same model fluid and the same model laws for reproducing the waves, cur- 

 rents, and bottom material for both regimes) is impossible. Other effects, 

 such as edge waves, rip currents, the directional spectrum, the long-wave 

 environment, and the prototype sediment-size distribution and amount of 

 sorting (Longuet-Higgins, 1953), must all be considered at least to the 

 point of showing that they are unimportant at the particular location 

 being studied. Other questions also arise concerning both scale effects 

 (Bijker, Stapel, and de Vries, 1957; Diephius, 1957; Saville, 1957) and 

 operational techniques that should be used in the model. 



An alternative to a completely movable-bed model is the fixed-bed 

 model using relatively small quantities of tracer material to qualita- 

 tively indicate shoaling patterns. Such model studies are less expen- 

 sive to conduct and, depending on the problem studied, may be only 

 slightly less useful than completely movable-bed models. Tracer studies 

 can be particularly well adapted to existing fixed-bed models (perhaps 

 originally constructed for another type of study) where shoaling infor- 

 mation is desired. 



At the present time, movable-bed scale modeling of coastal sediment 

 transport should be considered an art rather than a science. Although 

 the development of this art is difficult, it can be useful when fully 

 understood and appreciated. Le Mehaute (1962) expresses some interesting 

 and informative thoughts on this subject and Simmons (1950) discusses the 

 contribution of hydraulic models to coastal sedimentation studies. As 

 discussed below in the similitude for movable-bed models, a number of 

 similitude relations have been developed, each containing its own partic- 

 ular assuirptions and constraints. The derivation of these scaling rela- 

 tions varies from completely empirical to completely mathematical. Hence, 

 the investigator is immediately at the crossroads, because many choices 

 are available and probably one of several will suffice provided the cor- 

 rect art is applied to model operation. The objective of additional 

 applied research on coastal erosion should not be to develop the artist's 

 techniques but to further the science. Several areas where additional 

 study is necessary are delineated in this section; however, this does not 

 mean that sufficiently accurate model studies cannot now be performed, 

 but rather it indicates that comprehensive planning and serious thought 

 are required to conduct such studies. More importantly, it means that 

 the conduct of this type of model should not be rushed. Adequate time 

 and funds must be allocated to assure that accurate model results are 

 obtained. 



2. Similitude Relations . 



The similitude relations for movable-bed models are discussed in 

 numerous sources; some of the more detailed discussions include Allen 

 (1947); Sedov (1959); Goddet and Jaffry (1960); Valembois (1960); 

 Langhaar (1962); Bijker (1967), Fan and Le Mehaute (1969); Le Mehaute 

 (1970); Yalin (1962, 1963, 1971); Kamphius (1972); and Noda (1972). 

 Only the main points relative to similitude relations, rather than com- 

 plete discussions of the subject, are presented in this section. This 



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