scales for surface heat exchange in heat dispersion studies are 1:1,000 

 horizontally and 1:100 vertically (10:1 distortion); however, if the 

 model is subjected to movable-bed testing, the maximum desirable distor- 

 tion is about 5:1. Therefore, the model designer must exercise a con- 

 siderable amount of judgment in the selection of the "best" scales for 

 the model. 



In general, the scale selection is based on several rather practical 

 considerations. The vertical scale must be large enough to permit accu- 

 rate water level measurements and to provide sufficient water depth in 

 which to make measurements of velocities, salinities, etc., at various 

 depths. The horizontal scale should be small to minimize the cost of 

 model construction or to yield a model size which will fit an existing 

 site. The scale distortion should be small enough to permit satisfactory 

 reproduction of all phenomena to be considered in the model investigation. 

 Probably most importantly, it must be determined that the scales selected 

 will result in turbulent-flow conditions in the model throughout most of 

 the tidal cycle. 



The vertical scale most commonly used for estuarine models at WES is 

 1:100. With existing instrumentation, this is the smallest scale with 

 which it is possible to determine water surface elevations to within 

 ±0.1 foot prototype. The l:100-scale is also convenient for making depth 

 or elevation measurements on the model. In addition, this scale is nor- 

 mally on the order of the smallest vertical scale which will ensure that 

 model flow is turbulent. Thus, vertical scales smaller than 1:100 are 

 seldom, if ever, used. For models of very shallow estuaries, this scale 

 may result in model water depths which create undesirable capillary effects 

 and which are too small for use of existing velocity meters. In such 

 cases, increasing the vertical scale to about 1:60 or 1:80 is necessary. 



Horizontal scales usually vary from 1:300 to 1:2,000, depending on 

 the nature of the problems to be investigated, available space, and con- 

 struction costs. Since the area of the model increases with the square 

 of the horizontal scale, doubling the horizontal scale quadruples the 

 area of the model. 



The vertical scale is usually selected first, and selection of the 

 horizontal scale is based on the degree of scale distortion which can be 

 tolerated. Distortion ratios (horizontal :vertical) most commonly used 

 are 3:1, 5:1, 10:1, and 20:1. The higher distortion ratios obviously 

 result in smaller models and lower construction costs. Generally, a 

 distortion ratio of 3:1 to 5:1 is used if a part of the model will subse- 

 quently be converted to movable bed. These low distortion ratios are 

 also required if qualitative tests of wave climate are conducted in the 

 model. For most models not requiring movable-bed studies, a distortion 

 ratio of 10:1 is satisfactory for a wide range of investigations includ- 

 ing tides, currents, salinity intrusion, shoaling distribution, dye dis- 

 persion, heat dispersion, and hurricane surges. For this degree of 

 distortion, it is not desirable to conduct even highly qualitative wave 

 climate tests or movable-bed shoaling studies. If use of the model is 



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