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 is not alone. Many communities across the world are experiencing coastal erosion, 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 of 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, 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. Corrasion is caused by abrasion. Corrasion happens when materials like rock, sand, or sediment travel over a surface and wear that surface down. Rock, sand, or sediment can be carried in water, in glaciers, or even in wind. Abrasion is the term used to describe the ‘sandpaper’ action of the sand, rock, or sediment grinding down the coastline. So corrasion is the wearing away of a coastline by abrasion (the sandpaper action). Make sense? Before we move on, note that ‘corrasion’ isn’t ‘corrosion’ spelled incorrectly. They are different words with different meanings!


The last mechanism is called solution, and this occurs when the acids found in seawater dissolve some of the rocks making up the coastline (like limestone). Carbon dioxide in the air gets absorbed into the ocean, making the ocean slightly acidic, and some rocks are easily dissolved when exposed to acids.

Lesson Summary

Will the residents of Kivalina have to relocate due to coastal erosion (when the land making up the coast is worn away)? Relocation will cost an exorbitant amount of money and will change the way of life for its 400 inhabitants, but villagers may have little choice in the coming decade. Let’s review the five mechanisms of coastal erosion so that, one day, you might be able to help solve the problems associated with coastal erosion for communities like Kivalina. Remember: HAACS.

Type Description
Hydraulic Action Waves hit the coastline and weaken the rocks making up the coast, breaking them off.
Attrition Rocks crash into rocks on the coastline. This breaks them up into smooth pebbles, thus eroding the coastline.
Abrasion Rocks, sand, or sediment act like sandpaper and grind down the coastline.
Corrasion Rocks, sand or sediment are moved over the coastline (thus causing abrasion).
Solution Acids carried in seawater dissolve certain rocks, which erodes the coastline.