GRAIN SHAPE AND SIZE DISTRIBUTION 

 EFFECTS IN COASTAL MODELS 



by 

 J. Ian Collins 

 and 

 Charles B. Chesnutt 



I. INTRODUCTION 



Movable-bed scale models aid in the solution of many engineering 

 problems encountered in the nearshore zone. However, such models must 

 rely heavily on practical experience rather than on theoretical predic- 

 tions since it is not possible to model the complete physics of the 

 nearshore environment. 



In general, scale models are expected to reproduce the dominant 

 physical phenomena governing the behavior of the prototype. Hence, two 

 essential steps are necessary: (a) The dominant phenomena must be iden- 

 tified and measured; and (b) the feasibility of modeling these phenomena 

 must be demonstrated. In movable-bed coastal models it is uncertain if 

 either of these steps can be accomplished satisfactorily at present. 



The real need for an engineering model is to reproduce a known condi- 

 tion or history of events. Generally, it is unnecessary (or impossible) 

 to model all of the details and, in fact, such a modeling is probably 

 not desirable. 



The approach taken by the scientist is to establish a set of scale 

 relationships ("theoretical similitude") based on reproducing the 

 dominant physical phenomena. To the dissatisfaction of both the scien- 

 tist and the engineer, this approach has not worked. 



The engineer, confronted with a specific problem and an immediate 

 need for a solution, must attempt to reproduce a known history of events 

 ("practical similitude") and not be overly concerned with obtaining 

 theoretical similitude. 



This study is one of a series of reports prepared by Tetra Tech, 

 Incorporated, Pasadena, California, under contract to the U.S. Army 

 Coastal Engineering Research Center (CERC) . The first reports by 

 Fan and LeMehaute (1969) and LeMehaute (1970) reviewed all the conditions 

 of theoretical similitude and proposed some guidelines to aid the engineer. 

 Noda (1971, 1972), in a following report, proposed a general scale-model 

 relationship for coastal engineers based upon obtaining a practical 

 similitude of equilibrium beach profiles. Relationships between the 

 horizontal and vertical scales, and the sediment specific weight and 

 median-diameter ratios were developed empirically, based upon modeling 

 the distance between the Stillwater level (SWL) intercept and the beach 

 discontinuity on step-type profiles (Fig. la). Noda assumed that dupli- 

 cating this distance would provide a model applicable to all distances. 



