10.2 Erosion and Deposition by Waves

Lesson Objectives

• Explain how waves cause erosion of shorelines.
• Describe features formed by wave deposition.
• Identify ways to protect shorelines from wave erosion.

Vocabulary

• barrier island
• breakwater
• groin
• longshore drift
• sandbar
• sea arch
• sea stack
• spit

Introduction

Have you ever stood on a sandy ocean beach and let the waves wash over your feet? If you have, then you probably felt the sand being washed out from under your feet by the outgoing waves. This is an example of wave erosion. What are waves? Why do they cause erosion? And what happens to the sand that waves wash away from the beach?

What Are Waves?

All waves are the way energy travels through matter. Ocean waves are energy traveling through water. They form when wind blows over the surface of the ocean. Wind energy is transferred to the sea surface. Then, the energy is carried through the water by the waves. Figure below shows ocean waves crashing against rocks on a shore. They pound away at the rocks and anything else they strike.

Ocean waves transfer energy from the wind through the water. This gives waves the energy to erode the shore.

Three factors determine the size of ocean waves:

1. The speed of the wind.
2. The length of time the wind blows.
3. The distance the wind blows.

The faster, longer, and farther the wind blows, the bigger the waves are. Bigger waves have more energy.

Wave Erosion

Runoff, streams, and rivers carry sediment to the oceans. 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.

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.
• Sea arches form when waves erode both sides of a cliff. They create a hole in the cliff.
• Sea stacks form when waves erode the top of a sea arch. This leaves behind pillars of rock.

Over millions of years, wave erosion can create wave-cut cliffs (A), sea arches (B), or sea stacks (C).

Wave Deposition

Eventually, the sediment in ocean water is deposited. Deposition occurs where waves and other ocean motions slow. The smallest particles, such as silt and clay, are deposited away from shore. This is where water is calmer. Larger particles are deposited on the beach. This is where waves and other motions are strongest.

Beaches

In relatively quiet areas along a shore, waves may deposit sand. Sand forms a beach, like the one in Figure below. Many beaches include bits of rock and shell. You can see a close-up photo of beach deposits in Figure below.

Sand deposited along a shoreline creates a beach.

Beach deposits usually consist of small pieces of rock and shell in addition to sand.

Longshore Drift

Most waves strike the shore at an angle. This causes longshore drift. Longshore drift moves sediment along the shore. Sediment is moved up the beach by an incoming wave. The wave approaches at an angle to the shore. Water then moves straight offshore. The sediment moves straight down the beach with it. The sediment is again picked up by a wave that is coming in at an angle. This motion is show in Figure below and at the link below.

http://oceanica.cofc.edu/an%20educator'sl%20guide%20to%20folly%20beach/guide/driftanimation.htm

Longshore drift carries particles of sand and rock down a coastline.

Landforms Deposited by Waves

Deposits from longshore drift may form a spit. A spit is a ridge of sand that extends away from the shore. The end of the spit may hook around toward the quieter waters close to shore. You can see a spit in Figure below.

Spit from Space. Farewell Spit in New Zealand is clearly visible from space. This photo was taken by an astronaut orbiting Earth.

Waves may also deposit sediments to form sandbars and barrier islands. You can see examples of these landforms in Figure below.

Wave-Deposited Landforms. These landforms were deposited by waves. (A) Sandbars connect the small islands on this beach on Thailand. (B) A barrier island is a long, narrow island. It forms when sand is deposited by waves parallel to a coast. It develops from a sandbar that has built up enough to break through the water’s surface. A barrier island helps protect the coast from wave erosion.

Protecting Shorelines

Shores are attractive places to live and vacation. But development at the shore is at risk of damage from waves. Wave erosion threatens many homes and beaches on the ocean. This is especially true during storms, when waves may be much larger than normal.

Breakwaters

Barrier islands provide natural protection to shorelines. Storm waves strike the barrier island before they reach the shore. People also build artificial barriers, called breakwaters. Breakwaters also protect the shoreline from incoming waves. You can see an example of a breakwater in Figure below. It runs parallel to the coast like a barrier island.

A breakwater is an artificial barrier island. How does it help protect the shoreline?

Groins

Longshore drift can erode the sediment from a beach. To keep this from happening, people may build a series of groins. A groin is wall of rocks or concrete that juts out into the ocean perpendicular to the shore. It stops waves from moving right along the beach. This stops the sand on the upcurrent side and reduces beach erosion. You can see how groins work in Figure below.

A groin is built perpendicular to the shoreline. Sand collects on the upcurrent side.

Lesson Summary

• Ocean waves are energy traveling through water. They are caused mainly by wind blowing over the water.
• Sediment in ocean water acts like sandpaper. Over time, it erodes the shore. It can create unique landforms, such as wave-cut cliffs, sea arches, and sea stacks.
• Deposits by waves include beaches. They may shift along the shoreline due to longshore drift. Other wave deposits are spits, sand bars, and barrier islands.
• Breakwaters are structures that protect the coast like barrier islands. Groins are structures that help prevent longshore drift from eroding a beach.

Lesson Review Questions

Recall

1. What are waves?

2. How do ocean waves cause erosion?

3. Identify three types of landforms created by wave erosion.

4. What is a spit? How does it form?

Apply Concepts

5. Create a diagram to illustrate the concept of longshore drift.

Think Critically

6. Why are the smallest particles on a beach usually sand?

7. Explain how a barrier island helps protect the coast from wave erosion.

8. Compare and contrast how breakwaters and groins protect shorelines.

Points to Consider

Moving air, like moving water, causes erosion. Moving air is called wind.

• How does wind cause erosion? Does the wind carry particles in the same ways that moving water does?
• What landforms are deposited by the wind?