attainable, or a useful state to be trying to reach in experiments of 

 practical duration. Coastal engineering might be better advanced if 

 researchers were more concerned with trying to reach some constant rate 

 of profile change or a rate of profile change small in comparison to 

 other variables. 



IV. LABORATORY EFFECTS 



1. Definitions of Terms . 



Laboratory effects are the undesired differences between laboratory 

 and prototype conditions caused by the physical constraints which exist 

 in the laboratory, but not in the field. For example, the variations in 

 incident wave height discussed in Section II, 3 are laboratory effects; 

 i.e., the mechanical generator at one end of the wave tank caused a re- 

 reflection of the wave energy propagating away from the profile that 

 would not have occurred in nature. This project evolved from an investi- 

 gation of wave height variability and equilibrium profiles into a more 

 comprehensive examination of all laboratory effects. 



This section analyzes five laboratory effects based on results from 

 the 10 experiments. Other known laboratory effects are also identified. 



2. Test Length and Initial Slope Effects . 



a. Processes . Two physical processes are known to be affected by 

 changes in initial test length: re-reflection of waves from the wave 

 generator and secondary waves. 



(1) Re-Reflection . The height of the incident wave is a func- 

 tion of the height of the nominal (generated) and re-reflected waves and 

 the phase difference between the re-reflected wave and the wave generator 

 motion. The height and phase of the re-reflected wave are functions of 

 the height and phase of the reflected wave. The height of the reflected 

 wave is a function of the profile reflectivity. The phase of the reflec- 

 ted wave with respect to the generator motion is a function of the dis- 

 tance between the profile and the generator. The effect of initial test 

 length on re-reflection and incident wave height variability is discussed 

 in Section II, 3. The effect of incident wave height variability on the 

 profile is discussed in this section. 



(2) Secondary Waves . Secondary waves cause a spatial (longi- 

 tudinal) variation in wave height and a variation in the asymmetry of the 

 velocity distribution under a wave. The degree of asymmetry obviously 

 depends on the position along the tank. In this case the distance to the 

 toe of the initial profile from the generator is the controlling distance. 



b. Initial Test Length Effect . Four pairs of experiments are ex- 

 amined here. In two pairs (experiments 70X-06 and 71Y-06 and experiments 



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