Crest elevations for structures I, II, and III decreased most and 

 crest widths for structures II, III, and IV changed most during wave con- 

 dition a. Structure II with a crest submerged nearly 50 percent of the 

 tank depth, and structure III with a crest at the Stillwater level, lost 

 approximately 1.2 feet of height (17 and 9 percent of constructed height, 

 respectively) and gained crest width as the top layers were displaced 

 down the faces of the structures. In contrast, structure IV' s wide crest, 

 approximately 4 feet above the water surface, lost one-third of its width 

 but maintained its elevation. As expected, the first waves caused settle- 

 ment and bag movement in the loosely constructed breakwaters, with resulting 

 large changes in crest elevation and width. 



Large crest elevation changes also occurred during wave condition c, 

 the changes increasing with the increase in the constructed heights of 

 the breakwaters. Structure I lost only 0.1 foot (0.03 meter) or 3 percent 

 of constructed crest elevation, whereas structure IV lost 1.7 feet (0.52 

 meter) or 11 percent of constructed crest elevation, along with the other 

 two-thirds of the original crest. The maximum elevation changes from wave 

 conditions b and d were only 0.1 and 0.2 foot (0.03 and 0.06 meter), 

 respectively, and crest width changes were correspondingly small. The 

 largest crest elevation and width changes were caused by the two steepest 

 wave conditions. A crest width of two bags was inadequate to prevent 

 displacement of the entire crest layer during the steepest wave conditions. 



The front-face slopes of the breakwaters did not change uniformly 

 during the tests (Fig. 55). For each breakwater, the type of slope change, 

 flattening or steepening, varied with the wave conditions, but a given 

 wave condition did not cause the same type of change in all four breakwaters. 

 The largest changes occurred during wave conditions a and c, the steepest 

 conditions. The slopes of the submerged structures I, II, and III changed 

 most during wave condition c. For the emergent structure IV, slumping 

 during wave condition a caused the largest slope change, the slope flat- 

 tening from 1 on 3 to 1 on 4.2. During wave condition c, the slope 

 flattened further, from 1 on 4.2 to 1 on 5.3. The seaward face of an 

 emergent sandbag breakwater, especially the area near the Stillwater level, 

 is particularly vulnerable to severe damage from wave attack. 



d. Sources of Change . Compared to consolidation of sandbags and 

 settlement into the sand bed, bag movement Was primarily responsible for 

 the changes in the breakwater profiles. Bag movement was concentrated in 

 the area of strongest wave activity, on the crests of structures II and 

 III and near the Stillwater level on the front face of structure IV. Bag 

 movement and resulting changes in structure configuration were most exten- 

 sive during the steepest wave conditions a and c, when the top layers 

 of bags were displaced off the crests of structures and down the faces. 

 During tests III and IV, wave conditions a and c also produced the 

 largest amounts of wave setup, or water level increase, shoreward of the 

 structures, but the effect on bag stability was generally undetermined. 

 The only change clearly attributable to wave setup occurred during test 

 III, wave condition c, when the accumulation of setup appeared to stop 

 bag movement. 



62 



