Discussion of field simulation 



504. The numerical model best reproduced the measured response of the 

 beach profile with the most detailed, realistic input of wave height, wave 

 period, and water level variation. To correctly simulate the behavior of the 

 profile under changing forcing conditions, the variation in the input data 

 should have a time scale compatible with the profile response. Changes 

 between individual waves is not necessary or meaningful for use of the present 

 model , but differences occurring at a time scale of about an hour should be 

 represented for best results. Constant wave and water level conditions will 

 produce bars that are too steep and do not exhibit the smooth character 

 usually encountered in the field. 



505. The concept of breaking waves as a major cause of bar movement was 

 verified by the model simulation of the field profile change. Locations of 

 the bars were surprisingly well predicted considering the great variability in 

 water level during the modeled storm events . A mass conservation check 

 between measured initial and final profiles showed that none of the cases 

 simulated were free from three-dimensional effects. In Figure 79b the 

 difference in beach volume between initial and final volume was 45 m'^/m (a 

 loss in beach volume constituting 25 percent of the total absolute volume 

 moved across the profile). This difference is attributed mainly to differen- 

 tials in longshore sand transport, and, possibly, to limitations in the 

 surveys (spacing and accuracy). It is speculated that incorporation of long- 

 shore sand transport in the numerical model might produce a more pronounced 

 trough because a maximiom in the longshore sand transport rate is believed to 

 occur somewhat shoreward of the break point. 



506. Although the model was developed using laboratory data from situa- 

 tions with constant wave parameters, the capability to generalize and simulate 

 profile change on natural beaches with variable wave and water level condi- 

 tions was demonstrated. The steep foreshore and bar slopes produced in the 

 large wave tanks and well simulated by the numerical model were a product of 

 regular waves and constant water level. However, the important effects on the 

 profile of variable wave and water level could be represented fairly well by 

 superimposing regular waves with time changing height and period and stepwise 

 changes in water level. Thus, a single regular wave and fixed water level 



214 



