Operational procedures 



44. The representation of the sea state in scale models continues to 

 improve in modeling facilities because of enhancements of wave generating 

 equipment and improved understanding of the physics of water waves. The ear- 

 liest wave generators were capable only of a sinusoidal motion generating 

 monochromatic waves. The last decade has seen these facilities replaced in 

 many laboratories with wave generators capable of producing irregular waves 

 which simulate specified prototype energy spectra or irregular time series. 

 The techniques for application of monochromatic waves are somewhat standard- 

 ized, but presently there are widely differing opinions on the most appro- 

 priate application of irregular waves in scale models of rubble-mound stabil- 

 ity. The transformation of the waves from deep water to shallow water must be 

 arranged to be equivalent to that in the prototype for both monochromatic and 

 irregular waves. The shallow-water waves of interest for stability are usu- 

 ally taken to be those naturally transformed waves that would exist at the 

 site without the structure in place. This convention usually involves a cal- 

 ibration of the model facility before the model structure is placed in a flume 

 or basin. 



45. Complications with reflected waves arise after the structure is in 

 place. Techniques are available for analysis of model wave data which resolve 

 incident and reflected waves (Goda and Suzuki 1976). Some facilities are ca- 

 pable of compensating for reflected waves by modified motion of the wave gen- 

 erator. It is necessary in facilities without this capability to reduce wave 

 reflection as much as possible by various other means. Use of irregular model 

 waves can also result in spurious long-period waves (Jensen and Kirkegaard 

 1985) which must be compensated for by the generator or in the interpretation 

 of measured results. 



46. Model breakwater materials must reflect a number of prototype con- 

 ditions, including geometry, density, surface roughness, and orientation in 

 the structure. A number of recent tests have also involved attempts (the re- 

 sults of which remain in question) to estimate mechanical stresses within 

 armor units (Timco 1981 and Delft Hydraulics Laboratory 1985). Geometry, den- 

 sity, and surface roughness are controlled by careful choice of model mate- 

 rials and preparation of the units. Minor density scale effects due to use of 

 fresh water in a model of a saltwater site can usually be compensated for by 

 small adjustments to the weight of the model breakwater units. Orientation 



29 



