As sea waves approach the shore, friction with the ocean bottom causes them to become steeper and eventually break.
Understanding Friction's Impact on Waves
When wind-driven waves travel across deep water, their motion is largely unaffected by the seabed. However, as these waves move into shallower water, typically when the water depth is less than about half the wavelength, the influence of the ocean bottom becomes significant.
The primary way frictional interaction with the sea floor affects waves is by slowing down the lower portion of the wave. This drag effect changes the wave's structure and behavior.
Based on the provided reference, friction between the sea floor and the water causes the water to form increasingly steep angles. This means the front face of the wave gets steeper as it approaches the shore.
The Process: From Deep to Shallow Water
Here's a simplified look at what happens as waves transition from deep to shallow water where friction matters:
- Deep Water: Waves move freely; orbital motion of water particles is contained within the water column and doesn't interact with the bottom.
- Approaching Shore: As the depth decreases, the wave's orbital motion begins to touch the sea floor.
- Frictional Slowing: The friction between the bottom and the water slows the base of the wave compared to its crest.
- Wave Shortening and Steepening: This differential speed causes the wave to bunch up; its wavelength decreases, and its height increases, leading to the formation of increasingly steep angles.
The Result: Breaking Waves
The increase in wave steepness due to bottom friction has a critical consequence. Waves can only become so steep before they become unstable.
Waves that become too steep and unstable are termed “breakers” or “breaking waves.” This is the dramatic collapse of the wave crest, which releases a significant amount of energy and is a defining feature of the surf zone.
Think of it like tripping: the bottom of the wave hits resistance (friction from the seabed) and slows down, while the top keeps moving at a faster speed until it pitches forward and breaks.
Key Effects of Bottom Friction on Waves
- Increased Steepness: The angle of the wave face becomes steeper.
- Wavelength Reduction: The distance between wave crests decreases.
- Wave Height Increase: Energy is compressed, often leading to taller waves just before breaking.
- Breaking: Unstable, steep waves collapse, forming surf.
Below is a simple table summarizing the core effects mentioned in the reference:
| Interaction Type | Effect on Waves | Result in Shallow Water |
|---|---|---|
| Friction (Seabed) | Causes water to form steeper angles | Breaking Waves |
In summary, frictional interaction with the ocean bottom is a key process in shallow water that transforms incoming waves, increasing their steepness and ultimately leading to the phenomenon of wave breaking near the shore.
