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Geosciences LibreTexts

12: Coastlines

  • Page ID
  • Learning Objectives

    • Describe how energy is carried by waves and the relation of wave energy to features associated with the shoreline
    • Explain how breakers occur
    • Describe wave refraction and its contribution to longshore currents and the movement of sand along the coast
    • Explain how longshore currents cause the formation of spits and baymouth bars
    • Distinguish submergent and emergent coasts and describe coastal features associated with each
    • Describe the relationship between the natural river of sand in the littoral zone and human attempts to alter it for human convenience
    • Describe the pattern of the main ocean currents and explain the different factors involved in surface currents and deep ocean currents
    • Explain how ocean tides occur and distinguish among diurnal, semidiurnal, and mixed tide patterns.

    Coastlines are the great interface between the 29% of earth’s surface that is land and 71% of earth that is covered by the oceans. Therefore, it is the longest visible boundary on earth. To understand the processes that take place at this interface, we must first consider the energetic action at this boundary; namely, waves. More information on these topics can be found in a physical oceanography text. The importance of this interface is seen in the study of ancient shorelines, and particularly for natural resources, a process called sequence stratigraphy.

    Shoreline processes are complex, but important for understanding coastal processes. Waves, currents, and tides are the main agents that shape shorelines. Most coastal landforms can be attributed to moving sand via longshore drift, longterm rising sea levels, or longterm falling sea levels. Human intervention in beach processes, like jetties and groins, have negative consequences that need to be mitigated.

    • 12.1: Waves and Wave Processes
      Waves are created when wind blows over the surface of the water. Energy is transferred from wind to the water by friction and carried in the upper part of the water by waves. Waves move across the water surface with individual particles of water moving in circles, the water moving forward with the crest and moving backward in the trough. This can be demonstrated by watching the movement of a cork or some floating object as a wave passes.
    • 12.2: Shoreline Features
      Many different erosional and depositional features exist in the high energy of the coast. The coast or coastline includes all parts of the land-sea boundary area that are directly affected by the sea. This includes land far above high tide and well below normal wave base. But the shore or shoreline itself is the direct interface between water and land that migrates with the tides and with deposition and erosion of sediment. Processes at the shoreline are called littoral processes.
    • 12.3: Current and Tides
      Water in the ocean, when moving, can move via waves, currents, and tides. Waves have been discussed in chapter 12.1, and this section will focus on the other two. Currents in the ocean are driven by persistent global winds blowing over the surface of the water and water density. They are part of the Earth’s heat engine in which solar energy is absorbed by the ocean water (remember the specific heat of water). The absorbed energy is distributed by ocean currents.

    Thumbnail: The Cliffs of Moher, on the Atlantic coast of County Clare in Ireland, looking north from Hag's Head towards O'Brien's tower. Image used with permission (CC-SA-BY; Gdr).