in the tire bases. Holes, 1.5-inch diameter, were drilled in the truck 

 tire bases to prevent sand from accumulating in them. It was also hoped 

 that the high sidewalls of the larger tires would aid in keeping out 

 sand. Because the tires were substantially exposed at low tide, flota- 

 tion relied solely on air trapped in the tire crowns. Rubber conveyor 

 belting coupled the tires and was fastened by tying the end of the belting 

 into square knots. 



Site: 





SANTA ROSA SOUND 



^^M FTB 



N 

 A 







1 1 ^_ 



20 40m ^^5*5 















Installation: The breakwater was moored by six 11 kg (25 lb) danforth anchors 

 placed at each corner and in the middle of each end. The mooring lines 

 were of 0.5-inch nylon and were 9.1 m (30.0 ft) in length. 



Field Experience 



Operation: Coupling failure was a moderate problem with approximately 10 



percent of the square knots untying. To solve this problem, the operator 

 fastened the loose ends of the square knots together by means of 

 galvanized lag screws. Mooring- line failure was a minor problem due to 

 the nylon lines chafing. To resolve this problem, the operator led the 

 mooring lines through flexible plastic tubing at points where they were 

 wrapped around the tires. The steps taken to prevent sand accumulation in 

 the tire bases worked overall. The truck tires would occasionally ratchet 

 around under heavy wave action with some rotating to the point where the 

 hole was above the waterline thereby releasing trapped air. However, this 

 problem was not severe and adequate buoyancy was always maintained. 

 Finally, fouling growth was a moderate problem with a considerable weight 

 buildup noted. Under typical storm conditions with waves 0.8 m (2.5 ft) 

 in height and 10.2 m (33.5 ft) in estimated length, the transmission 

 coefficient was 0.30. The operator believed that the extra rigidity of 

 this FTB because of the use of truck tires was an important factor in 

 achieving such a low transmission coefficient. 



Benefits: The FTB performed admirably in its design role of accumulating sand 

 in its shadow of protection. During the project's brief life, accretion 

 built a 6.1-m (20.0 ft) sand point along the length of the breakwater. No 

 effect was observed on the neighboring shorelines. The operator noted 



37 



