V. COASTAL EROSION 



hy 

 Robert W. Whalin and C.E. Chatham^ Jr. 



1. Introduction . 



This section discusses hydraulic models of coastal erosion. A model 

 study of this type is a three-dimensional investigation which involves 

 littoral transport, onshore-offshore transport, and possibly scour or 

 erosion around structures. The discussion is limited to coastal ero- 

 sion problems not involving tidal inlets; thus, the major forcing func- 

 tion of concern is that due to wind-wave action. A movable-bed model 

 study is needed to investigate the effect of coastal or offshore struc- 

 tures on shoreline evolution, the stability of beach modifications (e.g., 

 the construction of perched beaches; Chatham, 1972; Chatham, Davidson, 

 and Whalin, 1973), and the design of functional structures for preventing 

 coastal erosion. Shoreline dynamics and engineering problems resulting 

 from coastal erosion are well documented (Bagnold, 1940; Krumbein, 1944; 

 Keulegan, 1945; Brown, 1950; Bascom, 1951; Beach Erosion Board, 1954; 

 Shinohara, 1958; Le Mehaute and Brebner, 1961; Eagleson, Glenne, and 

 Dracup, 1961, 1963; Eagleson, 1965; Inman, 1965; Inman and Frautschy, 

 1965; Caldwell, 1966; Ippen, 1966; Johnson, 1919, 1956, 1957, 1959, 1966; 

 Inman, Komar, and Bowen, 1968; Bijker and Svasek, 1969; Saville, 1969; 

 Einstein, 1948, 1971; Yalin, 1972; and Silvester, 1959, 1974). 



A completely quantitative movable-bed model investigation of coastal 

 erosion appears to be impractical within the present state-of-the-art. 

 However, movable-bed scale-model investigations of several types of 

 coastal erosion problems are feasible and can be conducted in such a 

 way that useful, and sufficiently accurate, information can be obtained 

 for design purposes. If adequate prototype data are available, and veri- 

 fication procedures in the model are successful, an investigator should 

 have confidence in the results of coastal erosion models (e.g., as in 

 the movable-bed river models investigation by Vemon-Harcourt). 



A satisfactory movable-bed scale-model investigation is perhaps the 

 most difficult type of model study to perform. Some examples of labora- 

 tory movable-bed model studies include: U.S. Army Engineer Waterways 

 Experiment Station (1943); Bagnold (1947); Saville (1950); Rector (1954); 

 Watts (1954); Shinohara (1959); Shinohara and Tsubaki (1959); Iwagaki and 

 Noda (1962); Savage (1959, 1962); Inman (1963); Sitarz (1963); Kalkanis 

 (1964); Bonnefille and Pemecker (1965); and Nayak, 1970. A study of the 

 physical processes involved in beach erosion by wave action (Manohar, 

 1955; Eagleson and Dean, 1961; Collins, 1963; Vanoni, 1964; Eagleson. 

 1957, 1959, 1965; and Calvin, 1967) would lead to an appreciation of 

 these difficulties. There is a transition from one basic regime of bound- 

 ary flow to another as sediment motion outside the surf zone is compared 

 to sediment motion in the surf zone; thus, exact dynamic similitude of 

 the dominant physical processes in both regimes simultaneously (using the 



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