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Catch a Wave

Instructions: Study the animation below and then scroll down to answer the questions that follow.

Questions

Now, answer the questions below.

  1. What happens to a wave as it moves into shallow water?
    Check Answer
    When the water depth decreases to one half of a wave's wavelength, the wave starts to "feel the bottom". That means that the deepest water molecules set into circular motion by the wave's energy run into the seafloor. This forces the wave to grow upwards, so wave height increases. The base of the wave is slowed down by friction against the sea bottom, while the top of the wave rushes ahead, so the wave crest begins to lean more and more forward until it topples over, and breaks on the shore.
  2. Describe how the slope of the seafloor controls the way a wave breaks.
    Check Answer
    On a gentle slope, waves begin to feel the bottom far from the shore. The waves grow slowly taller and lean forward, and foam spills down their fronts as they run up onto the beach. On a moderate slope, the waves get closer to shore before they break. Because the water shallows more rapidly, wave energy is rapidly concentrated into a small area, so the waves grow very tall and the crests curl far forward of the troughs. On a steep slope, the waves don't interact with the bottom until they are almost right at the shore, so they don't have time to build-up, and instead just surge far up the beach without breaking.
  3. Which type of breaker - spilling, plunging, or surging - will cause the most coastal erosion? Explain.
    Check Answer
    A surging breaker will cause the most erosion because it slams into the beach at full speed. Spilling and plunging breakers slow down as they drag across the seafloor, so their energy is dissipated over a wider zone.
  4. Which type of breaker - spilling, plunging, or surging - will deposit sand onshore and expand beaches? Explain.
    Check Answer
    Waves can move sand when their energy is in contact with the bottom. Spilling breakers spend the most time and energy dragging across the seabed, so they should be able to push more sand onto the beach. Surging waves hardly interact with the bottom at all, so they will have little effect on offshore sand.
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