The precise positioning of a breakwater usually requires careful study. In the absence of 

 other controlling factors, the alinement should be roughly normal to the primary direction 

 of wave approach to intercept the maximum amount of wave energy with the shortest 

 possible longshore length of structure. The breakwater should be as close to the shore as 

 possible, because, as the water depth increases, the structure cost usually increases. 

 However, the breakwater should not encroach on water area needed for an entrance and 

 fairway in its lee during normal and peak traffic conditions; it should be only as long as is 

 required to effect suitably quiet water for a safe entrance. An exception to these general 

 criteria is the occasional need to make an offshore breakwater long enough to trap littoral 

 drift and to hold it for periodic bypassing operations. A low-height breakwater may have to 

 be located farther off shore to allow for the turbulence that overtopping waves will generate 

 in its lee. A submerged reef may provide a good base for a lower-height structure required at 

 optimum distance from shore. Many coral reefs with wide lagoons in their lee fall in this 

 category. 



Where a harbor is built entirely offshore rather than in a basin behind the shoreline, its 

 entire outer perimeter except for the entrance must generally be a breakwater. The 

 breakwater location is then determined by the harbor configuration as well as by wave 

 characteristics and depth requirements. An example of such a harbor is shown in Figure 22. 

 In this event, transmitted and overtopping waves along the seaward leg of the perimeter may 

 require a second line of defense, such as a smaller inside breakwater or a revetted mole. 

 These secondary structures wiU usually be designed to protect the berthing areas in their lee 

 from surge and low-height waves that penetrate the outer structure. This configuration 

 leaves a parallel-to-shore entrance channel between the inner secondary structure and the 

 outer breakwater in which intermediate-height waves are acceptable. 



An important factor in breakwater design is construction planning. The designer must be 

 aware of construction problems and design within the capabiUties of the equipment that is 

 available. He must be aware of potential sources of materials and lead times required to get 

 them to the site, and he must study possible routes and means of transportation. For 

 example, an offshore breakwater must be constructed with a floating plant, or a temporary 

 trestle must be built to the offshore site for the use of land-based equipment. During early 

 stages of construction, there wiU be no protection against waves; therefore, the work should 

 be done during the most favorable season of the year. Provisions must be made to get 

 personnel and equipment to safety in a sudden storm or wave attack. 



In rubble-mound construction, the bedding layer and internal layers of smaller stone 

 must be placed before the armor layer is applied. The designer must know how this is done, 

 and analyze the difficulty of holding a slope of small stones exposed to sea and swell until it 

 can be protected with armor stone or concrete armor units. He must know boom lengths of 

 cranes and not design sections that can only be built with special equipment that wiU create 

 excessive costs. A transfer point will be required for moving land-source stone onto floating 



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