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Tales of the Coast
Coastal Dynamics

Coastal Effects of Tides

The importance of tides to coastal geological processes is threefold. First, the periodic change in water level results in different parts of the foreshore being exposed to wave energy throughout the day. In regions with large tidal ranges, the water may rise and fall 10 meters, and the shoreline may move laterally several kilometers between high and low water. This phenomenon is very important biologically because the ecology of tidal flats depends on their being alternately flooded and exposed. The geological significance is that various parts of the intertidal zone are exposed to erosion and deposition.

Second, tidal currents themselves can erode and transport sediment. Generally, tidal currents become stronger near the coast and play an increasingly important role in local circulation. Because of the rotating nature of the tidal wave in many locations (especially inland seas and enclosed basins), ebb and flood currents follow different paths. As a result, residual motions can be highly important in terms of transport and sedimentation. In inlets and estuaries, spatially asymmetric patterns of ebb and flood may cause mass transport of both water and sediment.

Third, tides cause the draining and filling of tidal bays. These bays are found even in low-tide coasts such as the Gulf of Mexico. This process is important because it is related to the cutting and migration of tidal inlets and the formation of flood- and ebb-tidal shoals on barrier coasts. The exchange of seawater in and out of tidal bays is essential to the life cycle of many marine species.


Coastal Effects of Tides

Tide Range: The difference in height between consecutive high and low waters. Davies (1964) applied an energy-based classification to coastal morphology by subdividing the world's shores according to tide range. Hayes (1979) expanded this classification, defining five tidal categories for coastlines:

  • Microtidal, < 1 meter
  • Low-mesotidal, 1-2 meters
  • High-mesotidal, 2-3.5 meters
  • Low-macrotidal, 3.5-5 meters
  • Macrotidal, > 5 meters