Tides

Introduction: (Initial Observation)

If you live near the ocean or some large rivers leading to the ocean, you may have noticed that the level of water changes almost every day. When the water rise, fishes and other marine animals enter the shoreline. When the water level falls, many of these animals remain on the beach. After each fall of water level, you can see varieties of aquatic plants and marine animals in shorelines.

Daily changes in water level are called tides. In this project you will learn about tides and make a model to show changes in water level in a shoreline. You may also make observations and record tides.

Before starting any project, you must have a project plan. A project plan is a list of task that you are planning to perform in order to complete your project.

Project Plan:

Although I have seen tides and I know that they occur every day, I would like to know what causes them and its effects on the environment. Like any other natural phenomena tides may also have benefits or cause damages. Knowing the potentials and harms of the tides will help us to utilize their benefits and and reduce its harms. Following is a list of tasks that I am planning to do as a part of my research:

1. Searching the Internet for the term “Tides”. This search may lead to documents, reports and books related to tides.
2. I will visit my local library and find books related to tides, oceans, oceanography, earth science and weather and look for chapters that may discuss tides. I am hoping to use the alphabetical index usually found at the end of the books and quickly find the pages that discuss tides.
3. Searching the internet with multiple keywords in order to find benefits and hazards of tides. Some of the keywords that I will use are “Tides, hazardous”, “Tides, benefit”, “Tides, dangerous”.
4. I will also try to find reports already available about tides. I may search the Internet for “Tides, Report”.
5. I will make a model to show changes in the water level.
6. I will try to observe and record tides in my area.

Information Gathering:

Find out about tides and the factors that may affect the them. Read books, magazines or ask professionals who might know in order to learn about the effect of tides on plants and animals of shore line. Find out if tides can be predicted and if tides have a daily schedule. Do tides have any benefits? Keep track of where you got your information from.

Following are samples of information that you may find.

What are tide pools?

Tide pools are areas on rocks by the ocean that are filled with seawater. Tide pools can be small, shallow puddles found high up on the shore or huge, deep holes nearer to the sea.

Tide pools form when the ocean covers the beach twice a day. Some of the plants and animals that live close to the sea are covered when the tide washes over them. They have to be able to survive in both wet and dry conditions. The tides bring fresh oxygen and food to them. Between tides, some of the smaller pools become too warm and begin to dry up. Many of the animals hide under cool, damp rocks and moist seaweeds so that their bodies do not dry out before the tide comes in again.

When do the tides occur?

The tides at a given place in the Earth’s oceans occur about an hour later each day. Since the Moon passes overhead about an hour later each day, it was long suspected that the Moon was associated with tides. Newton’s Law of Gravitation provided a quantitative understanding of that association.

Differential Forces

Consider a water molecule in the ocean. It is attracted gravitationally by the Earth, but it also experiences a much smaller gravitational attraction from the Moon (much smaller because the Moon is much further away and much less massive than the Earth). But this gravitational attraction of the Moon is not limited to the water molecules; in fact, the Moon exerts a gravitational force on every object on and in the Earth. Tides occur because the Earth is a body of finite extent and these forces are not uniform: some parts of the Earth are closer to the Moon than other parts, and since the gravitational force drops off as the inverse square distance, those parts experience a larger gravitational tug from the Moon than parts that are further away.

In this situation, which is illustrated schematically in the adjacent figure, we say that differential forces act on the body (the Earth in this example). The effect of differential forces on a body is to distort the body. The body of the Earth is rather rigid, so such distortion effects are small (but finite). However, the fluid in the Earth’s oceans is much more easily deformed and this leads to significant tidal effects.

A Simple Tidal Model

We may illustrate the basic idea with a simple model of a planet completely covered by an ocean of uniform depth, with negligible friction between the ocean and the underlying planet, as illustrated in the adjacent figure. The gravitational attraction of the Moon produces two tidal bulges on opposite sides of the Earth.

Without getting too much into the technical details, there are two bulges because of the differential gravitational forces. The liquid at point A is closer to the Moon and experiences a larger gravitational force than the Earth at point B or the ocean at point C. Because it experiences a larger attraction, it is pulled away from the Earth, toward the Moon, thus producing the bulge on the right side. Loosely, we may think of the bulge on the left side as arising because the Earth is pulled away from the water on that side because the gravitational force exerted by the Moon at point B is larger than that exerted at point C. Then, as our idealized Earth rotates under these bulges, a given point on the surface will experience two high and two low tides for each rotation of the planet.

More Realistic Tidal Models

The realistic situation is considerably more complicated:

1. The Earth and Moon are not static, as depicted in the preceding diagram, but instead are in orbit around the common center of mass for the system.
2. The Earth is not covered with oceans, the oceans have varying depths, and there is substantial friction between the oceans and the Earth.

These make a more realistic description much more complicated, but the essential ideas remains as illustrated in the preceding diagram. Here are realtime links to the present tidal conditions in San Francisco Bay and Houston-Galveston and here is a link to a set of graphs for the tidal levels over current 24-hour periods for various tidal stations. Notice in comparing these graphs the differences in the detailed tidal fluctuations for different locations (for example, compare the graph for Tacoma, Washington, with that for South Pass, Louisiana). These differences are produced by the complicating factors mentioned above.

Spring Tides and Neap Tides

Another complication of a realistic model is that not only the Moon, but other objects in the Solar System, influence the Earth’s tides. For most their tidal forces are negligible on Earth, but the differential gravitational force of the Sun does influence our tides to some degree (the effect of the Sun on Earth tides is less than half that of the Moon).

Competition between the Sun and Moon in producing tides.

For example, particularly large tides are experienced in the Earth’s oceans when the Sun and the Moon are lined up with the Earth at new and full phases of the Moon. These are called spring tides (the name is not associated with the season of Spring). The amount of enhancement in Earth’s tides is about the same whether the Sun and Moon are lined up on opposite sides of the Earth (full Lunar phase) or on the same side (new Lunar phase). Conversely, when the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides. The figure shown above illustrates spring and neap tides.

Tidal Power

Tides can be used to produce electricity. Tidal power plants use the potential and the kinetic energy of sea water. The tides are caused by the rotation of the earth as well as the force of gravity of the moon and the sun.

The water of the tides is stored in natural bays which are separated from the ocean by an artificial dam. In times of rising tides the water passes the dam through several tubes and drives half of the installed generators. The other half of the generators are used when the water gets back to the sea passing through other tubes.

In order to get enough energy a tidal lift of more than three meters is necessary. Good coastal regions provide tidal lifts of 10 meters.

Some power plants do not store the water. These plants use only the tidal flow. The disadvantage is that they can only generate electricity in time of rising or falling tides.

The power that can be generated from storage tidal power plants is less because the pressure and the falling height of the water is smaller.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to observe and record coastal water level at a certain location and in a period of time and determine the maximum difference in water level from low tide to high tide.

Following are some specific questions that can be studied during this project.

• Do tides happen always at the same time or it changes during the month?
• How many tides do you have in your area in each 24 hours?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable (also known as manipulated variable) is the date and time of observation.

Dependent variable (also known as responding variable) is the water level.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Following are two sample hypothesis:

I have only observed tides waves in late afternoons; however based on my gathered information, tides may happen at different hours.

My hypothesis is that we will have one high tide and one low tide in each 24 hours.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Ocean water level at different times of the day in New York Harbor.

(Replace the name New York Harbor with the name of your location)

Introduction:

People who live in coastal areas or who look to the sea for their livelihood have been observing the tides and tidal currents for many years. They have used their observations and practical knowledge in a variety of ways to their advantage. For example, it has aided them in timing the sailing of ships to and from port. It has also aided them in maintaining aquaculture and fishery activities in the inter-tidal zone near their shores. Observing the tides is done using tide gauges. For your observation you will need to find a tide gauge nearby. Note that you can only do this if you live near ocean. You may find tide gauges next to bridges, docks and other water side constructions. Tide gauges may be as simple as a large graduated stick inserted in water.

Image on the right shows an early example of a “real-time” tide gauge. Mechanical tide gauges were first used in the United States in the 1850s. This old wooden station, used in 1897 at Fort Hamilton, New York, is one of the earliest examples of a real-time, tide-measuring device. When entering or leaving the port, mariners would view this station through binoculars. The pointer indicates the present level of the water while the vertical arrow indicates whether the tide is rising or falling.

You may have a more modern or even electronic type of tide gauge in your area.

Among mechanical water gauges, a stilling-well tide gauge has certain advantages. The gauge is connected to the sea below the level of the lowest tide via a narrow tube. Rapid fluctuations of sea level produced by wind waves cannot penetrate this tube because its small diameter does not allow rapid transport of water through it.

A float is connected to an indicator arrow.

Modern stilling-well gauges use non-mechanical means of measuring the water level in the well (acoustic or laser) and record the data electronically, but the principle remains the same.

Procedure:

Locate a tide gauge in your area and make arrangements to make observation and record the water level.

Observe and record the phase of the moon at the day of your observation. If you don’t see the moon, use the online Earth and Moon Viewer. At new and full phases of the Moon, you will see the lowest low tide and the highest high tide.

Observe the water level every hour for 24 hours.

Record your results in a table like this.

You may use your data table to draw a graph.

Make a simple tide gauge:

Introduction:

Construction of a tide gauge can be a valuable addition to any tides related Science Project. A tide gauge can be as simple as a wooden stick. You may insert and secure the wooden stick on the sides of a bucket of water and show how the water level can be measured. Such a simple tide gauge works well for calm rivers with no or very little water waves. For other locations where water waves are present, a stilling well tide gauge must be used. Following are steps to construct a simple stilling well tide gauge.

Material:

• 2 feet PVC pipe (1 1/2″)
• 6 inches PVC pipe (1 1/2″)
• T joint (1 1/2″)
• Cork
• 6 feet balsa wood stick.

Procedure:

Connect the short and long PVC pipes along each other to the T connector. T connector is used so that the water pressure can enter from the bottom and the side. In this way if the bottom hole gets blocked, your tide gauge will still work.

Get a cork that can easily enter the pipe. This will be the float. Instead of cork, you may use a small ball or an empty film canister. Just make sure that your pipe is large enough for your float.

Use glue to connect a 2 feet long, narrow balsa wood above the float.

Connect a 2″ and a 12″ balsa wood to the end of the first one so together they will form a U shape as shown in the diagram in the right.

Attach a colored arrow to the end of the last piece.

Test your tide gauge in a bucket of water.

Make a model to demonstrate tides:

Introduction:

A deep square cake pan can provide enough landscape to simulate a coastal area with water in one side and sands and rocks on the other side.

Procedure:

Obtain a long – deep tray or cake pan.

Mix some sands and pebbles and glue them to the bottom of the tray starting from the center to one side. Make slope similar to the slope of a natural shore.

Fill up the tray with about 1/2″ water. Your tides display model is ready now.

Lifting the tray about 1/2″ on the water side or on the land side can change the direction of gravity and force water to move to the opposite side. This will change the water level on these sides and demonstrates the tides.

Materials and Equipment:

Material that you need for experiment 1 are:

• Access to a local tide gauge or the Internet.
• Notebook and pen for recording your data
• PVC pipes
• PVC T joints
• Deep metal tray
• Sands and Pebbles
• Wood glue or hot melt glue

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

If you collect data of high tides and low tides for a period of 30 days, you may want to measure the changes in water level in different days of the month. To do that, you subtract the water height at low tide from the water height at high tide in different days.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

References:

List of References

• Interactive Tide Chart
• http://tidesonline.nos.noaa.gov/
• Observe water level
• http://co-ops.nos.noaa.gov/levelwhy.html