(1) Model Boundaries . Any model must have some artificial 

 boundaries; however, this presents several problems in a movable- 

 bed model. Sediment must be injected artificially at the proper 

 rate and at the proper location (relative to the shoreline) 

 along one lateral boundary and the sediment must be removed at 

 the other boundary. The other alternative (usually impractical) 

 is to make the model area large enough to study the area of 

 interest before the model walls introduce any undesirable model 

 effects. Lateral model boundaries are also responsible for 

 introducing a model circulation (due to interruption of the long- 

 shore current and transport). This circulation can be compen- 

 sated for by providing sufficiently effective energy absorbers 



at the model boundaries or perhaps by allowing a return channel 

 for water to flow from one model boundary to the other. Inject- 

 ing and removing water at the model boundaries through the appli- 

 cation of a type of manifold system has been postulated by some 

 investigators as a method of solving this problem; however, the 

 method usually introduces more problems than it solves. 



The seaward boundary of the model may also provide a possible 

 source of undesirable model effects. If the wave generator pro- 

 duces a wave behind it, this energy must be effectively absorbed. 

 If there is a nonnegligible reflection coefficient from the model 

 beach and underwater topography, then reref lection of this energy 

 by the wave generator must be avoided. In most cases this can be 

 accomplished by using wave filters in front of the wave generator. 



Since the linear scales of the model will be distorted, re- 

 flection coefficients from the shoreline will be exaggerated. 

 The grain size of the model material will also be too large, 

 and the permeability of the model sediment is expected to be 

 too large. Thus, the model bed will dissipate too much wave 

 energy. Therefore, these two scale effects tend to compensate 

 each other. 



Another possible model boundary effect is the reinforcement 

 of model edge waves due to the lateral boundaries. Edge wave 

 reinforcement tends to increase model rip currents. The effect 

 of linear-scale distortion on rip currents should be further 

 investigated, as well as the effects of artificial bottom 

 material on rip current velocities. 



(2) Wave Generator Characteristics . In practice, all wave 

 generators have certain inherent limitations. Waves can be gen- 

 erated over a definite range in wave periods without gear changes 

 or modifications, and there is a finite limit to a generator 

 stroke, which limits the wave height that can be generated. Wave 

 generators designed to produce monochromatic waves usually tend 

 to generate more energy at harmonics of the fundamental frequency 

 as the stroke increases. Spectral wave generators have the same 



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