specific scaling relationships will make the physical model into a useful tool for studying scour, but care 

 must be taken to assure that the entire regime being modeled behaves in a manner consistent with the 

 assumptions of the modeling guidance. Careful thought must also be given to proper scaling of structural 

 attributes, such as flow through rubble mounds, if scour might be influenced by such interactions. 



Scaling Requirements 



18. Many investigators have expressed opinions regarding the important physical parameters and 

 scaling requirements to be considered in formulating guidance for movable-bed models of coastal 

 sedimentary processes. Rather than exhaustively reviewing the literature, only those parameters or 

 requirements that appear to be predominant (to the authors) will be discussed. 



19. Important sediment-related parameters are the mean (or median) grain size, immersed weight of 

 the bed material, the sediment fall speed (settling speed of the grain's centroid), and the Shield's 

 parameter (indicator of the fluid velocity necessary to initiate sediment movement). Additional modeling 

 parameters are wave height, wave period, water depth, initial bottom configuration, and process duration. 



20. The most common scaling problem arises when the prototype grain size is so small that geometric 

 scaling of the sediment results in model bed material below the size considered the boundary between 

 cohesive and noncohesive sediment (about 0.08 mm), thus altering the sediment transport mechanism in 

 the physical model. Some researchers have developed dimensionless parameters by combining several of the 

 sediment parameters listed above. Then, instead of decreasing grain size, similarity of the dimensionless 

 parameter is maintained by using a bed material having, for example, a smaller specific weight than the bed 

 material in the prototype. Unfortunately, lightweight materials introduce another set of problems, so that 

 many investigators now recommend that the same type of sediment in the prototype be used in the model. 



21. Distortion of the scale model, i.e., different vertical and horizontal length scales, has also been 

 suggested as a means for overcoming the inability to geometrically reduce the sediment to model scale, and 

 many scaling laws have been proposed that require model distortion; but this practice is still viewed with 

 skepticism by some. Dean (1985) reviewed several studies and concluded that the state of knowledge on 

 movable-bed models was largely based on empirical observations. Further, he argued against the use of 

 dissimilar bed materials in scale models and also against distorting the model as required by many scaling 

 relationships. This guidance essentially constrains the coastal processes movable-bed physical model to 

 being geometrically undistorted using (most likely) fine-grained sand as the model sediment. 



22. Perhaps the most relevant requirement for modeling coastal scour, as well as nearshore beach 

 dynamics, is to attain similarity of the equilibrium beach profile between prototype and model, particularly 

 in the surf zone. Parameters that appear to correspond to features of the equilibrium profile are similarity 

 candidates for developing scaling criteria. 



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