predicted and measured results shows an overestimation of the significant wave 

 height, H s , by this prediction technique. For 13 September at 1200 a devel- 

 oping sea is dominant and the comparison shows good agreement. However, at 

 2100 the same day the seas had grown to more than 2.4 meters and the irregular 

 wave technique overpredicts H s for the deeper water and underpredicts for the 

 relatively shallower water depths. A more quantitative description of these 

 trends can be obtained through analysis of all the data for each of these cases. 



The entire data set for the irregular wave technique is shown in Figure 2. 

 The ordinate is the ratio of the predicted significant wave height, H s _ rec j, 

 to measured significant wave height, H s _ obs . This ratio would be equal to one 

 for perfect agreement between the model and measured results. The ratio of the 

 water depth to measured Kg-obs is plotted along the abscissa. The SPM method 

 (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1977) pre- 

 dicts that the ratio of breaker depth to breaker wave height is between 1.5 and 

 1.2 for the conditions encountered for these field measurements. The scatter in 

 the data shown in Figure 2 is of the same magnitude as seen by Goda (1975), Fig. 

 12) in data from Sakota Port. Subsets of the data are plotted in Figures 3, 4, 

 and 5, which show the results for the three wave categories: sea, swell, and 

 Hq > 1.8 meters. An analysis of these plots leads to the following conclusions 

 on the performance of the irregular wave prediction technique for estimating H g : 



(a) At or below the breaker limit (i.e., d/H s < 1.5) the model 

 underpredicts H s by as much as 40 percent. 



(b) For sea conditions (i.e., waves are still in the stage of 

 generation by local wind) and H Q < 1.8 meters, there is good agree- 

 ment between predicted and measured results. The predicted H g is 

 on the average 13 percent greater than the measured H s . The ratio 

 Hs-pred/H s -obs ^ as mean 1.13 with a standard deviation of 0.14. For 

 all available cases of this type d/H s _ obs > 2. 



(c) For swell waves where d/H s -obs > 2 there is significant over- 

 prediction by the model. The predicted H s exceeds the measured H s 

 by as much as 150 percent with the average on the order of 50 percent. 

 The best agreement was for the measurements made at the Baylor farthest 

 offshore in about 7.6 meters of water. The ratio H s _ pre< i/H s _ obs has 

 mean of 1.5 with a standard deviation of 0.38. 



(d) For Hq > 1.8 meters, the model performance is mixed. For 

 observations at the Baylor gage in the deepest water (^7.6 meters) 

 the model shows good agreement with the gage measurements. For the 

 other locations, there is a tread where the model performance de- 

 creases as d/H s _ obs increases. At d/H s _ obs - 1.5 the agreement 

 is good. As d/H s _ obs increases the overprediction of H s by the 

 model increases rapidly until at d/H s _ obs = 4.0 the model is over- 

 predicting by about 80 percent. 



To determine how the SPM method compares to the field data, the SPM method, 

 given the deepwater wave height H and wave direction, can be used to predict 

 the wave height at points close to shore (decreased water depth) . This method 

 applies the linear wave theory for refraction and shoaling to obtain wave 



10 



