The magnitude of other sources of configuration changes could not be 

 measured except in rare instances when an area in a breakwater changed 

 elevation with no apparent bag movement. Such an area was found when the 

 surveys of structure II were inspected after emptied, torn bags were dis- 

 covered in the structure. Comparison of the surveys showed that the bags 

 probably leaked until empty during the steepest wave condition, reducing 

 the elevation in the area by 1.5 feet. Significant bag leakage also 

 occurred in structure I, but the effects could not be isolated. 



Attempts to estimate the amount of settlement into the sand bed were 

 hampered by changes in the elevation of the sand bed near the structures. 

 Sand levels against the bottom layer of bags at both toes at the beginning 

 and end of each test were compared using corresponding photos and surveys. 

 Any settlement that occurred did not exceed the change of sand elevation 

 due to sand movement near the toes. The average profiles indicate that 

 wave conditions a and c caused erosion, and wave conditions b and d 

 caused accretion at the toes of the structures. The sand elevation 

 changes during each wave condition were about 0.5 foot for tests I and II, 

 and 0.25 foot (0.08 meter) or less for tests III and IV. Since erosion 

 and accretion alternated as the wave conditions were changed, the total 

 amount of scour was small except at the rear toes of structures I and II. 

 Scour in and around structure I during the damaging midtest tank draining 

 caused appreciable settlement into the sand bed, lowering the crest eleva- 

 tion 0.3 foot or 10 percent of constructed crest elevation. 



e. Final Structure Configurations . The configuration of each struc- 

 ture at the end of a test was the most stable configuration for a sandbag 

 breakwater in water 12 feet deep and subject to the range of wave condi- 

 tions used for testing. Sandbags alone are an unstable structural material 

 when used in a wave environment; consequently, the total change in the 

 configuration of a sandbag breakwater before reaching stability is an 

 important design consideration. Noting the initial and final crest eleva- 

 tions (Table 4), the reduction in elevation from the constructed config- 

 uration to the final configuration (and the change as a percentage of the 

 constructed crest elevation) was 0.7 foot (0.21 meter) or 24 percent for 

 structure I (0.3 foot of this occurred during the midtest tank draining 

 and the following 4 hours of wave action); 1.7 feet or 25 percent for 

 structure II; 2.1 feet (0.64 meter) or 18 percent for structure III; and 

 1.8 feet (0.55 meter) or 11 percent for structure IV. For structures II, 

 III, and IV, 1.1 feet of change was due to loss of a bag layer. 



To ensure that a submerged breakwater constructed in the field will 

 maintain a stable crest elevation, the crest must be wide enough to prevent 

 loss of the entire crest layer of bags. For emergent breakwaters, the 

 front slope is more critical than the crest width. The crest of structure 

 I remained about one row of bags wide, despite the disruption of the bag 

 arrangement by the midtest tank draining and subsequent wave action. Struc- 

 ture ,11 lost bags from the single row across its crest until the remaining 

 bags formed a crest approximately 14 feet or two rows of bags wide. Bags 

 lost from the crest of structure III, originally two rows of bags wide. 



63 



