conditions, a stable, permanently submerged breakwater would require a 

 crest width of at least 21 feet (6.40 meters), or three rows of bags and 

 a seaward slope no steeper than 1 on 5. Structure IV, 16.4 feet (5 meters) 

 high in 12 feet of water at the beginning of the test, was 14.8 feet (4.51 

 meters) high at the end with a front slope of 1 on 5.3 and a crest width 

 of 9 feet. An emergent breakwater in similar depth and wave conditions 

 would require a crest at least 14 feet, or two rows of bags wide and a 

 front slope no steeper than 1 on 5.5. 



The configurations mentioned above could be unstable on most open coasts 

 where wave conditions are more severe than those used in the tests. If 

 tidal changes alternately exposed and submerged the crest of a sandbag 

 breakwater, both the crest width and seaward-face slope would have to be 

 chosen to withstand maximum wave impact. The crest would have to be at 

 least 21 feet wide and the seaward slope could be no steeper than 1 on 5.5. 

 Wave conditions higher than 6 feet (1.83 meters) could require additional 

 increase in the breakwater's cross section. In comparison, rubble break- 

 waters built in similar water depths to control beach erosion and able to 

 withstand wave heights greater than 6 feet have crest widths of 12 to 15 

 feet and seaward slopes as steep as 1 on 1.5. 



70 



