III. FIELD DATA 



1. Layout . 



The site of the floating breakwater instrumented in this study is 

 located at Friday Harbor, Washington, on San Juan Island, just east of 

 Victoria, British Columbia (Fig. 23) , The breakwater is 25 feet wide, 

 904 feet long, anchored in approximately 40 feet of water, and was in- 

 stalled in October 1972. The structure is made of Polyolefin flotation 

 tanks linked together by a matrix of large wooden timbers. It is laid 

 out in an expanded L-shape, the inside angle being 115°, with the short- 

 er leg (227 ft.) directed toward shore and the longer leg (627 ft.) 

 toward magnetic north. The site itself is protected on three sides 

 by San Juan and Brown Islands off the harbor entrance. This leaves an 

 0. 25-mile-wide channel into the harbor with a northeasterly fetch of 

 about 1.7 nautical miles. Southeasterly winds can also generate waves 

 of importance parallel to the shorter leg where the fetch is about 1 

 nautical mile. 



2. Instrumentation . 



The shorter leg was instrumented in this study for two reasons: 

 (a) the most frequent winds are out of the southeast, and (b) barges were 

 to be tied to the longer leg during the winter months for added protec- 

 tion. However, the wave gages are positioned to give the proper inci- 

 dent and transmitted wave data for all relative wind directions (Figs. 

 24 and 25) . 



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

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

 direction; (b) wave heights at key locations; (c) anchor cable forces; 

 and (d) directional acceleration and angular motions of the breakwater. 

 The locations of the measuring sensors are shown on Figure 25. Signals 

 from the sensors were carried by underwater cable to the recording system 

 which was located in a small building mounted near the center of the 

 short leg. 



3. Wind Data . 



Windspeed and direction were measured by Weather Measure Corpora- 

 tion's W121 sensor. Some additional circuitry was required to record 

 the windspeed, and the sensor was recalibrated to this circuit. The 

 sensor was mounted on the breakwater at the intersection of the two legs 

 at 20 feet above the water surface. 



Waves . 



Wave characteristics were measured at four locations with the second 



49 



