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Shoreline Science: Exploring the Erosive Energy of Waves

A sandy science activity from Science Buddies



George Retseck

Key concepts
Oceans
Beaches
Geology
Erosion

Introduction
A day at the beach is a wonderful way to spend time with your family and friends. You can swim, play games and build sand castles. But have you ever wondered how the beach you are standing on came to be? How, for example, did all of that sand get there? Beaches are formed and continually changed by the ocean's waves moving rock particles onshore, offshore and along the shore. In this activity, you can investigate how beach formations are made by some parts of a beach that can resist erosion from the waves more than other parts.

Background
A beach is a geologic formation made up of loose rock particles such as sand, gravel and shell fragments deposited along the shoreline of a body of water. A beach has a few key features. The berm is the part that is mostly above water; this is the active shoreline. The top of the berm is known as the crest, and the part that slopes toward the water is called the face. At the bottom of the face there may be a trough and, further seaward, there may be sandbars parallel to the beach.

The erosion of rock formations in the water, coral reefs and headlands create rock particles that the waves move onshore, offshore and along the shore, creating the beach. Continual erosion of the shoreline by waves also changes the beach over time. One change that erosion can cause is the appearance of a headland. This is land that juts out from the coastline and into the water and affects how the surrounding shoreline is eroded.

Materials
•    Paint-roller pan
•    Measuring cup
•    Sand
•    Water
•    Timer
•    Digital camera
•    Plastic 500-milliliter water bottle (empty)
•    Adult volunteer to help take pictures
•    Small gravel, such as aquarium gravel

Preparation
•    Cover the bottom of the paint-roller pan with five cups of sand. Build up a beach with most, but not all, of the sand at the shallow end of the pan.
•    Slowly pour six cups of water into the deep end of the pan. Let the water and sand settle for five minutes. How has the beach changed during this time?

Procedure
•    Take a picture of your beach so that you have a record of how it looked in its original state. Where is the shoreline (the area where beach and water meet)?*
•    Lay a plastic bottle horizontally so it is floating in the water in the deep end of the pan.
•    For two minutes bob the water bottle up and down with your fingertips to create waves. If the waves get so big that water splashes out of the pan, make them smaller. How does the water swirl? How does the shoreline change after one minute? What about after two minutes?
•    After two minutes of bobbing the bottle, take a picture of the beach. How does it look compared with the first picture?
•    Empty, clean and dry the paint-roller pan. Prepare a "beach" again, as you did for the preparation. When the beach is complete, make a "headland" by creating a mound out of two cups of small gravel in the middle of the shoreline. The headland should be partly in the water and partly on the beach. Take a picture of the beach with the headland.
•    Again, lay the plastic bottle horizontally so it is floating in the water. For two minutes, bob the water bottle up and down with your fingertips. Again, if the waves are so big that water splashes out, make them smaller. How does the water swirl? How does the shoreline change after one minute? What about after two minutes?
•    After two minutes, take a picture of the beach. How does it look compared with the previous picture?
•    How does the headland affect where the water goes? How does it affect how much the shoreline erodes?
•    Extra: Repeat this activity at least two more times with a ruler taped to the side of the pan. Exactly how much shoreline erosion occurs with and without a headland?
•    Extra: Try increasing or decreasing the speed of bobbing the bottle. Does this affect how the beach changes over time?
•    Extra: Pour a large volume of water all at once into the deep end of the pan to simulate a storm surge or a tsunami. What happens to the beach?

*Correction (6/26/12): The sentence was edited after posting to clarify meaning. In this activity, experimenters are asked to observe changes in the shoreline.


Observations and results
Did the shoreline erode, or recede from the water, after you bobbed the water bottle up and down for two minutes? Did most of the shoreline erode less when there was a headland, especially the shoreline closest to it?

As waves hit the shoreline over time they erode it and push it further inland. When larger and stronger waves hit the shoreline, such as in a storm, more shoreline is eroded. On a beach that is made up of a mixture of small sand grains and larger, dense rocks, the sand will be eroded away first, leaving behind the larger rocks. Over time this can create a headland—an outcropping of the larger rocks—and a bay nearby. The headland receives most of the waves' energy and consequently protects the bay from erosion. Artificial headlands are sometimes created for this purpose: to prevent coastal erosion. In your model, you should have seen that less shoreline eroded near the headland, but further away from the headland, along the sides of the pan, more erosion occurred and the shoreline was pushed farther inland because the more distant shoreline was not as well protected by the headland.

More to explore
What Causes Beach Erosion? from Scientific American
Shoreline erosion and migration from the State of Delaware
A guide to managing coastal erosion in beach/dune systems: Artificial headlands from the Scottish Natural Heritage
Restore Your Shore from the Minnesota Department of Natural Resources
Building Beaches from Science Buddies

 

This activity brought to you in partnership with Science Buddies
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