Presque Isle, Pennsylvania (July 1980) 



Figure 5-27. Offshore breakwaters with asymmetric cuspate spits (oblique wave 

 attack) . 



structure, forming a tombolo. Thus, breakwaters provide protection to the 

 backshore property not only by reducing incident wave energy, but also by 

 building a wider protective beach which acts as a buffer during storm events. 



3. Shoreli ne Response. 



The shoreline response to the construction of any offshore breakwater is 

 predominantly governed by the resulting alterations in the longshore transport 

 of material in the vicinity and, to a lesser extent, by the onshore-offshore 

 transport rate. The placement of a breakwater causes the shoreline to adjust 

 to the new conditions and seek an equilibrium configuration. 



If the incident breaking wave crests are parallel to the original shore- 

 line (which is a condition of no longshore transport) , the waves diffracted 

 into the offshore breakwater's shadow will transport sand from the edges of 

 this region into the shadow zone. This process will continue until the shore- 

 line configuration is essentially parallel to the diffracted wave crests and 

 the longshore transport is again zero. In this instance the cuspate spit (or 

 tombolo) will have a symmetric shape, with the tombolos featuring concave 

 sides and the cuspate spits exhibiting a more rounded convex shape. 



For obliquely incident waves the longshore transport rate in the lee of 

 the structure will initially decrease, causing deposition of the longshore 



5-63 



