volume and the direction of transport. The transport calculation has to be 

 performed in the same direction as the direction of transport. Therefore, two 

 independent algorithms, one calculating the transport rates from grid cells 

 1 to N+ 1 and one calculating in reverse order, are needed. These algo- 

 rithms should give the same transport rate. However, for large values of the 

 stability parameter or because of the presence of detached breakwaters, 

 especially if they are transmissive , the two algorithms may give slightly 

 different results. This message is issued if the difference in the two 

 calculated transport rates is greater than 0.0005 m 3 /sec at any cell along- 

 shore. This condition is remedied by decreasing the stability ratio, which in 

 turn is done by decreasing the time step, increasing the grid cell size, or 

 decreasing the wave height. Extremely high angles of wave incidence may also 

 produce this error. In addition to the reporting the actual transport rate 

 difference, the shoreline change resulting from this difference is also 

 reported. 



41. WARNING. UNPHYSICAL DEEPWATER WAVE STEEPNESS. The input offshore 

 wave data may be manipulated, for example, to investigate model sensitivity or 

 the effect of extreme conditions. In these investigations the wave height is 

 often increased, and, if care is not taken, it is possible to accidentally 

 specify waves of unphysically large steepness. GENESIS checks that the 

 offshore wave steepness does not exceed the value of 0.142, and, if it does, 

 reduces the deepwater wave height to satisfy this condition. This message is 

 issued if the wave steepness exceeds 0.142 and is remedied by decreasing the 

 wave height or increasing the input wave period in the WAVES file. 



Error Messages Issued by the Computer 



42. Even though much effort was devoted to making the data input proce- 

 dure as straightforward and error-free as possible, it is inevitable that 

 mistakes will made in preparing input files. As a result of mismatch errors 

 between read instructions in GENESIS and the improper content of a data file, 

 a computer will issue error messages that may be obscure and difficult to 

 interpret. Experience indicates that the most common input errors occur in 

 the START file. In this case the computer system may issue a message about an 



C13 



