(c) after the wave breaks offshore of the bar, the bar is quickly 

 scoured (Fig. 19, d) and the beach profile subsequently returns 

 to that of Figures 19, b or 19, e and the breaker location 

 returns closer to shore (about 1 hour and then the cycle repeats) 



The apparent instability occurs with beach profiles developed using 

 uniform (periodic) waves, a uniform grain size, or smooth, spherical 

 grain shapes. The appearance of this phenomenon when only the grain 

 shape is different occurs less frequently. 



3. Effects of Channel (Test Repeatability) . 



Laboratory effects are the undesired differences between laboratory 

 and prototype conditions caused by physical conditions unique to the 

 laboratory, and would include such conditions as wave paddle to beach 

 face distance, left- or right-hand channel, effect of channel side and 

 bottom convergence, and the effect of finite water depth at the toe of 

 the beach. 



Initially, the hydrodynamics of the surf zone in the presence of a 

 movable bed were thought to be unique; hence, the equilibrium beach 

 obtained would be independent of the initial beach profile. However, 

 the profile shape ahead of the point at which sediment moves was found 

 to affect the wave, the breaker characteristics, and the resultant 

 profile (Fig. 20). 



An indication of laboratory effects can be derived from a cross- 

 comparison of several runs. Test repeatability was checked at intervals 

 since a "control" sediment was used in several of the shape and distribu- 

 tion tests. Comparisons of such tests are shown in Figures 21, 22, and 

 23. Some apparent differences are present but the general beach profiles 

 show some repeatable characteristics, e.g., in all profiles the beach 

 face has the same slope. Some laboratory effects are believed to be 

 present, but the profile differences due to other phenomena (see Figs. 

 7 to 18) are not masked by the laboratory effects. 



The position of the bar varied considerably under nearly identical 

 wave conditions (Figs. 21 and 22). Noda's (1971, 1972) model law was 

 based upon the horizontal distance from the SWL intercept to certain 

 profile features, e.g., the bar. Modeling that distance (if not 

 repeatable) is not easily achieved. Noda's law and most other profile 

 modeling laws are based upon reproducing an equilibrium profile. If the 

 shape of the stable (unchanging) profile which developed for a test 

 series with identical wave conditions is not repeatable, then the concept 

 of equilibrium must be qualified, and all modeling laws based upon the 

 usual equilibrium assumptions are open to question. 



These few tests are not sufficient to prove or disprove the concept 

 of profile equilibrium. However, the lack of repeatability suggests that 

 the concept of equilibrium should be reevaluated. 



46 



