occur because of the movement of the inclined barge acting essentially as a 

 wave generator. Also, transmission occurs through the gap which periodically 

 opens under the bottom edge of the barge. The transmission coefficient with 

 and without the spring mooring system (and without the bow angle) is approxi- 

 mately the same. This suggested to Raichlen (1978) that the dynamics of the 

 barge mooring system are important with respect to forces but apparently not 

 very important in connection with the wave transmission. 



(6) Effect of Bow Angle on Mooring Forces. There appeared to be a 

 slight increase in the mooring force due to the installation of the bow angle; 

 however, the difference was too small to differentiate from experimental 

 scatter. The mooring force tended to decrease from the larger values for the 

 long waves to relatively small values for the shorter (deeper water) waves 

 (Fig. 62). Figure 62 also displays the variation with kd of the forces 

 which would be associated with the mooring line if the line were infinitely 

 stiff. For very, long waves, this force appears to be the same as the force 

 associated with the spring line. Hence, the dynamics on the mooring line for 

 these long waves are little affected where the ratio of depth-to-wavelength is 

 of the order of 0.048. However, in the region of kd at approximately 0.75, 

 the mooring line force without the spring is nearly three times that with the 

 spring. The spring system allows the barge to attain an orientation that is 

 conducive to minimal hydrodynamic force. For the inextensible spring, the 

 body moves but cannot orient itself to significantly reduce the mooring force. 

 Raichlen 1 s (1978) experiments tend to demonstrate that the more flexible the 

 spring system, the smaller the mooring force, presumably because the barge is 

 allowed to move proportionally more and assume an orientation which minimizes 

 the restraining forces. However, the disadvantage of the softer spring is the 

 increased motion of the barge which tends to generate larger waves on the lee 

 side, compared to the stiffer mooring. 



b. Structure Resting on Supports (Bottom Clearance) . Raichlen (1978) 

 conducted two-dimensional experimental studies with a spacer mounted to the 

 end of the barge resting on the bottom where a bottom gap of 4.7 5 feet pro- 

 totype existed (Fig. 63). Since the length of the mooring line remained 

 unchanged, the angle of inclination of the barge with the bottom was decreased 

 to approximately 12.2°. 



(1) Transmission Coefficient . A greater transmission for the same 

 wave height was anticipated when the structure was supported above the bottom 

 of the wave flume (see Fig. 64). A comparison with Figure 57 (the correspond- 

 ing figure when the structure was not elevated above the bottom) reveals that 

 indeed there is a distinct difference in transmission although not to the 

 degree anticipated. A certain amount of disturbance occurred during the 

 transmission process, particularly at the seaward edge of the barge. Bubbles 

 were observed just after the wave trough had passed, as the wave became 

 detached from the seaward edge. The end of the barge near the bottom moved 

 almost 5 feet (one gap height) off the bottom. If a gap at the bottom is to 

 be maintained in the prototype, the forces involved when the barge drops back 

 to the seabed should be determined. The downward movement of the seaward end 

 of the barge during the overtopping process was probably due to the water mass 

 associated with extreme wave tests, and was not as readily apparent for 

 smaller waves. Although tested for transmission, the larger waves are prob- 

 ably not realistic in terms of either the design or the operation of the 

 mooring system for the sloping-float breakwater. 



106 



