Oakton Community College Name _________________________________________ EAS 100 - Intro to Earth Science Lab 8 - Tsunamis, Tides and Longshore Drift* *Modified from the original lab written by J....

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Oakton Community College Name _________________________________________ EAS 100 - Intro to Earth Science Lab 8 - Tsunamis, Tides and Longshore Drift* *Modified from the original lab written by J. Cumpston, OCC In the aftermath of the great 2004 Indian Ocean tsunami, it is useful to learn and understand how and why it happened. Based on what a girl from Australia had recently learned in her science class, she saved everyone on the beach! There are two different scenarios during which a tsunami can form. Both are related to movement on the ocean floor due to plate tectonic activity. Here is what happens: 1a. Vertical displacement on ocean floor. A. Down-drop of the ocean floor along a fault. B. The surface of the ocean dips down in response this lower ocean floor. C. Water rushes in to this lower area resulting in a bulge of water above the down-dropped area. D. Gravity takes over and pulls this high area of water downward, causing a wave form to develop which flows outward in all directions. OR 1b. Convergent plate tectonic activity along a boundary on the ocean floor shoves an incredible amount of water in the direction of plate movement. A. The moving mass of water causes a wave form to develop on the surface. 2. This surface wave has a wavelength of many kilometers. 3. The wave can travel at speeds of 500 - 950 kilometers per hour (kph)! 4. Recall that water particles move in a circular motion as the wave energy passes through them. Once the wave reaches shallow water, the wave begins to "feel the bottom". Heights can build up to 30 m (90 ft). 5. At the shoreline - the first thing that occurs is a rapid withdrawal of water away from the beach plus bubbling of the water at the edge. This is what the Australian girl saw in Thailand. 6. The water surges back between 5 and 30 minutes later followed by another retreat. Picture water sloshing about in a bowl. 7. These waves can travel great distances - a quake in Japan can reach Hawaii and California. 8. There is greater danger along shallow slopes than along steep slopes. 2 Part 1: Understanding Waves Match Each Term to the Figure 1. Wave Height ________ 2. Trough ________ 3. Wave Base ________ 4. Crest ________ 5. Wavelength ________ 6. Amplitude ________ 7. Draw a cross-section of a wave that has dragged along the bottom and is in the surf zone. What is different about the wave, compared to when it was in open water? Why has this change occurred? 8. To what depth do you have to dive in-order to feel undisturbed water if the wavelength (λ) is 46 m? 9. At what water depth will a wave begin to feel the ocean floor if it has a λ of 52m? 3 10. Waves begin to break when the water depth is 1/20th of the wavelength. If you cannot swim would it be safe to walkout to where the waves are breaking for a 60m λ wave? Why or why not? The period (T) of a wave is the amount of time it takes for two consecutive crests to pass by a fixed position. The velocity (V) of a wave can be calculated with the following formula: V = λ / T. Make sure to show your work for all calculations! 11. What is the velocity in m/sec of a wave with a wavelength of 20 m and a period 5.8 seconds? 12. What is the velocity in m/sec of a wave with a λ of 600 m and a period of 10 minutes? 13. What is the velocity in m/day of a tsunami with a wavelength of 400 km and a period of 25 minutes? Is your answer a realistic number? Why or why not? 4 Ocean Tides The height of the ocean rises and falls in tides as the moon moves around the Earth in a 29.5 day orbit. The water rises on the planet closest to the moon due to gravitational pull; the gravity of the Sun also rises tides, but not nearly as much as the moon. The tidal range is the difference in height between high and low tides on a given day. In addition to regular high and low tides, there are spring and neap tides. Spring tides are during the new moon phase and full moon phase where the gravity of the sun and moon are working together to form higher high tides and lower low tides. Neap tides are during the first and third quarter moons where the sun and moon are not aligned and result in lower high tides and higher low tides. In a perfect world there would be two spring tides and two neap tides per cycle each lasting about a week. Tides occur at different rates on different parts of the planet. Diurnal tides have one high and one low per tidal day, semidiurnal have two highs and two lows per tidal day, and mixed tides have two different highs and two different lows per day. Tidal Data for a Three Locations Across the Planet Profile One Profile Two Profile Three Time (days) Height (m) Time (days) Height (m) Time (days) Height (m) 0.2 3.5 0.2 3.2 0.2 3.7 0.4 2.2 0.4 1.7 0.4 1.2 0.6 3.5 0.6 3.2 0.6 2.8 0.8 2.2 0.8 1.6 0.8 2.5 1.2 3.6 1.2 3.1 1.2 3.8 1.4 2.1 1.4 1.7 1.4 1.3 1.6 3.5 1.6 3.2 1.6 2.7 1.8 2.2 1.8 1.8 1.8 2.4 2.2 3.6 2.2 3.3 2.2 3.9 2.4 2.1 2.4 1.7 2.4 1.1 2.6 3.5 2.6 3.2 2.6 2.8 2.8 2.3 2.8 1.7 2.8 2.6 Plot the data in these tables on the graph at the top of the next page. Use a different color for each profile. 5 14. What is the tidal pattern of each profile? 15. What is the tidal range of each profile? 16. What factors affect tidal height and occurrence? 6 Events Along the Shoreline The prevailing wind in an area continually blows waves at an angle to the shore, this creates a “longshore current” that pushes sand down the beach (longshore drift). 17. Draw a line from A to B to make a breakwater protecting the coast from the longshore current. There is a pocket of calm water behind the breakwater and slightly increased erosion on the shoreline next to it, draw how the shoreline will change behind the breakwater. Why does the shoreline change shape? 18. Draw a line from C to D to make a groin out into the water perpendicular from the shoreline. Sediment is stopped on the updrift side of the groin, and as water goes around it there will be much increased erosion on the downdrift side. Draw the new shoreline on the updrift and downdrift sides of the groin. 19. Which structure is better at preventing coastal erosion? Explain your reasoning. 20. Based on your answer to #3 above, why are both structures still used?