base of the structure during condition c was moved back during condition 

 d, forming mounds at both toes of the structure (Figs. 17, 18, and 19) 

 and flattening the front slope, but ca.using no change in crest elevation. 

 Wave- induced changes in the structure were small compared to the disrup- 

 tion when the tank was drained. 



8. Test II. 



a. Construction . After the completion of test I, the tank was drained 

 and an 8-inch-diameter pipe was installed to drain water from behind the 

 test structures (Fig. 20). To install the pipe, attempts were made to 

 remove some bags intact, but some half -buried bags tore when grasped by 

 the crane's bag grip, spilling sand on the structure. After installation 

 of the pipe, the sand bed was regraded, the first layer of bags for struc- 

 ture II was placed (Fig. 21), and the tank was refilled. The successful 

 construction of structure I through waves indicated that wave action during 

 construction had little affect, on the constructed shape of the breakwater; 

 consequently, the remaining four layers for structure II were dropped into 

 place through still water, resulting in a breakwater 7 feet high. 



b. Wave Changes . The transmitted wave height for small waves was 

 slightly greater than the incident wave height, but the large waves under- 

 went 5 to 10 percent attenuation. Waves of condition a plunged over the 

 structure and continued to spill until reformed about 80 feet (24.38 meters) 

 behind the structure. Waves of condition b peaked-up like those in test 

 I. Waves of condition c and d peaked-up and plunged as they passed the 

 structure, then spilled shoreward of the structure (Figs. 22 and 23). 



c. Structure Changes . During wave condition a the top layer of 

 bags was displaced off the structure crest, the backface slumped and the 

 front face settled, resulting in a decrease in crest elevation, an increase 

 in crest width, and a flattening of both slopes despite scour at the rear 

 toe (Fig. 24). Changes during wave condition b were negligible. Although 

 more scour occurred at both toes during wave condition c, bags in the 

 backface were displaced off the structure or, if damaged, stayed in place 

 and leaked sand, flattening the back slope. The front face settled and 

 bags from the crest were displaced onto it, flattening the front slope, 

 widening the crest, and lowering the crest elevation. Wave condition d 

 caused accretion of sand at the front toe, but the lower half of the front 

 face settled, steepening the front slope slightly. No change in elevation 

 occurred. After completion of test II, two torn and empty bags were dis- 

 covered on the crest of the structure (Figs. 25 and 26) and large sand 

 ripples were observed in the sand bed indicating active sand movement (Figs. 

 27 and 28). 



9. Test III. 



a. Construction . After the sand bed was regraded and the torn bags 

 from the second structure were replaced, the first layer of new bags for 

 the third structure was placed before the tank was filled (Figs. 29 and 



34 



