the ratio of that crest elevation to the water depth, the data points 

 follow the two curves in Figure 56, approximately the upper and lower 

 bounds of the data. Data for the 4- and 3.9-foot (1.22 and 1.19 meters) 

 wave heights of wave conditions a and b and for the 5.8- and 5.7-foot 

 (1.77 and 1.74 meters) wave heights of wave conditions c and d appeared 

 to be grouped together along the curves; however, no relationships based 

 on crest width, wave steepness, or wavelength were apparent, perhaps due 

 to the scarcity of data or to wave reflection and setup effects. The 

 curves show that submerged breakwaters attenuate the high waves more than 

 the low waves, but this effect is reversed as the crest elevation 

 approaches or exceeds the Stillwater level. The transmitted wave height 

 is larger than the incident wave height over segments of both curves, 

 most prominently for wave conditions a and b in the region of data from 

 test II. The negative attenuation occurred for waves which did not break 

 or broke in a complex manner over a distance shoreward of the breakwater. 



To effect more than 30-percent attenuation in wave height, the crest 

 elevation to water depth ratio of a submerged sandbag breakwater must be 

 greater than 0.70, requiring the crest to be at a depth where wave forces 

 are most severe. Of the three submerged breakwaters, structure III, with 

 its crest in the severe wave action region, underwent the largest changes 

 in crest elevation and crest width. The data are sufficient to conclude 

 that an effective sandbag breakwater producing significant changes in wave 

 height will be susceptible to damaging amounts of bag movement and must 

 be designed and constructed carefully to maintain a stable configuration. 



V. GENERAL OBSERVATIONS 



1. Sandbag Dimensions and Weights . 



The nylon bags measured 8 by 5 feet when empty and 7 by 4 by 1.1 feet 

 when filled with damp sand to 75 percent of capacity. When bags were 

 immersed and undisturbed by other bags landing on top of them, they 

 retained an air pocket that slowly leaked until, after 17 hours, a stable 

 air volume was reached before all air had escaped. The time required to 

 reach the stable state increased with increasing material weight or weave 

 "tightness" and decreased with increasing immersion depth. When six bags 

 were weighed to determine the effects of air trapment, the average weight 

 was 2,629 pounds before immersion and 1,488 pounds while immersed after 

 puncture to release all trapped air. After release of trapped air, indi- 

 vidual bags weighed as much as 325 pounds more than their weight when 

 first immersed and 85 pounds (39 kilograms) more than their immersed weight 

 with a stable air volume immediately before puncture. 



2. Sandbag Problems and Improvements . 



Besides buoyancy from trapped air, other problems encountered during 

 testing were actinic deterioration, closing filled bags, and handling 

 filled bags, especially when frozen. A single sand-filled, uncoated nylon 

 bag exposed to direct sunlight for 18 months tore open. Since the time of 



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