c. The direction of sand transport alongshore must be preserved in 

 accordance with the natural direction of the potential local 

 transport. 



11. Although the above-listed principles are easy to understand, their 

 implementation in a computer program is considerably involved, in particular, 

 for b and c. The present report describes well-tested algorithms for imple- 

 menting the seawall boundary condition in a general manner. 



Limitations of the Method 



12. The seawall constraint should be formulated on the same level of 

 idealization as the shoreline model. Thus, it is not appropriate in the model 

 to consider wave reflection and sea bottom scouring, and settling, flanking, 

 and collapse of the seawall (for further discussion on one or more of these 

 topics, see Sato, Tanaka, and Irie 1969; Silvester 1977; Toyoshima 1979; 

 Walton and Sensabaugh 1979). It should be stressed that the procedure de- 

 scribed here possesses the same limitations as well as the same advantages as 

 the shoreline model. The seawall boundary condition is only valid to the ex- 

 tent the shoreline model is valid. 



13. One of the most restrictive assumptions made in deriving the shore- 

 line model is that the beach profile remains unchanged and moves seaward or 

 shoreward in parallel to itself (an assumption of equilibrium of the profile). 

 In nature, however, if a beach erodes to reach a vertical or nearly vertical 

 seawall, due to wave reflection and scouring, the beach slope immediately in 

 front of a seawall is expected to become steeper than the slope on the adjoin- 

 ing beach without structures or steeper than the original beach before the 

 seawall was built. 



14. The above discussion notwithstanding, examples can be found in the 

 field of the growth and recovery of formerly eroded beaches fronting rough- 

 faced sloping seawalls (Toyoshima 1979); nearly vertical seawalls (O'Brien 

 1985); and even a vertical seawall (Berrigan 1985a, b). Because of an apparent 

 lack of data in these cases, however, cause and effect have not been clearly 

 distinguished. That is, it is not known with certainty whether the seawalls 

 promoted growth of the beaches in front of them or if, e.g., sediment trans- 

 port conditions changed to bring back the beaches with no relation to the sea- 

 wall, the seawall only initially functioning to protect the land behind it. A 

 combination of the two scenarios is also possible. Toyoshima (1979) states 



