e. Analysis and Interpretation of Results . Although many questions 

 remain unanswered relative to the conduct o£ satisfactory movable-bed 

 scale models of coastal sediment transport, such investigations are con- 

 sidered feasible. The degree of confidence for such studies is largely 

 dependent, for each individual case, on the success of the prototype data 

 acquisition program, flume tests required for scale selection, and the 

 model verification phase. 



5. Fixed-Bed Tracer Models . 



Fixed-bed hydraulic models have long been recognized as an extremely 

 valuable tool in studying the effects of coastal construction projects on 

 wave, tide, and current conditions. In recent years, the use of rela- 

 tively small quantities of sediment tracer material in fixed-bed models 

 has generally been accepted as the most reliable and least expensive 

 method of studying sediment transport due to wave and tidal action. In 

 practically all cases, the results of such studies are considered qual- 

 itative rather than quantitative. 



In general, movable-bed model laws require distorted scales unless 

 the sediment in the model is the same as in the prototype (i.e., sand). 

 However, these laws can be adapted to undistorted-scale fixed-bed models 

 for the selection of tracer materials, and the scaling relations of Noda 

 (1972) are used as an example. 



Noda indicates a relationship or model law among the four basic scale 

 ratios: the horizontal scale, X; the vertical scale, y; the sediment- 

 size ratio, n^; and the relative specific weight ratio, n^i (see Fig. 

 5-1). These relationships were determined experimentally from a wide 

 range of wave conditions and beach materials and are valid mainly for 

 the breaker zone. Therefore, if there is an interest in longshore and 

 onshore-offshore sediment transport (which occurs mostly in and around 

 the breaker zone), this appears to be a most appropriate scale relation. 



Tracer material is selected for undistorted-scale models by the 

 following procedure. Using the characteristics of the prototype sedi- 

 ment (grain-size distribution, specific weight, etc.), the vertical model 

 scale is assumed correct and, for a specified material, the median grain 

 size and horizontal scale are computed. Next, the horizontal scale is 

 assumed correct and, for the same material, the median grain size and 

 vertical scale are computed. This procedure is repeated for several 

 materials and results in a range of median grain sizes for each material. 

 Preliminary model tests are conducted with the different sizes and based 

 on these test results, judgment, past experience, and knowledge of sedi- 

 ment movement in the prototype, the most realistic tracer material is 

 selected (i.e., the engineer is practicing art rather than science). 

 This procedure has proven effective in evaluating the movement and 

 subsequent deposits of sediment in several three-dimensional models 

 at WES; typical case histories are described below. 



Examples of Model Studies Conducted . 



(1) Port Orford Harbor, Oregon . 



296 



