piled up at the back edge of the crest, widening it to 26 feet (7.92 

 meters), about the width of four rows of bags. Although structure IV 

 lost both rows of bags from its crest, the final crest width was only 9 

 feet (2.74 meters) or one row of bags due to the extensive slumping of the 

 front face. The constructed crest width required to prevent bag movement 

 cannot be determined from these test results, but for submerged breakwaters 

 the width should increase with increased structure height. For a struc- 

 ture crest near the Stillwater level, as in the case of structure 111, 

 widening the constructed crest will not prevent displacement of the seaward 

 row of bags onto the front face; therefore, a minimum crest width of three 

 bag rows is recommended. 



Experience in construction of test breakwaters has shown that achieving 

 a design slope for a structure built in overlapping layers by dropping 

 bags through water is extremely difficult. The m.ethod of bag placement 

 and the instability of the individual bags ensure that some change in the 

 slope will occur as a result of wave action immediately after construction. 

 The total changes in the front-face slopes of the test structures (Fig. 

 55) are, when compared with the final slopes, indicative of the design 

 slopes needed to minimize change. The front slope changed from 1 on 4.2 

 to 1 on 2.8 in test 1, 1 on 2.2 to 1 on 2.6 in test II, 1 on 3.6 to 1 on 

 4.2 in test III, and 1 on 3 to 1 on 5.3 in test IV. The final slope for 

 test IV is not completely comparable to the final slopes from the other 

 tests since structure IV was not subjected to wave condition d. Based 

 on changes in structures II and III during wave condition d, the final 

 slope of structure IV would be 1 on 5.5 or flatter, but the large amount 

 of slumping on structure IV compared to the other structures, the result 

 of the different forces on an emergent breakwater, makes such an estimate 

 questionable. Neglecting test I and the effects of midtest tank draining, 

 the higher the breakwater, the flatter the final slope. The flattest slope 

 from structure I, measured during wave condition b before the tank was 

 drained, was 1 on 4.6, between the final slopes of 1 on 4.2 for structure 

 III and 1 on 5.3 for structure IV. Slope changes by the wave conditions 

 in those three tests might have been minimized by building the initial 

 front slope at 1 on 5. 



3. Wave Transmission . 



To determine the breakwater design configuration which will be adequate 

 to provide a required degree of wave protection, the designer must know 

 the relationships of the design height and crest width of the structure to 

 the wave attenuation by the breakwater and to the wave damage to the struc- 

 ture. Although the definition of these relationships was a goal of the 

 testing program, the tested combinations of structure height and crest 

 width were insufficient to isolate the effects of crest width changes from 

 the effects of structure height and wave property changes. Only general 

 conclusions can be drawn from the data. 



The transmission coefficient, H^/H^, and attenuation. A, for each 

 wave condition in each test are given in Table 5. When plotted as func- 

 tions of the crest elevation at the beginning of the wave condition and 



64 



