wave H; 



Figure 32. Wave and force record for shallow-water waves (d = 2.0 m, 

 T = 5.5 s). 



throughout the run, and a minimum value, which remains essentially constant. 

 The individual force peaks occur as the breakwater surges shoreward during 

 the passage of each wave crest, but is prevented from moving too far in this 

 direction by the mooring-line restraint. On the other hand, the seaward 

 movement of the breakwater is not similarly opposed, since no force cantilever 

 was installed on the leeward side of the breakwater. Instead, only a constant 

 negative restoring force or preload of approximately 113 kilograms was exerted 

 on the breakwater via the shoreward mooring line and pulley-weight arrangement 

 shown in Figure 18. The zero-force reference position recorded at the begin- 

 ning of each run always corresponds to this static preloaded condition of the 

 cantilever force gage. Negative force values up to the magnitude of this 

 preload can consequently be obtained as the breakwater surges seaward; these 

 constitute the stable lower limit of the force records. 



A time-series analysis of the force data was not performed because the 

 experiments were limited to regular waves and because the level of effort 

 required did not make it feasible. For practical purposes, each force record 

 is therefore characterized by a single force value that is considered most 

 useful for design purposes — the peak force, F, occurring during the length 

 of record (excluding wave generator start-and-stop transients, which have no 

 counterpart in nature). Typically, this implies that the first 5 to 10 waves 

 were not included in the analysis, nor were those last waves propagating down 

 the tank after shutdown of the wave generator. Each run consists of at least 

 50 waves. In addition to the peak mooring force, F, an approximation to the 

 drift force, F, J.s also obtained, as is the significant peak force, F . 

 The drift force F is the net, time-averaged force acting on the seaward 

 mooring line; it was determined "by eye" as shown in Figure 33 and is there- 

 fore subject to larger errors. The significant force, F , represents the 

 average of the largest one-third of the force peaks, again excluding stop-and- 

 start transients; it is obtained manually, directly from the data trace. 



If stop-and-start transients are included in the determination of the peak 

 mooring force, as has been done by other investigators (Giles and Sorensen, 



36 



