the lee of the structure will have the same period as the incident waves. A 

 properly designed segmented detached breakwater provides very effective storm 

 protection. The protected beach can accrete sufficiently to survive storm- 

 generated erosion while maintaining the natural longshore transport rate 

 during normal wave conditions. 



51. The amount of wave energy reaching the lee of a segmented structure 

 is controlled by gap size and diffraction around the breakwater ends. The 

 length of each structure segment should be on the order of one-half the dis- 

 tance to shore. After selecting the ratio of total segment length to total 

 project length, it is necessary to design the number and lengths of the gaps. 

 Gaps should be at least two wavelengths wide relative to those waves which 

 cause the average, nonstorm sediment transport. Wide gaps will cause the 

 shoreline to respond with various spaced salients and embayments. The plan- 

 form relief (i.e., the distance perpendicular to shore from the salient tip to 

 the shoreward-most point on the eroded embayment) will be great, and therefore 

 will not provide uniform storm protection along the project. Increasing the 

 number of gaps and shortening the length of each will promote a shoreline 

 planform of less relief, and thus provide more uniform protection (Figure 19). 

 Figure 20 shows a segmented breakwater constructed near Rome, Italy. Note the 

 more uniform accretion behind the shorter segments. The gaps between the 

 longer segments are probably too small. 



52. Only under certain circumstances will the shoreline directly behind 

 a gap stabilize in a position seaward of its original or filled location. The 

 gap size and the distance offshore must be such that there is significant 

 reduction in wave energy opposite the gap. If the volumetric retreat behind 

 the gaps is less than accretion behind the segments, the project will cause a 

 net increase in the local quantity of littoral material. Unless fill is 

 placed, this increase will come from the existing littoral system and may 

 damage adjacent shores. 



53. If a uniform level of shoreline advance is necessary, segmented 

 breakwaters separated by large gaps should not be used. A single long 

 structure designed to permit wave transmission by either overtopping or per- 

 meability, or a segmented structure with numerous small gaps, possibly in- 

 cluding underwater sills, should be considered to achieve a more uniform 

 shoreline advancement and formation of a perched beach. 



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