What is Coastal Erosion? - Definition & Causes

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  • 0:04 Coastal Erosion
  • 1:16 Hydraulic Action
  • 1:41 Attrition
  • 2:05 Abrasion and Corrasion
  • 2:56 Solution
  • 3:16 Lesson Summary
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Lesson Transcript
Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Communities falling into the ocean? It sounds like a scene from a science fiction movie, but it happens in the real world. This lesson will examine the five mechanisms that cause coastal erosion and put some coastal communities at risk.

Coastal Erosion

Let's travel to a remote Alaskan village called Kivalina. This Inupiat village of 400 people is located on the coast of the Chukchi Sea. For generations, the people of Kivalina have used the sea ice in the winter to hunt bowhead whales; however, due to warmer temperatures in the Arctic, the sea ice is no longer what it once was. Now, venturing out onto the ice has become dangerous. But that's not the biggest concern. With less sea ice present, stronger waves are reaching and eroding the coastline, and Kivalina is in danger of disappearing into the Chukchi Sea.

The Alaskan village of Kivalina is in danger of disappearing into the Chukchi Sea

Of course, Kivalina isn't alone. Many communities across the world are experiencing coastal erosion, which is when land is worn away due to numerous culprits that we'll discover shortly. Scientists predict that Kivalina will be gone by 2025, and the residents will be forced to relocate. Let's check out the five processes that cause coastal erosion, which you can remember with the letters HAACS (each letter stands for a mechanism). While HAACS doesn't roll off the tongue, remembering the letters will help you to remember the five mechanisms.

Hydraulic Action

The first mechanism of coastal erosion is called hydraulic action, which is where waves or flowing water are able to remove rocks and sediment from a coastline. Imagine a cliff, and picture air trapped in the cracks of the rocks that make up the cliff. As waves hit that cliff, the air is squeezed and may actually escape. This process weakens the rocks of the cliff, and some of these rocks will actually break off.

Waves hitting the coast cause hydraulic action


The second type of coastal erosion is called attrition. This happens when rocks are brought onto the coastline and smash into the rocks that make up the coast. Waves washing up on the coast carry more than water - they can carry sand, rocks, and sediment, all of which can break down the rocks that are situated along the coastline. With attrition, the rocks smash into each other, break apart, and become smoother.

Attrition results in rocks breaking apart and becoming smoother

Abrasion and Corrasion

The next two types of erosion, abrasion and corrasion, are a lot alike. In fact, some sources even combine them as one, but we won't here. However, we'll show how one causes the other.

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Additional Activities

Coastal Erosion

Coastal erosion is the eroding of beach sand due to wind and wave action. If you have even been on a beach and the wind starts blowing hard, you may have noticed the stinging feeling as the sand particles hit you. Over time, this can shift a lot of sand from one location along the beach to another location. Wave action is constant at a beach. When the waves strike the beach at an angle other than 90° it causes sand to be shifted down the beach. To slow down the erosion process sea oat plants are planted on beaches. The plants' roots help hold the sand in place. Today, let's set up an experiment to show how sand is eroded on beaches due to wind an waves.


  • A few cups of play sand
  • A straw
  • A shallow, but wide and long plastic storage container
  • Water
  • A small bucket
  • A 2 inch thick strip of wood as long as the storage container, or close to it
  • Smart phone or digital camera to take pictures


  1. Place the play sand against the long end of the storage container extending no further than half-way to the other side of the storage container.
  2. Take a picture of the setup.
  3. Take the straw and start blowing on the sand making a sweeping motion all of the way along the long length of the storage container.
  4. Take another picture of the setup.
  5. Place the strip of wood underneath the long edge of the storage container on the side where the sand is located.
  6. Fill the small bucket with water and add the water to the storage container so the water just starts moving up onto the sand.
  7. Push down on the water in the storage container making waves.
  8. Take a final picture of the setup.

Follow Up Analysis

  1. What did you notice about the sand from picture 1 to picture 2?
  2. What do you notice about the sand from picture 2 to picture 3?
  3. Which type of erosion, wind or wave action caused the biggest shift of sand?

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