Landforms of erosional coasts

There are two major types of coastal morphology: one is dominated by erosion and the other by deposition. They exhibit distinctly different landforms, though each type may contain some features of the other. In general, erosional coasts are those with little or no sediment, whereas depositional coasts are characterized by abundant sediment accumulation over the long term. Both temporal and geographic variations may occur in each of these coastal types.

Erosional coasts typically exhibit high relief and rugged topography. They tend to occur on the leading edge of lithospheric plates, the west coasts of both North and South America being excellent examples. Glacial activity also may give rise to erosional coasts, as in northern New England and in the Scandinavian countries. Typically, these coasts are dominated by exposed bedrock with steep slopes and high elevations adjacent to the shore. Although these coasts are erosional, the rate of shoreline retreat is slow due to the resistance of bedrock to erosion. The type of rock and its lithification are important factors in the rate of erosion.

Landforms of depositional coasts

Coasts adjacent to the trailing edge of lithospheric plates tend to have widespread coastal plains and low relief. The Atlantic and Gulf coasts of the United States are representative. Such coasts may have numerous estuaries and lagoons with barrier islands or may develop river deltas. They are characterized by an accumulation of a wide range of sediment types and by many varied coastal environments. The sediment is dominated by mud and sand; however, some gravel may be present, especially in the form of shell material.

Depositional coasts may experience erosion at certain times and places due to such factors as storms, depletion of sediment supply, and rising sea level. The latter is a continuing problem as the mean annual temperature of the Earth rises and the ice caps melt. Nevertheless, the overall, long-range tendency along these coasts is that of sediment deposition.

All of the processes discussed at the beginning of this section are in evidence along depositional coasts. Waves, wave-generated currents, and tides significantly influence the development of depositional landforms. In general, waves exert energy that is distributed along the coast essentially parallel to it. This is accomplished by the waves themselves as they strike the shore and also by the longshore currents that move along it. In contrast, tides tend to exert their influence perpendicular to the coast as they flood and ebb. The result is that the landforms that develop along some coasts are due primarily to wave processes while along other coasts they may be due mainly to tidal processes. Some coasts are the result of near equal balance between tide and wave processes. As a consequence, investigators speak of wave-dominated coasts, tide-dominated coasts, and mixed coasts.

A wave-dominated coast is one that is characterized by well-developed sand beaches typically formed on long barrier islands with a few widely spaced tidal inlets. The barrier islands tend to be narrow and rather low in elevation. Longshore transport is extensive, and the inlets are often small and unstable. Jetties are commonly placed along the inlet mouths to stabilize them and keep them open for navigation. The Texas and North Carolina coasts of the United States are excellent examples of this coastal type.

Tide-dominated coasts are not as widespread as those dominated by waves. They tend to develop where tidal range is high or where wave energy is low. The result is a coastal morphology that is dominated by funnel-shaped embayments and long sediment bodies oriented essentially perpendicular to the overall coastal trend. Tidal flats, salt marshes, and tidal creeks are extensive. The West German coast of the North Sea is a good example of such a coast.

Mixed coasts are those where both tidal and wave processes exert considerable influence. These coasts characteristically have short stubby barrier islands and numerous tidal inlets. The barriers commonly are wide at one end and narrow at the other. Inlets are fairly stable and have large sediment bodies on both their landward and seaward sides. The Georgia and South Carolina coasts of the United States typify a mixed coast.