30). Ten more layers o£ bags were dropped through still water to produce 

 a structure 12.1 feet high above the sand bed with a top layer two rows of 

 bags wide (Fig. 31). 



b. Wave Condition Problems . With the structure crest at the still- 

 water level, the structure effectively blocked the tank, creating, during 

 various phases of testing, wave reflection problems between the generator 

 and the structure on the seaward side and a wave setup condition of 0.5 



to 1 foot (0.15 to 0.30 meter) on the shoreward side. As these difficulties 

 became obvious during wave condition a, different procedures for minimiz- 

 ing wave reflection effects and setup were tried. The most effective 

 procedure was generating bursts of 10 to 20 waves with 2-minute intervals 

 between bursts. 



c. Wave Changes . Waves broke on the seaward edge of the structure's 

 crest; then, the transmitted waves reformed with the primary period of the 

 incident waves and two or three secondary waves, making visual measurement 

 of wave height difficult. Wave attenuation decreased abruptly when the 

 top layer of bags was removed by wave action during condition a, but 

 increased with increasing wave steepness afterward, even when another layer 

 was removed during wave condition c. The wave setup also increased with 

 increasing wave steepness and the crest width widened as the structure lost 

 height, perhaps contributing to the changes in attenuation. 



d. Structure Changes by "Low" Waves . During the first hour of wave 

 condition a, the seaward row of the top layer of bags was displaced onto 

 the front face and the shoreward row of the top layer was displaced into 



a pile on the backface, steepening both slopes, widening the crest, and 

 lowering the crest elevation below the Stillwater line (Figs. 32 and 33). 

 A single row of four bags was used to rebuild the crest of the structure 

 to its original height. During the next series of 90 waves, run in bursts, 

 the new layer was removed onto the back slope (Fig. 34). The wave condi- 

 tion was continued for an additional 1 hour and 50 minutes (1,100 waves) 

 with little additional change to the structure. Wave condition b caused 

 a small amount of bag movement at the seaward edge of the crest. 



e. Structure Changes by "High" Waves . When wave condition c was run 

 continuously, bag movement occurred over all of the visible crest and front 

 face (Fig. 35) until setup behind the breakwater stabilized. When waves were 

 run in bursts of 20 (Fig. 36), extensive movement of the bags continued 

 until a new stabilized profile was attained after the top layer of bags 



was displaced onto the pile of bags on the backface and the bags on the 

 upper part of the front face underwent small displacements. Wave condi- 

 tion c flattened the front slope and left the structure with a lower, 

 wider crest. During wave condition d, the pile of bags on the backface 

 slumped, decreasing the crest elevation, and bags on the front face contin- 

 ued to settle and undergo small displacements, flattening the slope slightly 

 (Figs. 37, 38, and 39). Changes in the sand bed elevation at the back toe 

 during test III were negligible; changes at the front toe were only about 

 50 percent as large as changes during tests I and II. 



43 



