How Waves Travel

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How Waves Travel

This lesson aligns with NGSS PS4.A

Introduction

A wave is a disturbance that propagates through space and time, transferring energy from one location to another without causing a permanent displacement of the medium through which it travels. In the case of water waves, the medium is water. This article explores the mechanics of wave motion in these two distinct environments, highlighting the differences in behavior and the factors that influence them.

Water Waves

Water waves can be classified into two main types:

Transverse waves: These waves move perpendicular to the direction of energy transfer. For example, when a stone is dropped into a still pond, waves radiate outward in a circular motion.
Longitudinal waves: These waves move parallel to the direction of energy transfer, as seen in sound waves. Water waves at the surface of the ocean primarily exhibit a combination of transverse and longitudinal motion, creating a circular motion of water particles. This motion varies significantly depending on the water's depth.

Wave Motion in Deep Water

Characteristics of Deep Water Waves

In deep water, typically defined as water depths greater than half the wavelength of the waves, water waves exhibit specific characteristics. The motion of water particles in deep water is primarily orbital; this means that as waves pass, water particles move in circular paths.

Particle Motion

Circular Motion: When a wave travels through deep water, the water particles beneath the surface move in circular orbits. The diameter of these orbits decreases with depth, meaning that the motion is most pronounced at the surface. Below a certain depth, the particle motion effectively becomes negligible.
Energy Transfer: As a wave propagates through deep water, it carries energy across the surface without significant movement of the water itself toward the shore. Instead, while the water surface rises and falls, the water particles return to their original positions after the wave passes. This characteristic allows deep-water waves to transport energy over long distances.

Wave Speed and Wavelength Deep-water waves are influenced by wind speed, duration, and fetch. The relationship between wave speed (v), wavelength (λ), and period (T) is given by the equation:

v=λ/T

In deep water, as waves gain energy from the wind, they can grow larger in amplitude and wavelength. The speed of deep-water waves increases with wavelength, meaning that longer waves travel faster than shorter ones.

Wave Motion Near the Shore

Characteristics of Shallow Water Waves

As waves approach the shore and enter shallower water, their behavior changes dramatically. Shallow water is defined as water depths less than half the wavelength of the waves.

Particle Motion

Elliptical Motion: In shallow water, the motion of water particles transitions from circular to elliptical orbits. As waves approach the shore, the circular motion becomes increasingly flattened due to the interaction with the seabed. This effect causes the orbits of water particles to become more elliptical, especially near the bottom.
Wave Breaking: As waves enter shallower depths, they slow down due to the friction with the ocean floor. This results in an increase in wave height and a decrease in wavelength, leading to the phenomenon known as wave steepening. Eventually, when the wave height exceeds a certain threshold relative to its wavelength, the wave becomes unstable and breaks, crashing onto the shore.

Energy Transfer Toward the Shore

Unlike in deep water, where waves primarily transfer energy without significant water movement, near the shore, waves cause water to move toward the beach. This movement is a result of the wave's energy being transformed into kinetic energy of the water particles. The breaking waves generate currents that can move significant volumes of water landward, contributing to beach erosion and sediment transport.

Surf Zone Dynamics

The area where waves break is known as the surf zone. In this region, the waves become increasingly chaotic, and water moves in various directions. Swells can create strong currents, such as longshore currents that flow parallel to the shore, which can carry sand and other materials along the coastline.

The Transition Zone: From Deep to Shallow Water

As waves travel from deep to shallow water, they undergo several transformations:

Wave Speed Decrease: As waves enter shallower depths, their speed decreases due to friction with the seabed. The decrease in speed is directly related to the water's depth.
Wave Height Increase: As waves slow down, they gain height (amplitude) due to conservation of energy. This increase can lead to dangerous conditions for swimmers and surfers if the waves become too steep and break.
Wavelength Decrease: The wavelength of waves shortens as they approach the shore, contributing to the increased wave height.

Conclusion

The motion of water particles in deep water is primarily orbital; this means that as waves pass, water particles move in circular paths.
As waves approach the shore and enter shallower water, their behavior changes dramatically. In shallow water, the motion of water particles transitions from circular to elliptical orbits.
As waves approach the shore, the circular motion becomes increasingly flattened due to the interaction with the seabed.

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