equation for beach material, and the equation must be properly expressed in 

 numerical form (item b(4)). 



51. Requirement c(i) states that the numerical formulation must be 

 capable of representing all important morphological features. The scheme must 

 produce a profile which is in equilibrium with imposed waves of constant 

 properties (c(2)). This requirement is important because it is often 

 possible for a numerical scheme to appear to be stable for a small number of 

 calculation time-steps but will diverge if allowed to continue for a longer 

 simulation interval. This situation implies that the intermediate computed 

 results may be spurious. Naturally, any model must be calibrated and 

 verified by suitable field data (item c(3)). Only then can it be confidently 

 used for predictions. 



52. Concerning the requirement for extension to a probabilistic 

 description (item d(l)), it has been shown in Part II that a relatively wide 

 range in natural variability exists in beach profile change. In addition, an 

 artificial variability exists because the initial conditions required to set 

 up a numerical model of profile change (wave conditions, initial profile 

 shape, etc.) are not accurately known. The most logical approach would be to 

 calculate a range of values for profile change, within which the actual 

 change would be expected to lie. This approach leads to a stochastic or 

 probabilistic description. For item d(2), it is known that random waves 

 exhibit somewhat different properties from those of monochromatic waves. At 

 a later stage in modeling, the random nature of the wave field should be 

 taken into account as well as sediment transport under random wave action. 



Required Characteristics of Dune Erosion Models 



53. A pragmatic engineering simulation model for dune erosion need not 

 possess all the characteristics of a theoretically ideal beach profile change 

 model. In this subsection, a subset of parameters, expected to comprise a 

 first-stage dune erosion model, is listed. 



a_. Offshore wave height and wave period. (Local wave height may 

 or may not be necessary. ) 



b. Water level, including astronomical tide, meteorological tide, 

 and wave setup. 



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