transmitted gage being used as a backup. Two spar buoys instrtimented 

 to measure wave elevation were located outboard of the breakwater and 

 positioned so that one measured the incident wave field, and the other 

 measured the incident plus reflected wave field. Two stationary gages 

 were attached to pilings behind the breakwater to measure transmitted 

 wave height. All four gages were of the resistance type. The spar 

 buoys were used outside the breakwater to help reduce navigation hazards 

 and because of the costs and logistics of placing stationary piling at 

 these locations . 



The buoys were made of two sections of PVC pipe, the lower section 

 being 6 inches in diameter and 15 feet long, and the upper section of 

 3- inch diameter and 12 feet in length with the upper 8 feet wound with 

 a resistance wire. Four feet were exposed above the water surface, and 

 a 2. 5- foot-diameter disc was attached to the bottom to damp vertical 

 motions. The natural periods in heave and roll, respectively, are 18 

 and 14 seconds, well above the anticipated maximum wave period of about 

 4 seconds. See Appendix J for a complete description of the wave staff 

 and buoy designs. 



5. Cable Forces . 



Anchor cable forces were measured using a bonded strain gage-type 

 load cell that was placed in the anchor chains beneath the water surface. 

 These cells and the associated electronics were designed and built for 

 this project. They have an overall system accuracy of 0.75 percent of 

 the designed or rated total load cell capacity over a temperature range 

 of 10° Celsius (design load 12,500 pounds) , These load cells employ a four- 

 arm wheatstone bridge circuit which has two strain gages in each leg of 

 the bridge and are self-temperature compensating. The units are 0-ring 

 sealed and wired directly to the bridge amplifier circuitry mounted in 

 the recording package. 



6. Motion Package. 



Breakwater accelerations were measured using three Kistler servo- 

 accelerometers (Model 303T) . One accelerometer, oriented horizontally, 

 was mounted at the center of the breakwater to measure the sway accelera- 

 tion. The other two were oriented vertically and mounted at opposite 

 outboard edges of the breakwater to measure the vertical accelerations. 

 The heave acceleration was obtained by taking the average of the signals 

 from the two outboard accelerometers; the roll acceleration was obtained 

 by taking the difference of these two signals and dividing by the distance 

 between them. The accelerometer locations are indicated in Figure 25. 



7 . D ata Acquisition System . 



The data recording and electronic package was built around the Sea 



53 



