EM 1110-2-1100 (Part III)
30 Apr 02
h. Three-dimensionality of shoreline features.
(1) The three-dimensionality of noncohesive shoreline shape and its corresponding underwater
expression are important to various aspects of engineering design. Dunes are more susceptible to
breakthrough where the beach width fronting the dune is narrow due to the diminished protection afforded
by the berm. Noncohesive shorelines are typically neither straight nor of smooth curvature. They often have
isolated "bumps" or more regularly spaced shoreline features. Migration of such shoreline features can
undercut or flank the landward ends of coastal structures such as seawalls or groins. For this reason any
regularly spaced (rhythmic) topographic features inherent in the beach/shoreline structure as reflected in its
planform shape may be of importance to the engineer for consideration of risk factors in evaluation of
projects. Little is known about these features from a quantitative standpoint, although considerable
qualitative descriptions of the features are documented.
(2) Typically, shoreline rhythmic features are classified by their planform (longshore) spacing or
approximate wavelength if they are of reasonably regular form and hence are often referred to as "sand
waves" by many authors. The term "sand wave" in this context should not be confused with underwater sand
waves that are ubiquitous in most marine environments. Planform amplitude or height of the shoreline
features (defined as cross-shore distance from embayment to cusp point) often is correlated with the
wavelength. That is, longer shoreline planform wavelength (or alongshore spacing) suggests larger planform
amplitudes (Sonu 1969). Along most shorelines, a spectrum of rhythmic shapes and sizes is present, making
the underlying shoreline planform characteristics somewhat confusing. Due to the continuous range of scale
in observed shoreline rhythmic features it is impossible to completely separate discussion of the various
features by size since the physics governing the various length scales may be similar.
(3) At the small end of the scale of rhythmic shoreline features (and consequently of lesser engineering
significance) are "beach cusps." Figure III-2-23 is an example of a beach with developed beach cusps.
Russell and McIntire (1965) compile observational statistics on beach cusps from a number of ocean beaches
and show cusp planform wavelengths (or longshore spacing) from 6 to 67 m. Numerous authors have
postulated theories for the conditions of formation as well as spacing and amplitude of these smaller-scale
features.
(4) At present, though, an explanation that would encompass all the numerous small-scale rhythmic
features noted along the shoreline is lacking. Extended discussion on these smaller-scale shoreline features
can be found in Komar (1976).
(5) As an example of larger-scale rhythmic topography, Figure III-2-24 shows a shoreline from Tokai
Beach, Japan (Mogi 1960), in which two predominant wavelength scales of rhythmic sinuous topography
dominate over a 3-month period of study. The shorter planform wavelengths in this example are on the order
of 250 m while the longer planform wavelengths are on the order of 2.5 km. Although phasing changes are
evident, the rhythmic feature length scales appear to have prevailed throughout the different survey periods.
Lippmann and Holman (1990) have documented the conditions for rhythmic bars along one section of the
North Carolina shoreline (Duck, NC). They found that rhythmic bars were a predominant feature observed
in 68 percent of video imaging records and noted that during the strongest wave activity the rhythmic features
were destroyed but that the features returned 5-16 days following peak wave events.
(6) Along beaches in Japan, Hom-ma and Sonu (1963) observed that under certain conditions crescentic
bars with a regular alongshore spacing would weld to the shoreline with consequent large cusps formed at
the attachment points (see Figure III-2-25). Sonu (1973) notes a second type of rhythmic topography in
which rhythmic shoreline features are associated with the presence of rip current cell circulation (see Fig-
ure III-2-26). Sonu (1973) noted that both types of rhythmic topography could be present independently
Longshore Sediment Transport
III-2-49
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