PART V: MODELING SCOUR AT COASTAL STRUCTURES 

 USING MOVABLE -BED PHYSICAL MODELS 



General 



49. The following sections provide a brief discussion of physical 

 hydraulic modeling as abstracted from Fowler and Smith (1986) . 



Model and Prototype Similarities 



50. When conducting scaled physical model tests of prototype phenomena, 

 for exact reproduction, three types of similarity are required between model 

 and prototype (i.e., the model and prototype must be geometrically, 

 kinematically , and dynamically similar). Without similarity, results from the 

 model tests cannot be extrapolated to render meaningful prototype results. 



51. For geometric similarity, the ratio of model-to-prototype lengths 

 must be the same for all corresponding lengths. Dynamic similarity is 

 achieved when all relevant forces which act on corresponding masses in the 

 model and prototype occur in the same ratio (F p /F m = constant) throughout the 

 flow fields. For precise modeling of any fluid prototype, ratios of all of 

 the above forces must be equivalent in model and prototype. Short of modeling 

 at a 1:1 (prototype) scale, no fluid exists with viscosity, surface tension, 

 and elasticity that will satisfy this equivalency requirement. Fortunately, 

 only one or two of these forces are dominant in a given phenomenon and the 

 other forces may be neglected with little error. For coastal modeling 

 studies, inertia and gravity forces are dominant and Froude scaling guidelines 

 are used for hydraulic parameters. Also, since turbulent flow exists in most 

 prototype situations, the scale is selected such that the model Reynolds 

 number, R = pUl//x , where U, the average velocity, is greater than the critical 

 Reynolds number (so that turbulent flow is obtained) . Kinematic similarity 

 requires that fluid flow patterns in model and prototype be similar. If all 

 force ratios and geometric length ratios are similar in model and prototype, 

 kinematic similarity is ensured. 



49 



