As a result of these two-dimensional experimental studies, Chen and Wiegel 

 (1969) deduced that the slope-floating beach with pontoons, the twin-pontoon 

 breakwater, and the fixed-dissipator breakwater were the most effective of 

 those concepts tested. Scale effects, mooring forces, and mooring line elas- 

 ticity were not evaluated in these investigations. 



XIV. SUMMARY AND CONCLUSIONS 



1. General Considerations. 



A need exists in coastal and semishielded bay regions for a means of 

 protecting certain operations and shoreline features from the effects of 

 excess wave energy. Permanently fixed breakwaters provide the highest degree 

 of protection and can be exceedingly expensive to construct under certain 

 conditions. Floating breakwaters, however, provide a lesser degree of pro- 

 tection, but they are generally less expensive and movable from one location 

 to another or replaceable if circumstances such as floe ice dictate. Floating 

 breakwaters should be evaluated as a viable alternative when the cost of a 

 fixed structure exceeds the economic return to be gained at that location. 



The construction cost of a fixed rubble-mound breakwater increases expo- 

 nentially with water depth, while a floating breakwater requires essentially 

 the same structural features regardless of the depth. The cost of a floating 

 system is only slightly dependent on water depth and foundation conditions. 

 The interference of a floating breakwater with shore processes, biological 

 exchange, or circulation in bays and estuaries is minimal. Floating break- 

 waters also have a great multiple use (if pontoon-type structures) of serving 

 as boat docks, moorings, or walkways. 



There are certain disadvantages with floating breakwaters to be weighed in 

 an evaluation. The design of a floating system must be carefully matched to 

 the site conditions, with due regard to the longer waves which may arrive from 

 infrequent storms. The floating breakwater can fail to meet its design objec- 

 tives by transmitting a larger wave than can be tolerated without experiencing 

 structural damage. Since this is a mobile system, cumulative fatigue stresses 

 may eventually lead to system collapse. Uncertainties in the magnitudes of 

 applied loads and lack of maintenance cost information dictate conservative 

 design practices which increase initial project cost. 



Adequate wave reduction or energy attenuation can be attained by a float- 

 ing breakwater only if the incident wave is of a relatively low height. A 

 reasonable magnitude appears to be an incident wave height not exceeding 4 

 feet, with a corresponding wave period not exceeding 4 seconds. Floating 

 breakwaters can attenuate waves with these incident characteristics to a 

 magnitude tolerable in a small-craft mooring area (wave heights up to 1.5 

 feet). The location of a floating breakwater should not be subjected to 

 design waves exceeding the 4-foot-high, 4-second condition, except for open- 

 ocean applications of a distinctly different concept formulated to withstand 

 substantial increases in the incident wave characteristics. The structure and 

 mooring system should be designed to withstand maximum probable forces from 

 the maximum storm wave which occurs very infrequently. 



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