discussion of model scales is based on Noda (1972) and Le Mehaute (1970) ; 

 two tables and a figure taken directly from Noda and Le Mehaute contain 

 their symbols which were not converted to conform with the other sections 

 of this report. (Symbols used only in this section are specifically 

 identified in the Symbols and Definitions of the Appendix.) 



The basic philosophy for movable-bed scale-model investigations 

 is founded on the physical laws responsible for the dynamic processes 

 involved and the understanding of these phenomena to ensure that the 

 relative magnitudes of all dominant processes are the same in model 

 and prototype. This is an impossible task for movable-bed scale models, 

 since most of the fluid processes involved are complicated by nonlinear 

 fluid behavior, turbulence, and complex boundary conditions. Thus, the 

 complicated combination of forces that occur in the prototype cannot 

 always be reproduced exactly in the model. In such instances, an attempt 

 is made to reproduce the dominant processes with the anticipation that 

 other forces are small. In attempting to develop similitude relations, 

 the idea of reproducing the dominant physical processes may be abandoned 

 and attention turned to an attempt to maintain similitude of the beach 

 profiles and longshore transport rates. 



This section discusses some of the pertinent coastal processes and 

 important parameters in deriving similitude relations. The first of 

 these is the beach profile where considerable effort has been expended 

 in explaining the existence of summer and winter beach profiles. Motion 

 of the water itself is important in determining the beach profile. Sedi- 

 ment characteristics are acutely important in determining the motion in- 

 duced by wave action. Accurate modeling of cohesive sediments is assumed 

 to be beyond the present state-of-the-art; therefore, attention is focused 

 on noncohesive sediments. A sediment is described by its median diameter, 

 DgQ, and the specific weight, y. The relative specific weight y' of 

 a material is defined by y' = (y^ ~ Yf)/Yfj where y is the specific 

 weight of the sediment and Yf the specific weight of the fluid. Hydro- 

 dynamic properties of the sediment are usually represented by the fall 

 velocity which is related to the drag coefficient, fluid density, particle 

 volume, and projected area of the particle normal to its direction of 

 motion. Initiation of sediment motion is extremely important in modeling 

 sediment transport. For steady-flow conditions, the classical Shields 

 criterion is the required similitude relation; however, it is questionable 

 whether the Shields criterion is valid for initiation of sediment motion 

 in the coastal zone since the processes are much more complicated than 

 that of steady uniform flow. 



Four basic parameters must be chosen in the construction of a movable- 

 bed scale-model law: the horizontal scale A; the vertical scale p; 

 the sediment size, D50 (median diameter); and the specific weight of 

 the sediment y^. The functional relationships among these four para- 

 meters, which result in identical model and prototype beach profiles, 

 changes in beach profiles, and longshore transport rates for identical 

 spatial and temporal wave and tide conditions, are the desired model laws. 

 Numerous model laws can be postulated from various assumptions regarding 

 the physical processes governing sediment motion. 



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