PART II: BACKGROUND FOR THE SEAWALL BOUNDARY CONDITION 

 Action of a Seawall on a Beach 



20. There is remarkably little quantitative information available on 

 the behavior of real beaches backed by seawalls. It has been long known that 

 under certain wave conditions, a vertical seawall will accelerate erosion of 

 the beach in front of it (see Russell and Inglis 1953; Sato, Tanaka, and Irie 

 1969). Scour is the primary cause of this erosion. Sand is scoured from the 

 sea bottom in front of a vertical seawall by the standing wave system produced 

 by wave reflection at the wall. Any current, such as the longshore current, 

 can then transport the mobilized sand out of the area. If there is a contin- 

 ued net loss of sand over a long period of time, the end result is that the 

 beach in front of the seawall can no longer maintain the natural equilibrium 

 profile and the beach slope will become steeper. Walton and Sensabaugh (1979) 

 discuss this and other processes believed to enhance erosion of beaches backed 

 by vertical or nearly vertical seawalls. 



21. On the laboratory scale, it has been amply demonstrated that a sea- 

 wall does not always produce erosion when introduced in the active wave zone 

 of a beach in equilibrium with the existing waves. A brief discussion will 

 now be given of three experiments (Dorland 1940, Chestnutt and Schiller 1971, 

 and Hattori and Kawamata 1977) performed using sand beaches in two-dimensional 

 wave flumes . 



22. Dorland (.19^0) used moderately steep waves in an attempt to repro- 

 duce storm conditions. He placed a vertical seawall at the shoreline of a 

 beach which had been allowed to attain equilibrium under constant wave action, 

 scooped out part of the bed in front of the seawall, and then continued 

 applying the. waves. In the two such experiments performed, the outer bar moved 

 landward and the scooped out area partially filled with sand from the offshore. 

 In a third series of runs- using three sets of wave conditions varying cycli- 

 cally, Dorland similarly found that the scooped out beach partially recovered. 



23. Chestnutt and Schiller (.1971) found that maximum erosion occurred 

 if a seawall was placed on an equilibrium beach in a "critical" region lying 

 from about 0.5 x^ to 0.67 x^ , where x^ is the width of the surf zone, as 

 measured from the shoreline. When the seawall was moved to a position shore- 

 ward of the critical region, the profile immediately seaward of the wall began 



