because 

 the energy in the higher harmonics can be dissipated more easily by im- 

 perfect reflections from the irregular boundaries. Standing wave systems 

 with periods in the intermediate- and long-wave range have large horizon- 

 tal excursions in the nodal areas, even when the vertical amplitudes are 

 as small as 6 inches to 1 foot. Ships moored elastically in or near the 

 nodal areas of such oscillations in a harbor basin are subjected to oscil- 

 latory forces that can cause mooring lines to break and damage the ship 

 or pier or, at a minimum, make loading or unloading of cargo difficult 

 or impossible. The resonant motion of a moored vessel is a function of 

 the vessel's mass and the length, number, position, and elasticity of 

 the mooring lines. Thus, resonant ship motions may occur when the water 

 masses in the harbor basin are not oscillating in a resonant mode. How- 

 ever, the worst conditions for the shipowner would be when both the harbor- 

 basin waters and the moored ship were oscillating in resonance with the 

 forcing oscillations from outside the harbor. 



The above discussion shows that the designer of a harbor faces a for- 

 midable task where relatively large vessels are moored elastically for 

 loading and unloading cargo, and v/here the harbor is exposed to rela- 

 tively long-period waves. From a practical approach, the designer should 

 determine the wave energy spectrum that exists outside the harbor and then 

 select water depths and horizontal dimensions of the harbor basins to de- 

 tune the basins from the peaks in the intermediate- and long-period part 

 of the energy spectrum of the outside waves. The detuning of the harbor 

 basins is difficult, if not impossible, except for one or two of the inci- 

 dent wave periods, because the basin geometry and dimensions are largely 

 determined by the requirements of ship navigation into and out of the 

 different liarbor basins, the required turning basin areas, and the water 

 depths necessary to accommodate the larger, deep-draft vessels. For 

 optimum mooring conditions, the harbor designer should be able to detune 

 the oscillating characteristics of the moored ships from both the periods 

 of the outside forcing function and the resonant periods of the water 

 masses in the harbor basins. This stage of the detuning process, to be 

 successful, would require that the energy spectrum of the forcing oscil- 

 lation contain a minimum of peaks, and that the harbor designer has both 

 the authority to establish the method of ship mooring and the technical 

 information and ability to determine the characteristics of the mooring 



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