PART II: SCALING GUIDANCE 



14. Water- related scour of noncohesive sediments in nature is caused by various environmental forces 

 such as wave- and tide-induced currents and turbulence that act to mobilize and transport sediment grains. 

 The interaction of the fluid with the solid boundaries of coastal structures increases the turbulence level, 

 which is typically accompanied by increased local scour. 



15. Scour can develop gradually over a long time span, such as the enlarging of a scour hole at the tip 

 of a jetty, or rapidly during intense storms, such as at the toe of a seawall during severe wave conditions. It 

 is reasonable to assume the dominant scour mechanisms associated with these two time scales are bed 

 shear stress-induced sediment transport for the case of long-duration scour, and turbulence-induced 

 sediment transport for the short-duration case. Although it is recognized that this generalization may not 

 be strictly true, and in some situations a combination of these two mechanisms will govern, it is beneficial 

 to have a broad framework with which to classify scour processes for the sake of developing scaling criteria. 



16. This study focused on developing scaling guidance for modeling turbulence-dominated scour 

 occurring over relatively short periods. Although the physics of this scour mechanism may be more difficult 

 to express in terms of mathematical representations than the case of bed shear stress-related scour, 

 favorable experience by others in the parameterization of beach erosion led to the belief that proper scaling 

 criteria can ultimately be developed for this situation. Success in developing such a tool for studying 

 storm-related scour has potential for great cost savings in scour prevention at coastal projects. Types of 

 projects that could be examined with a valid movable-bed physical model include storm response of beach 

 fills, scour at the toes of structures, and storm impacts to the fronting beach caused by seawalls. 



Movable-Bed Modeling Considerations 



17. A limited number of studies have validated movable-bed modeling guidance for scour with 

 prototype-scale data, and the observation hcis been made that although most guidance does well with the 

 data used to establish the relationships, they do not fare as well with other data (Fowler and Smith 1986; 

 Dette and Uliczka 1986; Lappo and Koshelnik 1988; Penchev, Sotkova, and Dragncheva 1986; Dean 1985). 

 Consequently, no clear consensus presently exists regarding appropriate scaling relationships for small-scale 

 movable-bed models of coastal scour, particularly in proximity to coastal structures. However, it can be 

 stated that one set of universal scaling criteria covering all types and causes of coastal scour will never be 

 developed; instead, there will be multiple sets of scaling criteria, each set specific to a particular genre of 

 scour and the associated forcing functions, sediment characteristics, and boundary condtions. These 



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