4. The shore can be protected against erosion through the use of 

 coastal structures, nonstructural procedures, such as beachfill, or a combina- 

 tion of structures and nonstructural methods (SPM 1984; US Army Corps of Engi- 

 neers 1981 ). In situations where extensive damage may occur because of storm 

 waves and water intrusion, or where nonstructural procedures are not feasible, 

 then seawalls, bulkheads, and coastal dikes are commonly constructed for beach 

 erosion control and for preventing inundation. If the word "seawall" is used 

 to describe any man-made or natural object which functions as a nonerodible 

 barrier along the shoreline, the concept of "seawall" encompasses true sea- 

 walls, coastal dikes, storm surge barriers, shore-connected breakwaters, bulk- 

 heads, revetments, and rocky coastal cliffs. A coast may contain several such 

 seawalls, and their presence must be taken into account when assessing the 

 long-term (order of years) evolution of the shoreline. 



5. It is also necessary to estimate the impact of a proposed seawall in 

 the design process for shore protection. Even a wide sandy beach cannot erode 

 indefinitely; at some point in time the beach material will be exhausted, and 

 permanent structures and resources will become exposed to wave action and in- 

 undation. In such a situation, emergency protective measures will be taken, 

 most likely by the construction of a revetment, bulkhead, or seawall. A nu- 

 merical model of shoreline change must allow for the real world situation of 

 the ultimate presence of a seawall. 



Seawalls and the Shoreline Change Model 



6. Numerical models provide a powerful means for making quantitative 

 estimations of shoreline evolution. In particular, the so-called "one-line" 

 numerical model, originating from the work of Pelnard-Considere (1954), has 

 been widely applied in recent years. Kraus (in preparation) gives an anno- 

 tated bibliography of the literature on one-line models. The term "one-line" 

 typically refers to the shoreline; therefore, this model is often called the 

 "shoreline" model. Despite the large number of applications of the shoreline 

 model, representation of the action of a seawall in the model has received 

 little attention. A seawall imposes a constraint, or boundary condition, on 

 the solution (shoreline position) obtained with the model. 



7. The most obvious boundary condition imposed by a seawall is that 

 the beach fronting the wall cannot move landward of it. Also, a seawall 

 prevents the sediment contained behind it from entering the littoral system, 



