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Depositional shorelines can be subdivided conveniently into 3 groups on the basis of tidal range: hypotidal, 0-5 ft; mesotidal, 5-10 ft; and hypertidal, > 10 ft. In terms of worldwide distribution, the 3 groups are relatively equal in occurrence. This paper deals only with mesotidal barrier beaches and is based on field studies in New England and southeastern Alaska and on a literature survey of the barrier beaches of the world.
Tidal range is significant in the formation of beach structures in that it determines the distribution and concentration of wave energy over the beach profile and generates topography that affects ebb- and flood-current systems. Beach profiles change markedly with changes in tidal phase. The most dramatic changes in the beach profile, and the most rapid sediment migration (exclusive of storm conditions), occur at spring tides. Neap tides produce unique morphologic features such as neap berms and berm-ridges.
A large diurnal inequality of the tides, such as occurs in southeastern Alaska, has a striking effect on beach morphology and on the disposition of primary structures over the beach profile. This inequality results from 4 levels of concentration of wave energy during a 24-hour period.
The most fundamental unit in producing primary structures on mesotidal beaches is the ridge-and-runnel system. High-angle beds that dip landward are produced as the ridge migrates toward shore. A complex association of structures is affiliated with the migrating ridge. Some models of the associations of primary structures produced under differing conditions of tide, beach composition, and wave climate are derived from these field and literature studies.
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