Exploring Our Fluid Earth

Materials

• Table 5.2
• Flat wave tank or large flat plastic bin
• Water
• Wind generating source (e.g., fan) or paddle or other object to displace water
• Sand
• Toothpicks
• Assortment of small rocks
• Sponges or other waterproof objects to represent coral reefs
• Towels
• Ruler (optional)
• Protractor (optional)

Procedure

Safety Note: If using an electrical device to generate waves, keep it far away from the water. Promptly clean up splashes to prevent slipping. Consider doing this activity outside if the weather permits.

A. Waves influence the coastline.

1. (Optional) Wash the sand and rocks before the activity to remove fine particles that will make the water in your tank muddy.
    
2. Fill the tank with water to about one-third full.
    
3. Practice generating waves of different frequencies and amplitudes, either with a wind source or by displacing water using a paddle or other object.
    
4. Create a beach by carefully pouring sand into one end of the tank, so that a portion of the sand extends above the waterline.
    
5. Predict the effects on the shoreline of changing each of the following variables related to waves.
   
   1. wave amplitude
   2. wave frequency
   3. wave direction (Fig. 5.16)

Image

Image caption

Fig. 5.16. Wave direction (top view)

Image copyright and source

Image by Byron Inouye

6. Test your predictions about how waves influence the coastline.
   1. Change only one wave variable at a time. See Special Feature Practices of Science: Variables for information on variables.
   2. Mark the shoreline with toothpicks.
   3. Sketch profile and top views of your coastline before generating waves.
   4. (Optional) Use a ruler and protractor to determine the height, width, and slope of the beach.
   5. (Optional) Take “before” pictures of the wave tank and shoreline.
   6. Generate waves for 5 minutes. This amount of time should be sufficient for waves to move the sand in your tank. However, depending on your set-up, you may have to generate waves for longer, or may be able to stop sooner.
      
      1. Record exactly how long you generate waves.
      2. Try to keep the waves consistent in amplitude and frequency
   7. Observe the wave patterns that are created. Make sketches of the wave patterns.
   8. At the end of the trial, draw profile and top views of the coastline. To observe changes in the shoreline, compare the location of the waterline after five minutes with the location of the toothpicks placed at the waterline before the experiment.
   9. (Optional) Measure the height, width, and slope of the beach with a ruler and protractor.
   10. (Optional) Take “after” pictures of the wave tank and shoreline.
        
7. Reconstruct the beach between each trial.

B. The coastline influences waves.

Fig. 5.17. Coastal and nearshore landforms and beach slopes

Image by Byron Inouye

1. Refer to Fig. 5.17 for beach slopes and naturally occurring coastal and nearshore features. Based on how waves behave at boundaries, including reflection, refraction, and diffraction (Table 5.2), predict how waves will interact with each of the features.

2. Test your predictions about how waves will interact with coastal features. Follow Part A Step 6 for each coastal feature you investigate. Reconstruct the beach between each trial.
   1. Shape the sand to make beach features.
   2. Use rocks or other dense objects to form offshore natural features.
   3. Use sponge pieces weighed down with rocks to make submerged coral reefs.

C. Simulate waves on the windward and leeward coast of an island.

In Part B of this activity you looked at how an offshore island affected coastal and nearshore features. In this part of the activity you will be focusing your attention on the island.

1. Rearrange the sand to simulate an island located about two-thirds of the way down the length of the tank (remove the beach at one end of the tank from Parts A and B).
   1. The island should be about five cm higher than the waterline and narrow enough so that water can flow freely around all sides.
   2. The windward coast is the coast facing the prevailing winds (i.e., the winds most common to an area). As winds create waves, the windward coast of your island is on the side of the tank were you generate waves.
   3. The leeward coast is the coast on the opposite side of the island and is protected from the prevailing wind. 
       
2. Predict how waves will influence the windward and leeward coasts of your islands.
    
3. Follow Part A Step 6 to test your predictions. Remember to mark the shoreline of the entire island with toothpicks and observe changes on both the windward and leeward coasts.
    
4. Dry out the sand before storing it by spreading it out in a pan or on plastic tarps in the sun.