levels against the toes of the structures fluctuated as the wave conditions 

 were changed, there was little movement of the original bags at the toes. 

 The only significant, distinctly measurable scouring occurred when the tank 

 was drained during testing of structure I and water trapped on the shore- 

 ward side of the structure drained out under it. 



2. Breakwater Performance . 



During testing under four different wave conditions, the crest eleva- 

 tions of the four structures decreased and, excluding the lowest breakwater, 

 structure I, the slopes flattened. None of the breakwaters maintained the 

 constructed configuration under the first wave condition, a result of the 

 unstable performance of the bags. The largest changes in configuration 

 occurred during the first wave condition and the steepest wave condition. 

 Excluding the lowest structure, as the constructed height of the breakwaters 

 was increased, the total change in the front-face slope was larger and 

 the final slope was flatter. The largest slope change for structure IV 

 was from 1 on 3 to 1 on 5.3. The change in crest elevation and widthi 

 increased with structure height for the three submerged breakwaters. The 

 crests of structures II and III lost 1.7 and 2.1 feet of elevation, respec- 

 tively (25 and 18 percent of the respective constructed crest elevations) , 

 and widened from 7 to 13 feet (2.13 to 3.96 meters) and from 14 to 26 feet 

 respectively. In contrast, the emergent crest of structure IV narrowed 

 from 14 to 9 feet (4.27 to 2.74 meters) wide while losing 1.8 feet of 

 elevation or 11 percent of the constructed crest elevation. 



The combinations of breakwater elevation, crest width, and seaward- 

 face slope constructed and produced during testing were inadequate to fully 

 evaluate the effects of configuration on attenuation. Although crest width, 

 wavelength, and wave steepness were expected to affect wave attenuation, 

 the relationship between crest elevation and attenuation was described by 

 only two curves, one through the low wave data of wave conditions a and 

 b and one through the high wave data of wave conditions c and d (Fig. 

 56). In general, the emergent structure attenuated the low waves more than 

 the high waves, while the submerged structures had the opposite effect. 

 For some submerged crest elevations the transmitted wave height was greater 

 than the incident wave height. This negative attenuation was prominent in 

 the low wave data for structure II and occurred to a minor extent in the 

 high wave data for structure I. Only the breakwaters with crests above 

 or slightly below the Stillwater level, structures III and IV, produced 

 wave attenuation greater than 30 percent, but they also sustained the most 

 severe wave damage. 



3. Design Considerations . 



The final configurations of the most effective breakwaters, structures 

 III and IV, are guides to proper design practices producing stable sandbag 

 structures. At the end of testing, structure III, originally 12.1 feet 

 high in 12 feet of water, was 10.5 feet (3.20 meters) high with a front 

 slope of 1 on 4.2 and a crest width of 26 feet. For similar depth and wave 



69 



