superimposed on the long-term evolution for which an analysis can be 

 done independently. 



Among the significant recent reports leading toward understanding of 

 surf zone circulation and related bottom topography are: Bowen and 

 Inman (1969] who advocate the presence of edge waves as a cause of rip 

 currents and beach cusps; Hino (1974) who states that rip currents are 

 the result of mobility of the sand bed and hydrodynamic instability; 

 Sonu (1972) and Noda (1972) demonstrated that a perturbation on bottom 

 topography causing waves to refract and have varying intensity along the 

 shore induces a variation in radiation stress which ^in turn enhances rip 

 currents; finally, Liu and Mei (1976) applied the radiation-stress 

 concept to a groin perpendicular to shore and to an offshore breakwater. 



These investigations offer at least partial answers to a number of 

 important problems, important in understanding shoreline processes. It 

 definitely indicates that the radiation-stress approach holds the poten- 

 tial key to understanding many types of nearshore currents, heretofore 

 unexplored. It is also evident that the study of surf zone hydrodynamics 

 will rapidly reach a plateau if sand-water interaction problems are not 

 mastered, and at this stage, these can only be considered empirically. 

 Determinism leaves off with the inception of turbulence. 



Even though the dynamics of nearshore currents hold the key to 

 understanding of beach processes, application of the methodology based 

 on radiation stress to investigate shoreline evolution mathematically is 

 still beyond the state-of-the-art. 



Both approaches could be pursued in parallel and the results of the 

 scientific approach could slowly be incorporated into a practical 

 engineering model. 



Conclusions based on the literature survey, as summarized in 

 Table 2, are: 



a. There is sufficient laboratory verification to give credibility to 

 a mathematical approach to the study of shoreline evolution for small 

 angles of wave approach. 



b. For large angles of incidence, there is a lesser chance at arriving 

 at a successful formulation since shorelines are then unstable and the 

 resulting shoreline evolution could not be predicted without the 

 initiation of more basic research beyond the present state of knowledge. 



c. Even though no field measurements subsequent to mathematical pre- 

 dictions have been found in the literature, many practicing engineers 

 have applied the theory of Pelnard-Considere (1956) to predict shore 

 evolution by taking into account variable wave climate. The method is 

 easy to apply and provides valuable information. 



52 



