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Oislonce Offshore (Grid Intervals AX = 25 m = 82 ft ) 

 Figure 6. Comparison of measured and calculated wave heights. 



Both model results and observations indicate that there is probably little 

 variation in wave height from the seaward boundary of the models to the region 

 near the end of the pier. Landward of this point the observations indicate 

 greater reduction of wave height than indicated in the model results. This 

 attenuation is assumed to be due to frictional and diffractive effects which 

 are not simulated by these models. 



Questionable values of wave height and direction can be generated by the 

 spectral model in areas of convergence just as with the ray model. If a value 

 of wave height or direction differs by what could reasonably be expected over 

 a distance of one grid block (82 feet in this case), it probably is not valid. 

 As a rough estimate, wave height changes of more than 20 percent in the dis- 

 tance of one wavelength should be considered questionable. To obtain a more 

 representative value, a neighboring value or average of values can be used. 

 Work is underway to improve both models in those cases where results are not 

 valid. 



IV . SUMMARY 



This report has discussed two numerical miodels which allow a wave field 

 to be transformed by the process of refraction and shoaling from offshore to 

 nearshore. One model (WAVE, 720X6R1CH0) gives results along wave rays; the 

 other model (SYLT, 720X6R1CF0) gives results on a horizontal two-dimensional 

 grid of points. Both models are based on linear wave theory and hence are 

 limited by the assumptions of the theory. 



Examples of model output 

 the CERC Field Research Facil 

 wave direction and height are 

 finite-difference grid model 

 coordinator, Kingman Building 

 Engineering Computer Program 

 Experiment Station, P.O. Box 

 tation to assist the user in 



are presented by an application to an area near 

 ity. Model results and field measurements of 

 compared. The ray model (720X6R1CH0) and the 

 (720X6R1CFO) may be obtained from the CERC ADP 

 , Fort Belvoir, Virginia 22060-5196 or from the 

 Library (ECPL) , U.S. Army Engineer Waterways 

 631, Vicksburg, Mississippi 39180. Documen- 

 running the models is also available. 



14 



