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# 9.15: Erosion by Waves

Difficulty Level: Basic Created by: CK-12
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Is this erosion or deposition?

The power of the ocean modifies landforms by erosion and deposition. Landforms modified by both erosion and deposition are seen in this photo. The cliff is being eroded by incoming waves. The beach is being created as sand is being deposited.

### Wave Erosion

Wave energy does the work of erosion at the shore. Waves erode sediments from cliffs and shorelines. The sediment in ocean water acts like sandpaper. Over time, they erode the shore. The bigger the waves are and the more sediment they carry, the more erosion they cause (See Figure below).

Waves erode sediment from sea cliffs. The sediment is then deposited on beaches. These sandy cliffs are in Greece.

Wave refraction either concentrates wave energy or disperses it. In quiet water areas, such as bays, wave energy is dispersed. This allows sand to be deposited. Land that sticks out into the water is eroded by the strong wave energy. The wave energy concentrates its power on the wave-cut cliff.

##### Landforms From Wave Erosion

Erosion by waves can create unique landforms (Figure below).

• Wave-cut cliffs form when waves erode a rocky shoreline. They create a vertical wall of exposed rock layers.
• Wave-cut platforms are level areas formed by wave erosion. Since they are exposed, sea level was higher relative or the rock was lower.

A wave-cut platform is exposed in Pembrokeshire, South Wales.

• Sea arches form when waves erode both sides of a cliff. They create a hole in the cliff, like the one seen in the Figure below.

A sea arch creates a natural bridge in California.

• Sea stacks form when waves erode the top of a sea arch. This leaves behind pillars of rock.

### Sediment Transport

Rivers carry sediments from the land to the sea. Sometimes the sediments are deposited in a delta. But if the waves are powerful, the water will transport the sediments along the coastline. Sediments eroded from cliffs near the shoreline may also be transported.

#### Wave Refraction

Most waves approach the shore at an angle. The part of the wave that is nearer the shore reaches shallow water sooner than the part that is farther out. The shallow part of the wave "feels" the bottom first. This slows down the inshore part of the wave and makes the wave "bend." This bending is called refraction.

Most waves strike the shore at an angle. This creates longshore currents, which are described in Concept Fresh Water.

### Vocabulary

• arch: An erosional landform that is produced when waves erode through a cliff.
• refraction: A change in the direction of a wave caused by a change in speed. Waves refract when they travel from one type of medium to another.
• sea stack: Isolated tower of rock that forms when a sea arch collapses.
• wave-cut cliff: A sea cliff cut by strong wave energy.
• wave-cut platform: Level area formed by wave erosion as waves undercut cliffs.

### Summary

• Ocean waves have a tremendous amount of energy and so they may do a great deal of erosion.
• Some landforms created by erosion are platforms, arches, and sea stacks.
• Longshore currents are created because water approaches the shore at an angle.

### Practice

Use this resource to answer the questions that follow.

What is Coastal Erosion? at http://www.youtube.com/watch?v=zUh3WeilFN4 (4:12)

1. What is coastal erosion?
2. What causes coastal erosion?
3. What is accretion?
4. What causes erosion to increase?
5. What determines the rate of erosion?
6. What are landslips?
7. Why are rates if erosion expected to increase?

### Review

1. Describe how a set of waves erodes a rocky headland.
2. How does wave refraction affect a shore?
3. What are the sources of sediment at a beach?

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

### Vocabulary Language: English

TermDefinition
arch Erosional landform that is produced when waves erode through a cliff.
refraction Change in the direction of a wave caused by a change in speed; waves refract when they travel from one type of medium to another.
sea stack Isolated tower of rock that forms when a sea arch collapses.
wave-cut cliff Sea cliff cut by strong wave energy.
wave-cut platform Level area formed by wave erosion as waves undercut cliffs.

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