A few diagrams tracing the divergence factors from deep water into the entrance for the 

 most critical wave periods revealed by statistical hindcast analysis will indicate the relative 

 effectiveness of any entrance plan for attenuating waves. Once the approximate layout plan 

 is determined by this method, it may need to be checked by model analysis and the 

 protective structures adjusted to achieve maximum attenuation of the waves. 



Some harbor sites have natural entrances that cause a reduction of the height of waves 

 entering from the main water body. This can occur as a result of island shielding in an 

 embayment or an entrance located within a series of prominences in a roadstead or sound. 

 In tliis case, the need for protection is reduced in proportion to the degree of wave 

 attenuation at the site. The historical records may show that only minimal protective works, 

 if any, are required. 



If the site is close to a large water body and an artificial entrance must be provided, a 

 model study may be needed to ensure that the required degree of wave attenuation in the 

 inner basins is achieved (Fig. 12). If wave height just outside the proposed artificial entrance 

 exceeds 10 feet, construction of the entrance and its protective structures will be costly and 

 a harbor capacity of several hundred boats must be provided to ensure enough revenues to 

 defray the initial cost outlay. Cost of such a harbor may exceed a million dollars and the 

 additional $50,000 to $100,000 cost of a hydraulic model study is worth the ensurance that 

 the inner basins wUl be free from troublesome wave action. 



Some harbors are so large that troublesome, short -period waves may be generated within 

 the harbor confines by strong winds blowing down long fairways or the wakes of numerous 

 power craft may combine to produce undesirable turbulence in the berthing areas. Where 

 small craft and large commercial vessels share the same harbor, the wakes of the large vessels 

 may penetrate the small-craft berthing areas. In such cases, the probability of troublesome 

 interior wave generation must be determined in the planning stage and corrective action 

 taken through the proper arrangement of moles or the provision of wave baffles as described 

 in Section V. 



b. Surge. Waves of long period and great length with low height are termed surge. The 

 most troublesome aspect of surge in a harbor is its horizontal water motion or oscillation, 

 which causes stress in mooring lines and anchorage systems and can make the maneuvering 

 of boats into slips difficult. Ordinary wind-generated waves tliat penetrate the entrance of a 

 harbor may acquire the characteristics of surge in the inner basins. If the period of the surge 

 oscillations in the basins correspond to the period of the sea or swell outside the entrance, 

 then the exterior sea or swell is the cause of the surge within the harbor. 



If the harbor surge period differs from the wave period outside, the oscillations may be 

 due to harbor geometry with respect to the wave input at the entrance. Most waves that 

 penetrate the entrance and reach the interior basins with a significant amount of residual 

 energy will usually have a length that is much greater than the water depths of these basins. 



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