The uncertain significance of Table 8 lies in the origin of the 

 resultant accreted sediment, and in the deposition of the eroded sediment. 

 In addition to the variable survey coverage, other possible sources of the 

 discrepancies between the net landward and net seaward quantities of 

 accreted and eroded sed iment, wh Ich should theoretically balance, can only 

 be qualitatively explained. One explanation Is the loss of suspended 

 sediment, seaward of the device, out Into the main part of the wave tank. 

 This loss most certainly occurred to some extent as coal was noted in the 

 form of sma I 1 ripples over the entire tank bottom after the tank had been 

 drained. Some of the sediment in several runs was contributed by the 

 shoreward erosion of the bottom profile. Some sediment was probably moved 

 to the measured bottom profile by the migration of the ripples into the 

 flume from the main wave tank. In addition, some sediment may have been 

 moved by the device from the seaward to the landward side and vice versa. 



Other possible contributing causes for the existing net discrepancies 

 may be error in measuring, plotting and In p I an imeter i ng the comparative 

 profiles. It is likely that some combination of each of these various 

 possibilities did occur. Nevertheless, a run resulting In a net accretion 

 on the landward side of the device should be viewed as successful, although, 

 with waves of extremely long period, such as in Runs 14 through 16, the 

 natural tendency is to build the beach. The contribution of sediment move- 

 ment by the device therefore remains somewhat indefinite, even though com- 

 pared to generally near-equilibrium conditions. Experience indicates that 

 for the waves of low steepness, more sediment would have accreted landward 

 of the position of the device had the device not been present. However, 

 in the case of impinging, shorter period (high steepness) waves, accretion 

 landward should be viewed as significant. 



3. Discussion and Results of Runs I through 16 



a. Run I . Figure 5 on page 34 illustrates the differences between 

 the molded slope and the profile as It had evolved In approximate equilibrium 

 with the prescribed wave conditions at least sufficiently far seaward to 

 cover the location at which the device was to be placed. The nearshore 

 concavity and offshore convexity of the equilibrium profile are normal for 

 waves of such a short period as 5 (prototype) seconds. The equilibrium 

 profile thus established, the device was placed about 237 feet offshore 

 from the intersection of the Stillwater line with the equilibrium profile, 

 in water having a depth of about 18.75 feet (see Figure 6). 



The device was set at its deepest setting so that its lowest flap 

 valve was touching the bottom. At this setting 4 flap valves were operat- 

 ing and the topmost rod was connected to the vertical valve supports through 

 the top hole set, one hole above the uppermost valve. On the landward side 

 of the device most of the 10 cubic feet of sediment that was eroded was de- 

 rived from the immediate vicinity of the device, while the accretion on the 

 landward side was composed of a rather thin, uneven layer of sediment, having 

 a volume of about 24 cubic feet, covering an area landward of the device 

 location for a distance of about 40 feet. The trough dug by the device, 

 which accounted for most of the erosion landward of the location of the 



14 



