10. Test IV . 



a. Construction and Preparation . After the tank was drained, a layer 

 of bags was dry-placed on the front face of the previous structure (Fig. 

 40). Then, the tank was filled and bags were dropped through still water 

 to form structure IV. The crest was 16.4 feet above the sand bed and two 

 rows of bags wide [Figs. 41 and 42). Since the bags in the earlier tests 

 appeared to change shape when wetted, before the initial survey the new 

 structure was subjected to "shakedown" waves of about 2-foot height and 

 12.8-second period in 13.2 feet (4.02 meters) of water, 1.2 feet (0.36 

 meter) deeper than the usual test condition, to saturate and consolidate 

 the part of the breakwater above the Stillwater line. An unrecorded number 

 of shakedown waves were run, soaking the seaward face of the structure with 

 little, if any, wave overtopping. Visible settlement occurred in the front 

 face above the Stillwater level and one bag in the front face was moved 

 out of place. The wet bags had trapped air, reacting like balloons when 

 stepped on. 



b. Wave Changes . In test IV, waves were run in bursts of 10 with a 

 1.5-minute wait between bursts. During wave condition c, after the crest 

 had been damaged, overtopping waves increased the water level shoreward of 

 the breakwater by 1.5 feet. After every four bursts, a 30-minute waiting 

 period was required to equalize the water level in the tank. The waves 

 shoreward of the breakwater were primarily the result of intermittent 

 overtopping from the waves breaking against the front face (Figs. 43 and 

 44) , the reason wave attenuation decreased as wave damage lowered the crest 

 elevation. Unlike the waves in tests I, II, and III which maintained the 

 incident period after passing or breaking over the submerged structures, 

 the transmitted waves in test IV differed from the incident waves in both 

 form and period. 



c. Structure Changes . During the first 60 waves of condition a, 

 many bags were dislodged from the front face of structure IV, the damage 

 starting in the third layer from the top and progressing upward to the top 

 seaward row of bags (Fig. 45). As the wave condition was continued, the 

 seaward row of bags in the upper part of the front face (i.e., from the 

 crest to 3 feet below the Stillwater level) slumped to the lower part of 

 the face (Fig. 46), leaving the shoreward row on the crest almost untouched 

 (Fig. 47). The front slope flattened dramatically and the crest lost 38 

 percent of its width but little of its height (Fig. 48). Wave condition 



b moved scattered bags on the front face downslope about 1 foot, but did 

 not change the height or front slope significantly. During the first burst 

 of waves of condition c, three bags from the remaining row on the crest 

 were displaced onto the backface, decreasing the crest elevation. After 

 the 60th wave, bags on the front face began moving with the wave action, 

 slowly sliding down the face. After 440 waves, the whole upper half of 

 the front face began slumping toward the toe and the bags remaining on the 

 top of the structure began moving shoreward. All bags visible above water 

 appeared to be moving. The slope of the upper half of the front face con- 

 tinued to flatten as the crest elevation continued to decrease. Wave 



49 



