that a rough-faced sloping and, ideally, permeable seawall will promote recov- 

 ery by dissipating wave energy, similar to the functioning of a natural beach. 



15. Based on the results of their laboratory experiments, Hattori and 

 Kawamata (1977) found that a necessary condition for the naturally occurring 

 restoration of an eroded beach backed by a seawall is that a surf zone exist 

 seaward of the wall. Essentially the same conclusion had been reached in an 

 earlier laboratory study by Chestnutt and Schiller (1971). Clearly, results 

 of simulations incorporating the seawall boundary condition in a shoreline 

 model must be interpreted with caution. 



16. In order to account for an alongshore variation in beach slope, a 

 mechanism to allow for cross-shore sand transport and a more complicated nu- 

 merical scheme than that used in the shoreline model are required. Numerical 

 models now exist which account for cross-shore transport in a schematic way. 

 The "2-line" model of Bakker (1969) and Bakker et al. (1971), and the "N-line" 

 model of Perlin and Dean (1978, 1983) are examples. Such models can, in prin- 

 ciple, more realistically represent the beach slope in front of a seawall than 

 can the shoreline model. 



17. At present, however, these models, although more sophisticated than 

 the shoreline model, have limitations for engineering use stemming from lack 

 of knowledge of the physical mechanism of cross-shore sand transport. Numeri- 

 cal instability and long computer run times are the main technical problems 

 encountered. Relatively short calculation time is an appealing feature of the 

 shoreline model. This feature, plus its demonstrated versatility for handling 

 a wide range of boundary conditions, ensures the use of the shoreline model as 

 an engineering tool in the foreseeable future. 



18. In summary for this section, to the extent that changes in beach 

 cross section can be neglected in comparison to changes in beach planform, the 

 shoreline model is a useful engineering tool for systematically investigating 

 and estimating shoreline evolution over time periods of several months to sev- 

 eral years. If seawalls are located along the coast, because of possible sig- 

 nificant changes in beach cross section, particular caution should be exer- 

 cised in interpreting model results. 



19. As progress is made, it will become desirable to incorporate the 

 seawall boundary condition in models more sophisticated than the shoreline 

 model. This task may prove to be difficult. Experience and familiarity with 

 the implementation of the seawall boundary condition in the shoreline model 

 should provide useful guidance. 



