heights measured without a structure in the tank to heights measured with 

 a structure in the tank. Structure stability was determined by comparing 

 survey profiles, and structure configuration was described by measuring 

 crest elevation and front- or seaward-face slopes from the survey profiles. 



2. Wave Tank Preparation . 



a. Tank Description . The breakwater tests were conducted in the large 

 wave tank, 635 feet (193.55 meters) long by 15 feet (4.57 meters) wide by 

 20 feet (6.10 meters) deep. The wave generator was a piston-type capable 

 of producing monochromatic waves. A rubble wave absorber was built in the 

 tank at a position 610 feet (185.93 meters) from the midposition of the 

 generator. * 



b. Tank Modification . In preparation for the breakwater tests, sta- 

 tioning was established along the tank with station 0+00 located 230 feet 

 (70.10 meters) from the 'absorber end of the tank (Fig. 5). A 3-foot 

 (0.91 meter) layer of unwashed sand (0.4-millimeter mean diameter) was 

 laid from the face of the wave absorber to a point 400 feet (121.92 meters) 

 along the tank. The tank was filled with freshwater to a depth of 15 feet 

 at the generator, leaving a 12- foot depth at the breakwater site. After 

 completion of structure I tests, an 8-inch-diameter (20.3 centimeters) 

 corrugated metal pipe was mounted along one side of the tank, running 

 through the structure site on the surface of the sand bed to aid draining 

 the tank without damage to the structure. 



c. Wave Condition Calibration . The presence of the sand bed required 

 calibrating the tank to determine the wave height at the breakwater site. 

 Step-resistance wave gages capable of measuring water level changes to 0.1 

 foot (0.03 meter) were installed at station 0+00 and at positions to sea- 

 ward and shoreward (Fig. 5). Wave periods of 6 and 10 seconds were chosen 

 for the breakwater tests. Wave heights for these periods were measured 

 both electronically and visually as generator settings were changed. 

 Calibration curves (Fig. 6) were drawn by graphing the average wave height 

 calculated from records for each generator setting and wave gage position. 

 The wave gage at station 0+00 was removed before construction of the first 

 breakwater. 



3. Breakwater Construction . 



a. Structural Scheme . The nylon bags, filled to 75 percent of capac- 

 ity, were dropped into place to represent field construction techniques. 

 The breakwaters were built in layers, four bags wide with the long axis of 

 each bag alined parallel to the wave direction. Rows of bags were placed 

 every 7 feet along the tank with each successive layer overlapping the 

 previous layer by one-half bag (Fig. 7). The design crest elevations of 

 the test structures graduated from the lowest to the highest, each struc- 

 ture being the base for its successor. Enough new rows of bags were placed 

 on the seaward side of the old structure to provide a base for the increased 

 crest elevation of the new structure. 



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