EM 1110-2-1100 (Part III)
30 Apr 02
of St. James Island, Florida (Niedoroda and Tanner 1970); 100 to several thousand meters in the case of
the example shown in Figure III-2-25 (Sonu 1973); up to 1,500 m for giant cusps noted along various
beaches (Shepard 1952, 1973); and on the order of hundreds of meters along the Atlantic bluff shoreline
of Cape Cod, Massachusetts (Aubrey 1980). Sonu and Russell (1967) noted, and later Dolan (1971)
measured shoreline rhythmic features along the North Carolina coast with alongshore spacing ranging from
150 to 1,000 m with the predominant spacing about 500 to 600 m. In the same study, Dolan (1971) measure
planform amplitudes from 15 to 25 m with large sand waves reaching amplitudes of 40 m. Numerous, well-
documented surveys of sand wave/giant cusp rhythmic beach planforms also exist along the Danish and
Dutch coasts (van Bendegom 1949, Brunn 1954, Verhagen 1989).
(8) Migration rates of rhythmic features vary widely with some studies reporting short-term fluctuations
in position but no net long-term migration. Van Bendegom (1949) documents the monitoring over an 80-year
period of large sand waves with amplitudes up to 200 m along the Dutch coast, and discusses the
corresponding cycle of beach erosion and accretion as these waves migrate along the coastline with an
average speed of 200 m/year. Dolan (1971) noted migration velocities of large sand waves along the North
Carolina coast ranging from 100 to 200 m/month during heavy weather seasons. Verhagen (1989) has
documented sand waves along the Dutch coast with amplitudes from 25 to 2,500 m and longshore speeds
ranging from 45 to 310 m/year. Sonu (1969) has suggested that migration velocities of such shoreline
features are inversely proportional to some power of the feature's alongshore spacing (i.e., the larger the
feature, the slower the movement).
(9) Edge waves and rip currents are often cited as the main contributing forcing functions to the
formation of rhythmic topography at various scales. A discussion of edge wave generation and hypothesized
effects on beaches can be found in Guza and Inman (1975), Huntley and Bowen (1973, 1975a, 1975b, 1979),
Huntley (1976), Holman and Bowen (1979, 1982), Wright et al. (1979), Guza and Bowen (1981), and Bowen
and Inman (1971). A discussion of rip current formation and its effects on beaches can be found in Bowen
(1969), Bowen and Inman (1969), Hino (1974), Dalrymple (1975), Dalrymple and Lanan (1976), Dolan
(1971), Komar (1971), Komar and Rea (1976), and Komar (1978). Conclusive evidence proving the
mechanisms for the formation of the many types of rhythmic topography is lacking.
(10) From an engineering standpoint the importance of rhythmic shoreline features (especially larger
ones) and their potential for migration should not be overlooked in planning engineering structures or in
analysis of design dune width for storm protection. For example, van Bendegom (1949) documents the
structural failure of a groin due to the erosion produced by a large, migrating sand wave along the Dutch
coast. Brunn (1954) described migrating sand waves along the Danish North Sea coast with observed
planform spacings on the order of 300 to 2,000 m and amplitudes on the order of 60 to 80 m in areas where
seasonal beach change was only 20 m/year and long-term shoreline recession only 2 m/year. In this regard,
Brunn (1954) also cites a case of a sand wave of 900 m wavelength and 60 m amplitude with a migration
speed of 700 m/year, and notes the difficulty of drawing definitive conclusions on average shoreline
movements in such areas. Dolan (1971) noted that the regular spacing of dune breaching on Bodie Island,
North Carolina, during the Ash Wednesday storm of 7 March 1962 correlated well with the rhythmic
topography seen in the shoreline. Dolan (1971) also documented erosion along the Cape Hatteras, North
Carolina, shoreline corresponding to embayments of rhythmic topography and suggested that when analyzing
beach variability for specific sites, in addition to the seasonal recession-progradation cycle, additional
variation (about 20 percent along the Outer Banks) should be considered to account for migration of the
rhythmic topographic features.
(11) In practice, aerial photography at a reasonable scale (1 in. = 100 m or larger) or shoreline surveys
are necessary to document the existence of rhythmic shoreline features. Sets of such aerial
photographs/shoreline surveys with common control points and interspersed over long periods of time should
Longshore Sediment Transport
III-2-53
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