vertical scale, and its value relates the order all slopes of the proto- 

 type are steepened in the model. In the study of tidal inlets, particu- 

 larly with movable-bed models, efforts are made to design models with 

 distortion values of five or less. Otherwise, the slopes required in 

 the movable-bed model for accurate reproduction of the prototype may be 

 steeper than the angle of repose of the model material, thus creating a 

 difficult scale effect to overcome. This point is introduced in this 

 section because inlets are often modeled with both a fixed bed and a 

 movable bed and with a distorted scale. Vertical scale ratios, model- 

 to-prototype, are generally in the order of 1:40 to 1:100; horizontal 

 scale ratios are generally in the order of 1:100 to 1:500. 



Distorted-scale inlet models are frequently constructed for multiple 

 purposes; e.g., an investigation of an inlet may be necessary where a 

 jetty is to be installed. A prediction will be required of the effects 

 of the jetty on tidal currents and water levels near the inlet and also 

 the degree to which the jetty interrupts the littoral drift and affects 

 deposition patterns near the inlet. In this case, a multipurpose model 

 is needed. This model would first be built with a distorted-scale fixed- 

 bed design and then adjusted and tested to determine the effects of the 

 jetty on tidal heights and currents, A segment of the fixed part of the 

 model surface would then be carefully removed and replaced with a movable 

 material to evaluate the effects of the jetty on the littoral drift. 



Model verification and testing in a distorted-scale, fixed-bed model 

 follow essentially the same procedures discussed for an undistorted-scale, 

 fixed-bed model. However, because of distortion effects the transference 

 equations from the model to a prototype situation are, in general, com- 

 pletely different. 



6. Movable-Bed Models . 



a. Theoretical Aspects of Movable-Bed Material Modeling . The ac- 

 cepted practice at many hydraulic laboratories experienced in the art of 

 movable-bed modeling is to construct the model to a manageable size based 

 on space limitations and instrumentation ability, and to use a readily 

 available material for construction (usually sand) which constitutes a 

 model scale distortion. Next, the empirical process of verifying the 

 model to reproduce prototype bed forms such as scour and deposition has 

 led to the distortion of a second parameter. This is usually accomplished 

 in the model by altering the wave climate, increasing or decreasing tidal 

 flow, or by changing the time scale from that resulting from the hydro- 

 dynamic scaling relations to an empirically selected one which reproduces 

 the sedimentology (referred to as the sedimentological time scale) . These 

 are empirical solutions based on the clever application of scale modeling 

 and the experience of the researcher; however, the mechanism of most sedi- 

 mentation phenomena is still not well understood. Several investigators 

 have attempted to derive formal scaling laws resulting in many varying 

 modeling formulas from which to choose, and, according to Kamphuis (1975), 

 owing to the variety and magnitude of scale effects, modeling coastal 

 movable-bed material continues to be an art rather than a science. 



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