the wave conditions in the offshore time series are grouped into the desig- 

 nated period-direction bands. The wave height on the nearshore reference line 

 calculated with unit offshore wave height is then given as the product of the 

 transformation coefficient alongshore and the input offshore wave height at 

 the time step, which is permissible by linear wave theory. Thus, although the 

 wave period and direction are constrained to lie in a finite number of bands, 

 the actual offshore wave height is used. Since it is doubtful whether 

 directional resolution greater than 11.25 or 22.5 deg can be achieved by 

 either a deepwater wave gage or hindcast, the described procedure is an 

 adequate representation of the data, yet it allows for efficient calculation. 

 Smaller increments in wave angle could be implemented, if appropriate. 



159. As an alternative to building a key for accessing the refraction 

 results, nearshore wave conditions on the reference line thus calculated can 

 be arranged in their order of occurrence in the offshore wave time series and 

 a large data file of nearshore wave conditions generated and stored for input. 

 In any case, manipulation of the wave data base requires substantial effort 

 and is one of the necessary tasks that must be performed as part of the data 

 preparation process if an external wave model is used. Practical details of 

 the use of an external refraction model with GENESIS are given in the GENESIS 

 Workbook. 



Limiting Deepwater Wave Steepness 



160. The input offshore wave data may be changed or manipulated for a 

 number of reasons, for example, to examine model sensitivity, to look at 

 extreme cases, and to run waves for storm (high-wave) conditions. In these 

 investigations the wave height is usually increased. In the process, if care 

 is not taken, it is possible to specify waves of unphysically large steepness. 

 GENESIS performs a check that the offshore input wave steepness satisfies the 

 Mitchell (1893) limiting wave steepness criterion: 



0.142 (21) 



76 



