to accrete, i.e., the previously wave-scoured region tended to be filled. 

 Chestnutt and Schiller point out that the surf zone width depends, in part, 

 on the wave period. Other factors being the same, the surf zone will be wider 

 for longer period waves. Therefore, whether or not a seawall will tend to 

 promote erosion or accretion depends on the wave conditions, which usually 

 have a marked seasonal variation. 



24. Hattori and Kawamata (1977) recorded beach profile changes on a 

 laboratory beach with and without a vertical seawall. For given wave con- 

 ditions, the beach was allowed to attain equilibrium before introduction of 

 the seawall. Incident wave steepness was varied for a fixed location of the 

 seawall relative to the initial shoreline. They found the existence of a 

 surf zone to be a necessary condition for recovery of an eroded seawall-backed 

 beach. This result is in agreement with the findings of Chestnutt and Schil- 

 ler (1971). The existence of a surf zone implies minimum wave reflection at 

 the seawall. Hattori and Kawamata also found that the restoring wave con- 

 ditions for a seawall-backed beach are similar to those for a natural labora- 

 tory beach without a seawall. 



25. Movable bottom laboratory experiments are difficult to interpret 

 because of scale effects, and longshore processes were absent in the experi- 

 ments under discussion. Nevertheless, a reasonable conclusion to be drawn 

 from the aforementioned work is that an eroded beach in front of a seawall 

 tends to recover when the mean water level is low, the waves have mild steep- 

 ness, and a sediment supply exists in the offshore. Toyoshima (1979), O'Brien 

 (1985), and Berrigan ( 1985a, b) give examples of prototype beaches backed by 

 seawalls which have become stable or have recovered to some degree. 



26. The interaction between beaches and seawalls is far from under- 

 stood. A focused and intensive field monitoring effort is definitely needed 

 as a first step toward achieving quantitative understanding of the influence 

 of a seawall on the shoreline and beach profile. Without data, quantitative 

 understanding and numerical modeling of the processes involved will be limited 

 and suspect. 



Seawall at Oarai Beach, Japan 



27. The physical picture for the seawall boundary condition formulated 

 by Hanson and Kraus (1985) is based on general observations of the shoreline 



10 



