Why do waves erode coastlines

Page Content. Two main processes are responsible for this; erosion and deposition. Coastal erosion is the breaking down and carrying away of materials by the sea. Deposition is when material carried by the sea is deposited or left behind on the coast. Destructive Waves Coastal erosion takes place with destructive waves. These destructive waves are very high in energy and are most powerful in stormy conditions.

The swash is when a wave washes up onto the shoreline and the backwash is when the water from a wave retreats back into the sea. Destructive waves have stronger backwashes than swashes. This strong backwash pulls material away from the shoreline and into the sea resulting in erosion. Constructive Waves Constructive waves, on the other hand, are low energy waves that result in the build-up of material on the shoreline. Constructive waves are low energy and have stronger swashes than backwashes.

This means that any material being carried by the sea is washed up and begins to build up along the coastline. The material that is deposited by constructive waves can most often be seen by the creation of beaches. Image credit: Jeff Hansen, U. Geological Survey. Hydraulic Action Hydraulic Action is the sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea.

Compression Compression occurs in rocky areas when air enters into crack in rock. This air is trapped in cracks by the rising tide, as waves crash against the rock the air inside the crack is rapidly compressed and decompressed causing cracks to spread and pieces of rock to break off.

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 Figure below. 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. Waves will spread the sediments along the coastline to create a beach. Waves also erode sediments from cliffs and shorelines and transport them onto beaches. Waves continually move sand along the shore and move sand from the beaches on shore to bars of sand offshore as the seasons change.

Seawalls are associated with reduced aesthetic value, and increased erosion at the ends and in front of the seawall. In Homer, where bluff erosion is rapidly reducing many beachfront properties, local homeowners have banded together to build a seawall to protect their land. These are the most obvious defensive methods. Sea walls are exactly that. They also produce a strong backwash in waves which undercuts the sea wall making their long term sustainability questionable.

A seawall provides a high degree of protection against coastal flooding and erosion. It fixes the boundary between the sea and land which can be beneficial if important infrastructure or buildings are located on the shoreline. After a volcanic island forms, it inevitably starts to subside, or sink under its own weight.

For an island, the more slowly it sinks, the more time the sea has to carve out the coastline at a particular elevation. In contrast, if an island sinks quickly, the sea has only fleeting time to cut into the coast before the island subsides further, exposing a new coastline for the sea to wear away. As a result, the rate at which an island sinks strongly affects how far the coast has retreated inland at any given elevation, over millions of years.

To calculate the speed of island sinking, the team used a model to estimate how much the lithosphere, the outermost layer of the Earth on which volcanic islands sit, sagged under the weight of each Hawaiian volcano formed in the past million years. Because the Hawaiian Islands are close together, the sinking of one island can also affect the sinking or rising of neighboring islands, similar to the way one child may bounce up as another child sinks into a trampoline.

The team used the model to simulate various possible histories of island sinking over the last million years, and the subsequent erosion of sea cliffs and coastlines.

They chose the 11 coastal locations in the study for their variability: Some sea cliffs face north, where they are battered by stronger waves produced by distant storms.

Other north-facing coasts experience tradewinds that come from the northeast and produce waves that are smaller but more frequent. The coastal locations that face southward experience smaller, less-frequent waves in contrast. The team compared erosion rates at each site with the typical wave power experienced at each site, which they calculated from wave height and frequency measurements derived from buoy data.

What they found was a rather simple, linear relationship between wave power and the rate of coastal erosion. The stronger the waves that a coast experiences, the faster that coast erodes. Specifically, they found that waves of a size that occur every few days might be a better indicator of how fast a coast is eroding than larger but less frequent storm waves. That is, if waves on normal, nonstormy days are large, a coast is likely eroding quickly; if the typical waves are smaller, a coast is retreating more slowly.

The researchers say carrying out this study in Hawaii allowed them to confirm this simple relationship, without confounding natural factors. As a result, scientists can use this relationship to help predict how rocky coasts in other parts of the world may change, with variations in sea level and wave activity as a result of climate change.