W. A. Price (1955), J.L. Davies (1964, 1973) and Miles Hayes (1975, 1979) are responsible for elucidating the relationship between hydrographic regime and coastal morphology. Using Davies' tidal classification of coast Hayes inventoried and diagrammed the distribution of features characteristic of coastal plain shorelines (fig. 1). On coastal plain shelves microtidal, mesotidal and macrotidal coasts each have a characteristic assemblage of features. Assumed in this classification is that with increasing tidal energy the importance of wave energy decreases. However this relationship is inferred and not quantified.
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Tides are responsible for carrying and flushing sediment perpendicular to the coast while wave driven littoral transport moves and redistributes sediment parallel to the coast. Barrier islands beaches are shore-parallel wave-dominant features.
On microtidal coasts barrier islands are several tens of kilometers long and inlets are few and far between. Because of the dominant littoral currents inlets carved during storms rarely remain open, but rapidly migrate down drift and fill. Sediment is transported into the back barrier region by storm overwash or through the sequential formation of flood tidal deltas during inlet migration and closure. Microtidal barriers are often backed by a bay or lagoon and flood tidal deltas are prominent on the shoreward side of inlet estuaries (fig. 2).
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Figure 2. Microtidal estuary model based on the barrier coast of the Gulf of Mexico. Wave generated sand bodies such as aligned beaches and recurve spits are common. Flood tidal deltas and storm generated washover fans are also prominent. Redrawn from Hayes and Kana, 1976) |
Inlets between barriers on mesotidal coasts are more abundant because of the increased flushing capacity of the tides. The large amount of sediment that is carried through the inlet is deposited on both the oceanward (ebb tidal delta) and shoreward (flood tidal delta) side. As flood tidal deltas build up they develop into vegetated marshland.
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Figure 2. Mesotidal barrier island inlet estuary. Oceanward
and shoreward accumulations of sand form the ebb and flood tidal deltas
respectively. Ebb tidal deltas are well developed. The strength of ebb
flow relative to wave energy controls the seaward elongation of the
delta. The back barrier region is typically occupied by marsh developed
in part on older flood tidal delta deposits. (Redrawn from Hayes
and Kana, 1976 ) |
In a marcotidal environment littoral transport is minimal and so is the ability to maintain sandy barriers and drift aligned beaches. Riverine and wave shaped river deltas that form on microtidal and mesotidal coasts are replace by funnel-shaped embayments on macrotidal coasts (figure 3). Here sand that would otherwise be carried along the coast is either deposited in tidal channels or carried off shore and deposited in linear sand ridges. Broad muddy tidal flats occupy the margins of macrotidal embayments. However, along tropical macrotidal coasts tidal flats are replaced by tide dominated mangrove swamps. On arid coasts broad salty tidal flats predominate.
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Figure 5. Macrotidal estuary. Typical funnel-shaped embayment with offshore linear sand bars and broad marginal tidal flats. Arrows show the direction of tidal flow. Redrawn from Hayes and Kana, 1976. |
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Key to Locations:
Figure 4. Shoreline classification based on both wave and tidal energy. From Hayes 1979. |
Lindley
Hanson/email /Gls214
Department
of Geological Sciences, Salem State
College, Salem, MA
