BULL RAIL, 



CAP 

 S£v\j.68m(26.6ii 



BREAKWATER PONTOON 



Figure 46. Cross section of floating breakwater at Friday Harbor, 

 Washington (after Adee, © 1975b). 



the point where the pontoons are attached to the structure. This failure 

 appears to be a result of fatigue from the cyclic wave loading. The current 

 method of protecting the breakwater is to moor unused barges to the seaward- 

 side of the breakwater during the winter months when severe storms are likely. 

 The construction cost of the Friday Harbor, Washintgon, breakwater (1972 

 dollars) was about $320 per linear foot. 



The Friday Harbor breakwater was instrumented by Adee, Richey, and 

 Christensen (1976) to obtain performance information from a field assessment 

 program. Four types of time-dependent data which are basic to describing the 

 response of the breakwater were collected: (a) wind velocity and duration, 

 (b) wave heights or water surface time histories at key locations, (c) anchor 

 cable forces, and (d) directional acceleration and angular motions of the 

 breakwater. Two spar buoys instrumented to measure wave heights were located 

 seaward of the breakwater and positioned so that one measured the incident 

 wave field, and the other measured the incident plus reflected wave field. 

 Two other stationary gages were located behind the structure to measure the 

 transmitted wave height. This was essentially the same instrumentation and 

 arrangement which had been previously used to monitor the performance of the 

 floating breakwater at Tenakee Springs, Alaska. 



A total of 95 records were obtained at the Friday Harbor breakwater 

 between December 1974 and May 1975. There was no known equipment failures or 

 breakdowns except for one of the load cells going off scale at low tide. The 

 average overall response, or transmission curves, for the events within each 

 wind direction window and for all the recorded data, are given in Figure 47. 

 These data were obtained by averaging the square root of the ratio of the 

 transmitted to the incident wave spectras for the records indicated for each 

 curve. Hence, they all have the same frequency resolution of 0.10195 hertz. 



All the anchor cable data showed a very dominant amount of energy at lower 

 frequencies. The exact location of these peaks varied for different records, 

 but in all records the peak of the force spectra was contained in the fre- 

 quency band of approximately 0.015 to 0.050 hertz. In most cases, however, a 

 relatively dominant peak appeared in the 56- to 63-second range. The anchor 

 forces measured were all quite low, with the largest range being only 628 

 pounds. The cables were spaced at 50-foot intervals along the structure. An 

 extreme storm event did not occur during the sampling season nor during two 

 winter sampling periods on the Alaska-type breakwaters at Friday Harbor, 

 Washington; however, the anchor forces were about an order of magnitude less 



83 



