Return flow through breakwater gaps 



61. Return currents through the gaps of segmented breakwaters can 

 increase offshore sand losses and possibly cause scour at the segment heads. 

 These currents may pose a hazard to swimmers who venture too close to the 

 structure's gaps. The currents usually occur when the structure is nearly 

 impermeable and low crested so that the water transmitted by overtopping can 

 return only through the gaps or around the ends of the structure. This prob- 

 lem may only arise during storm conditions when significant overtopping 

 occurs. The return currents can become particularly strong if the breakwater 

 is long and has only a few gaps. A method for estimating the magnitude of 

 these currents is presented in Seelig and Walton (1980). Return currents can 

 be reduced by raising the breakwater crest elevation, enlarging the gaps be- 

 tween segments, or increasing structure permeability. 



Beach user safety 



62. Coastal structures can present hazards to beach users who may be 

 inclined to swim too close to the structures or climb on them. A tombolo 

 allows easy access to the breakwater structure and invites such activity. The 

 number of people inclined to wade or swim out to the breakwater can be reduced 

 by placing it farther offshore. A more reasonable treatment of the problem is 

 proper beach supervision. A beach which is protected by a breakwater cannot 

 be used safely by surfers unless the breakwater is segmented and the gaps are 

 quite wide. 



Navigation 



63. If boats are commonly launched from a beach that is to be protected 

 by a long detached breakwater, gaps may be necessary to provide access to the 

 ocean, as was the case at Winthrop Beach. Detached breakwaters may need to be 

 marked with navigation aids if their location or distance offshore poses a 

 potential hazard to boaters. This is especially true if the structures are 

 low crested. 



Construction 



64. Because of the difficulties associated with predicting shoreline 

 changes caused by segmented detached breakwaters, it may be prudent to first 

 build a small prototype test breakwater, or the entire breakwater using seg- 

 ments with large gaps. In subsequent years, the structure could then be ad- 

 justed by partially closing the gaps. The expected shoreline change behind 

 the structure should be compensated for by placing a volume of sand equal to 



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