10: Waves

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Learning Objectives

After reading this chapter you should:

know the parts of a basic wave
know the terminology used to describe the motion of a wave (i.e. period, frequency, speed etc.)
understand the circular motion of water particles involved in wave motion
understand the difference between deep water waves and shallow water waves
know what factors influence wave speed in deep and shallow waves
know the three factors that determine the energy of wind-generated waves
understand the concept of restoring force
understand the difference between seas and swell
understand the concepts of destructive, constructive and mixed interference
understand why waves break as they approach shore
know the differences in the different types of breakers, and how the bottom topography impacts breaker type
understand why waves always approach parallel to shore, and why waves are larger off of points and smaller in bays
understand what causes tsunamis, and how they behave in the ocean

What was the largest wave ever recorded? 50 feet? 100 feet? Not even close. That record belongs to a wave created in Lituya Bay, Alaska, on July 9, 1958. On that day, a magnitude 7.8 earthquake caused a massive rockslide that slid down a mountainside and into the headwaters of the bay. The rockslide created a splash wave that was high enough to flatten vegetation up to 1722 ft (525 m) above sea level! The wave then moved through the narrow bay towards the sea, destroying a number of fishing boats along the way. Miraculously, a father and son on one fishing boat were carried above the trees by the wave, and survived to tell the story. This is by far the largest wave, a megatsunami, ever reliably recorded. The waves we will discuss in this chapter may not be quite that dramatic, but it is still important to know how they form, how they are propagated, and what happens to them as they interact with the shore.

A view of Lituya Bay taken a few weeks after the 1958 megatsunami. The rockslide occurred in the mountains at the head of the bay, producing the wave that them moved through the bay towards the sea (D.J. Miller, United States Geological Survey, [Public domain], via Wikimedia Commons).