GEOLOGY assignment

Lab 6: Tides and Coastal processes

Goals:

1. Understand what causes tides.

2. Understand how deep-ocean waves change and break when they reach a coastline.

3. Understand how shoreline transport of sediment occurs and how it is affected by human activities.

Tides

Tides are actually waves that move through the oceans and large lakes. In the oceans, they have periods of several hours and wavelengths of 1000s of kilometers. Depending on the wave period that dominates in different parts of the world, some places have only one high and one low tide a day (diurnal tides), whereas others have two high (about equal to each other) and two low tides (also about equal to each other) known as semidiurnal. A third type is mixed tides with two highs and two lows a day with different heights, (a high high, a low high, a high low and a low low). California, as the rest of the west coast has mixed tides.

The gravitational pull of the Sun and the Moon causes tides on Earth. The gravitational influence of an object is directly related to its mass and its distance from another object. So even though the Sun has much more mass than the Moon, it is over 380 times farther away from the Earth than the Moon. Therefore, our little Moon wins out and has the most influence on the tides on Earth.

As the Earth spins about its axis, centrifugal force keeps water balanced on all sides of the planet. However, the Moon's gravitational forces disrupt this balance by pulling the water towards the Moon. A double "bulge" is formed—one side pulling toward the Moon and the opposite side pulling away from it, just like swinging a water balloon causes it to stretch both toward AND away from your hand. The areas where the bulging occurs experience high tides.

Tides are predictable, but their periods do not coincide with the 24 hour Earth day. The Moon takes about 24 hours and 50 minutes to line up again exactly with the same point on the Earth. Therefore, the timing of the tides shifts almost an hour a day. Because the relative positions of the Sun, Earth and Moon change over the course a day, a month and a year, the absolute heights of diurnal, semidiurnal and mixed tides vary over these periods. For example, even within the same ~25 hour period, the heights of the semidiurnal tides are not exactly the same, although they are close. Even more significant changes are seen over a monthly cycle, with higher-than-normal spring tides (not named for the season but because the water "springs" higher than normal) and the lower-than-normal neap tides.

Exercise 1.

Let's investigate the causes of spring and neap tides. Go to this website http://aspire.cosmic-ray.org/Labs/Tides/tides_simulator.html . Pick the student version because the teacher version has an annoying note in the middle of the simulation screen—the teacher version will not give you any more information than the student version to answer these questions. Click on the tide simulator. Click the box to turn on spring/neap tides. Keep the particle number the same because it helps you to see what the oceans look like during different times of the month. As the simulation progresses, put the cursor over the stop button and when you see "neap" appear, stop the simulation and look at the position of the earth, sun and moon. Do the same with "spring." Try it enough times so that you are confident of the positions of the three astronomical bodies that cause the spring and neap tides.

Question 1. Draw two different pictures of the sun-earth-moon system that produce a spring tide. Label each of the astronomical bodies and show the envelop of the ocean water (the blue dots in the simulation). Move the circles around until they are in the places that you want them. The big circles are the sun, the medium size circles are Earth, and the small circles are the Moon. The blue oval is the water.

Question 2. Draw one picture of the sun-earth-moon system that produces a neap tide. Label each of the astronomical bodies. Move the circles around until they are in the places that you want them. The big circle is the sun, the medium size circle is Earth, and the small circle is the Moon.

Question 3. Let the simulation run and watch the month on the right and the neap and spring labels.

How many neap tides are there in a month? ______________

How many spring tides are there in a month? _____________

Question 4. Go to the National Oceanic and Atmospheric Administration (NOAA) tide predictions website for Santa Monica, California: http://tidesandcurrents.noaa.gov/noaatidepredictions/NOAATidesFacade.jsp?Stationid=9410840

Change the time range to monthly to see the tides from June 1-30, 2013. Fill in the first three columns of the chart using the information from this website.

How to find spring tides: Look for the dates that have not only high heights (add the two highs together), but also the a large difference between high and low tides. Make the best decision that you can using both factors. The two spring tides cannot be within the same week.

How to find neap tides: Look for smallest differences between the two high tides and smallest differences between high and low tides. Make the best decision that you can using both factors. The two neap tides cannot be within the same week.

Go to http://www.farmersalmanac.com/calendar/moon-phases/ and change the date to June, 2013. Mouse-over the dates that you chose to get the phases of the moon and add to the last column.

Date	combined spring tide heights of the day AND max difference between high and low tides (both in feet)	combined neap tide heights of the day AND max difference between high and low tides (both in feet)	phase of the moon

You have now discovered the relationship between phases of the moon and spring and neap tides!

Your biggest spring tide is unusually big! A special astronomical event occurred on this date.

http://www.scientificamerican.com/article.cfm?id=supermoon-to-rise. Re-read paragraph 2 under Tides on p. 1. Explain why this event would cause the spring tide to be larger than normal.

Interaction of the Ocean with Coastlines

This section of the lab looks at the interaction of the ocean with coastlines, particularly the development of longshore transport and what happens when human development along coastlines interacts with longshore transport. Although the video that you will be watching is quite old, it is still one of the best for illustrating coastal processes in southern California. Watch the video and answer the following questions. The Beach- A River of Sand http://www.youtube.com/watch?v=FqT1g2riQ30)

1. Where does sand that forms beaches in southern California come from and how does it get to the coast?

2. Along many California beaches, there is a big difference in appearance between winter and summer.

A. During which season is wave energy smallest?

1) winter 2) summer

B. Smaller waves move sand:

1) toward the beach (onshore) 2) away from the beach (offshore)

C. Is this picture typical of a summer or winter beach? 1) winter 2) summer

3. Most waves approach and strike the coast at an angle. Chose the picture that correctly shows the correct relationship between the direction of approach of the waves and the movement of sand in the surf zone and along the beachface.

( waves waves waves ) a. b. c.

4. Man-made structures rock groins and jetties interfere with the flow of sand generated by the longshore current, causing deposition of sand on one side and erosion of sand on the other. Based on this knowledge, which picture correctly shows the direction of longshore current?

a. b.

4. Breakwaters are built to provide quiet water for marinas where small boats can be safely harbored. What happens when the longshore transport of sand encounters the breakwater at the Santa Barbara marina? If this situation was just left alone, what would happen to the marina? What does the City of Santa Barbara do to prevent this?

5. At several places along the coast of southern California, beaches come to an end and only rocky shores continue for a portion of the coast. Why do the beaches end? What happens to the sand?

6. How does the building of dams affect the supply of sand along the beach?