65. In undistorted Froude models where the model sand has been reduced in size from the prototype 

 according to the fall speed ratio, deviations in the Ut/Umax velocity ratio between prototype and model 

 will also occur offshore of the breakpoint bar. However, these deviations will be less than in the case of a 

 distorted model employing prototype-size sand. The consequences of this offshore effect will be examined 

 further in Part IV using specific results from the present study. 



Applicability of Selected Scaling Criteria 



66. The selected movable-bed scaling criteria given by Equation 6 are for undistorted Froude models 

 where the sediment size is selected so that the fall speed parameter is held constant between prototype and 

 model. Past experience with these and similar scaling criteria, coupled with the assumptions used in 

 formulating the guidance, restricts application of this type of physical modeling to coastal sediment 

 problems and processes that are chiefly erosional in nature, with the erosion occurring in an energetic, 

 turbulence-dominated region such as the surf zone. Typically, the scaling is intended to replicate the 

 short-term response of the sea bed to storm-induced waves. Examples of situations that may be candidates 

 for modeling with the selected criteria include: beach and dune profile response to storm events, initial 

 beach-fill adjustment to larger waves, beach-fill response to storm events, and storm-related short-term 

 scour at the toes of structures. 



67. Most experience with these scaling criteria, including the present study, has been with 2-D wave 

 flumes; hence, applicability of the guidance to the 3-D situation is still in question, although Vellinga 

 (1986) has performed related tests in a movable-bed basin with encouraging results. 



68. An unfortunate aspect of the selected modeling criteria is that often a fairly large facility will be 

 required to successfully model prototype situations having fine-grained sediments. For example. Figure 1 is 

 a simple nomogram for estimating length scale ratio based on Equation 6. This estimate assumes a model 

 median sediment size of 0.13 mm, prototype and model water temperatures of 60 °F\ and quartz sand. It 

 is readily apparent that prototype sediment sizes representative of many beaches will require length scales 

 on the order of 10 or less. 



' A table of factors for converting non-SI units of measurement to SI (metric) units is found on page 7. 



24 



