4.3. LONG-WAVE HYDRODYNAMIC MODELS
153
best procedure is either to conduct large-scale tests to determine correct
wave dissipation properties of the basin boundaries, or to compare model
results with prototype measurements16.
16 Of course, prototype measurements can only be used when calibrating a model of an
existing facility with the intention of using the calibrated model to investigate proposed
changes.
Wave Breaking. Scale effects associated with the phenomenon of wave
breaking for geometrically undistorted models were discussed in reference
to short-wave models, and the reader is referred to that section for details.
Generally, it has been shown by Stive (1985) that scale effects are minimal,
or nonexistent, if the undistorted model wave height is greater than 10
cm. Below that wave height, surface tension may begin to impact the wave
breaking process. This will most likely be more of a problem for undistorted
long-wave models, which normally have small wave heights.
The wave breaking process in a geometrically distorted long-wave model
will not be representative of prototype wave breaking. This dissimilar wave
breaking is due to turbulent Reynolds stresses which are not in similitude
when geometric distortion is allowed in the model. Because wave breaking
is primarily a turbulent process, a scale effect will exist.
Prior to breaking, the processes of wave shoaling, wave refraction, and
wave diffraction are preserved for long waves scaled according to the distorted Froude criterion; but this scaling does not correctly reproduce wave
steepness, H/L, resulting in steeper waves in the model than in the prototype. Also, bottom slopes in a geometrically distorted model will be greater
than prototype slopes. Both of these factors will cause waves to break at
a different depth or even in a different mode (plunging instead of spilling)
than occurs in the prototype. Therefore, location of breakpoint and type
of breaker may not be accurately portrayed in a geometrically distorted
model.
Other Scale Effects in Long-Wave Models. The scale effects discussed in the previous sections comprise the more common effects encountered in long-wave physical models. However, there are several others scale
effects that may become important in certain instances, and the modeler
should be aware of their potential effect.
• Scale model experiments usually are conducted using fresh
water. If the prototype condition is salt water, there is
about a 3% difference in density and this changes wave
forces accordingly (Le Méhauté 1976).
153
best procedure is either to conduct large-scale tests to determine correct
wave dissipation properties of the basin boundaries, or to compare model
results with prototype measurements16.
16 Of course, prototype measurements can only be used when calibrating a model of an
existing facility with the intention of using the calibrated model to investigate proposed
changes.
Wave Breaking. Scale effects associated with the phenomenon of wave
breaking for geometrically undistorted models were discussed in reference
to short-wave models, and the reader is referred to that section for details.
Generally, it has been shown by Stive (1985) that scale effects are minimal,
or nonexistent, if the undistorted model wave height is greater than 10
cm. Below that wave height, surface tension may begin to impact the wave
breaking process. This will most likely be more of a problem for undistorted
long-wave models, which normally have small wave heights.
The wave breaking process in a geometrically distorted long-wave model
will not be representative of prototype wave breaking. This dissimilar wave
breaking is due to turbulent Reynolds stresses which are not in similitude
when geometric distortion is allowed in the model. Because wave breaking
is primarily a turbulent process, a scale effect will exist.
Prior to breaking, the processes of wave shoaling, wave refraction, and
wave diffraction are preserved for long waves scaled according to the distorted Froude criterion; but this scaling does not correctly reproduce wave
steepness, H/L, resulting in steeper waves in the model than in the prototype. Also, bottom slopes in a geometrically distorted model will be greater
than prototype slopes. Both of these factors will cause waves to break at
a different depth or even in a different mode (plunging instead of spilling)
than occurs in the prototype. Therefore, location of breakpoint and type
of breaker may not be accurately portrayed in a geometrically distorted
model.
Other Scale Effects in Long-Wave Models. The scale effects discussed in the previous sections comprise the more common effects encountered in long-wave physical models. However, there are several others scale
effects that may become important in certain instances, and the modeler
should be aware of their potential effect.
• Scale model experiments usually are conducted using fresh
water. If the prototype condition is salt water, there is
about a 3% difference in density and this changes wave
forces accordingly (Le Méhauté 1976).