developed for the littoral environment (e.g., Kraus 1983). However, generally 

 applicable models are only just becoming available. 



83. The simplest form of a numerical model is a diffraction analysis 

 with resultant energy flux calculations along the shoreline. A model which 

 computes shoreline response to wave conditions through time as the waves dif- 

 fract, refract, and shoal over a homogeneous offshore slope and around struc- 

 tures is called a "1-line" model (Kraus 1983). A "multi-line" or "N-line" 

 model can account for a downward-sloping, heterogeneous offshore bathymetry to 

 transform the deepwater wave and modify the bathymetry to develop an equilib- 

 rium profile and a stable beach planform configuration (Perlin and Dean 1983). 

 Field verification of the basic model theory is necessary before widespread 

 design application. 



84. Any model, physical or numerical, is only valid if the input param- 

 eters, particularly the wave conditions, are appropriate for the project site. 

 A few degrees of variation in approach angle, incorrect wave periods or 

 heights, or an improper distribution of the relative directional wave domi- 

 nance can all result in erroneous predictions. In addition, the assumptions 

 and limitations inherent in any model can greatly affect its results and how 

 they should be applied. The designer must understand how the model works in 

 order to develop the correct input and correctly interpret the model predic- 

 tions. There needs to be significant interaction between the designer and the 

 modeler to assure reliable site-specific adaptation of the model and a valid 

 interpretation of the model predictions. 



85. In summary, neither a numerical model nor a physical model should 

 be used as the only mechanism for determining a breakwater configuration. At 

 present, no single model can account for all the factors which might affect a 

 specific project site. Sediment transport on bars and bar migration, regional 

 and local current patterns, water-level changes, variability in the sediment 

 supply, wave reflection, rip currents, the interaction of long period swells 

 and local sea state, etc., can all significantly affect the actual sediment 

 response, but may be difficult or impossible to include in a single model. 



Field Tests of Breakwater Design 



86. Even with meticulous initial design and extensive physical or nu- 

 merical model tests, shoreline response to a detached breakwater project is 



58 



