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ACTIVITY: Eroding Beaches

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

The activity below draws from the content in the page The Forces of Waves.
Additional information on balanced and unbalanced forces can be found in the previous topic The Forces of Wind.

<p>Fig. 1. Waves cause severe beach erosion on the North Shore of Oʻahu.</p>Phenomenon:

The force of water can change the shape of a beach very quickly (Fig. 1)!

Inquiry:

How do wave forces affect sand movement?

Activity:

Explore the effect of wave forces on sand movement. 

Guiding Questions:

  1. Imagine a beach without any waves. Do you think the sand will move?
  2. What forces might cause the sand to move?
  3. How can the force of waves change the shape of the coastline?

Materials:

Teacher Recommendations:

Notes on materials:

  • Dish - about 20cm X 10 cm and 5cm high works well (need one per group)
  • Sand - about 3 cups per container (can be purchased from a local hardware store)
  • Water - about 3 cups per container
  • Spatula - or other type of paddle to generate waves (about 10cm across)
  • Food coloring - will need a few drops per container
  • Toothpicks - about 6 per container
  • Markers - to write on the side of the dish and mark the beach profile. Spare markers might be helpful in case they get wet.
  • Metronome - to help keep wave force consistent (recommend 60 waves per minute for two minutes)

Procedure:

Part A. Make a beach simulation tank (Fig. 2)

  1. Follow instructions on your worksheet to model beach erosion.
  2. Use the ruler to mark at each centimeter along the sides of your dish. This will help you measure beach erosion throughout the experiment.
  3. Add enough sand to your dish that it covers the bottom. Push it all to one side to form a sloping beach.
  4. Add water to your ocean model to cover the bottom of your dish and some of the sand.
    Note: There should be a section of beach above the water line (about 2 inches wide)
  5. Add a few drops of food coloring to your water so that it is easier to see.
  6. Reform your beach to ensure:
    1. That it has a uniform slope and shape from side to side
    2. There is at least a 2 inch wide beach above the water
  7. Use a permanent marker to trace the slope of your beach. Draw a small mark where the water and sand meet. This is the shoreline.
    Note: You will use this line to reform your beach in the same shape for each trial.
  8. Mark the shoreline with toothpicks.
  9. Make a "before" sketch of the side view of your beach. This is the beach profile. Note: you can use your ruler to take measurements of different parts of the beach.
    1. Use the diagrams in Fig. 2 as a guide.

<p>Fig. 2. Ocean model set-up with sandy beach slope marked in black marker, shoreline marked by tothpicks, and centimeters marked across the bottom.</p><br />

Part B. Practice making waves in your ocean model.

  1. Use a metronome or sound recording (search metronome online) set at 60 beats per minute (or similar).
  2. Put the large side of your paddle in the water the edge of your container on the opposite side of the beach. Start the metronome, and move the paddle gently towards the beach in time to the beat.
  3. What did you see? Record your observations.

Part C. Test the force of waves on your sandy beach.

  1. Reshape your beach so it lines up with your original profile line and the shoreline is in the same place.
  2. Follow the metronome sound and use the paddle to make a wave at each second for 2 minutes (Video 1).
    Note: Try to give each wave the same force with your paddle.
     
Video 1: Testing ocean erosion with wave forces on a sandy shore.
  1. After 2 minutes of making waves, observe the effect of wave force on sand movement.
  2. Use a different color marker to trace the new line of the beach and mark the shoreline on the side of your container (Fig. 3).
  3. Describe how your beach has changed.
  4. Make an "after sketch" of your beach profile following the wave action.

<p>Fig. 3. After wave-force, sandy beach slope marked in red marker.</p><br />

Part D. Test a Sea Wall!

  1. Reshape your beach so it lines up with your original beach profile line and the shoreline is in the same place (Fig. 4).
  2. Create a seawall by placing and stacking rocks at the shoreline (above the toothpicks).
  3. Draw another "before" sketch of the beach profile.
  4. Repeat part C to test the force of the waves against the seawall. (Video 2).
  5. Describe how your beach and seawall has changed.
  6. Make an "after sketch" of beach profile and seawall following the wave action.
Video 2. Testing the force of waves on the sandy shoreline with a seawall.

<p>Fig. 4. Ocean model set-up with rock wall. The sandy beach has been rebuilt so that the slope matches the original black marker line.</p><br />
<p>Fig. 5. After wave-force action on sand with balancing force of rock wall, the sandy beach slope is marked in purple marker. Notice the comparison to the original, black line slope, and the after slope, in red, of the plain beach trial.</p><br />


Activity Questions:

You have just experimented with balanced and Unbalanced Forces!

  • When two equal forces act in opposite directions, the result is balanced forces and there is no motion.
  • When one force is stronger than the other, the result is unbalanced forces that cause motion.
  1. Imagine that two teams are playing tug-of-war on a rope (Fig. 6).  <p>Fig. 6. These two teams are playing tug-of-war. Which team will win?</p><br />
    1. If their strength is equal and neither team is winning, are the forces balanced or unbalanced?
    2. If one team lets go, and the other team wins the rope, are the forces balanced or unbalanced?
  2. Think back to when you first set up your beach.
    1. What would the beach profile look like the next day if you did nothing to it? (Hint: would the sand move by itself?)
    2. If you return the next day, and the sand has not moved, were the forces acting on the sand balanced or unbalanced?
    3. If you return the next day, and the sand has moved, were the forces acting on the sand balanced or unbalanced?
  3. What happened to your beach profile when there was wave action? Explain.
  4. When waves interact with the shoreline to move sand, are the forces?
    1. Balanced  or Unbalanced (Circle one)
    2. What evidence do you have to support your answer?
  5. Why do you think that people build rock walls (or other hard structures) at the beach?
  6. After you added your rock wall, did waves affect the beach profile differently than before? Describe.
  7. Based on your observations, can you think of a reason why beach scientists often recommend against building sea walls in line with the beach? (hint: look at the entire beach profile, from under the wall and into the water).
  8. What other ways can people protect homes and property besides building seawalls parallel to the beach? (hint: think about how far property is built from the water, the amount of sand needed to protect property, the role of native plants in holding sand on a beach, and the various shapes and positions of seawalls compared to groins or piers.)
    Note: For more information on beach structures, check out the Exploring Our Fluid Earth topic on Beaches and Sand and scroll down to read "Sand Transport, Coastal Erosion, and Human Impact on Beaches"

Further Investigations: 

  1. Look for evidence of the effects of wave action at your favorite beach site. If possible, take photographs.
  2. Study beach erosion problems in your community.
    1. Find out what methods are being used to protect property from wave damage. Study designs for seawalls (revetments), jetties (bulkheads), and other features.
    2. What determines where oceanfront houses may be built? 
    3. Where did the old sand go?
    4. Where did the sand originally come from?
    5. Has additional sand been imported, and if so, where did it come from?

Table of Contents:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.