T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 1

Name: Tides Unit III: Real Tides (2 pts) Section:

Real Tides

Real tides can differ significantly from the predictions of the bulge theory of tides, because the bulge theory does not take into account a variety of factors like the presence of the continents, the depth of the ocean, weather conditions, and the shape of the shoreline. These factors can be strong enough to drastically alter the tides. For example, there are some places in the world with only 1 high tide and 1 low tide each day. This is the normal tidal pattern for these places. They are called diurnal tides.

There are two kinds of tidal patterns in which there are 2 high tides per day and 2 low tides per day. Semidiurnal tides have roughly the same heights each day: Both high tides have about the same height, and both low tides have about the same height. Mixed semidiurnal tides have significantly different heights on most days: One high tide is much higher than the other high tide, and one low tide is much lower than the other low tide.

T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 2

Semidiurnal and mixed semidiurnal tides are the most common tidal patterns. Diurnal tides are rarer. On the west coast of the United States has mixed semidiurnal tides. The east coast of the United States has semidiurnal tides. The Gulf (southern) coast of the United States has diurnal tides.

There are more than 2 tidal bulges and most move in circles

Perhaps the most obvious reason that the bulge theory of tides fails to accurately predict the tides is that the continents get in the way of the bulges. As the Earth turns, the bulges try to stay underneath the Moon (and on the side away from the Moon). In doing so, the bulges move over the ocean floor, but they cannot travel over the continents. Instead, the bulges bounce off the continents, split up into smaller bulges. These bulges are trying to reach the positions predicted by the bulge theory of tides. Most of the small bulges travel around the ocean basins in large circles. The circular motion is partially related to the Coriolis effect, something discussed in more detail in Unit 8A-1 “The Atmosphere and Winds.” In essence, the Coriolis effect causes objects moving over the surface of the Earth (like the tidal bulges) to bend off course owing to the Earth’s rotation underneath them. In addition, drag from the bottom of the ocean also slows down the bulges, keeping them from remaining under the Moon. Tides “feel the bottom” at all times, because they have huge orbitals (due to their very large wavelength, the distance between the bulges), so rubbing against the bottom is always a factor. Only at the Poles (where there is less distance to cover) can the tidal bulges move fast enough to stay under the Moon. In other places, the bulges are often “ahead” of the Moon, because the Earth rotates faster than the Moon orbits the Earth. The bulges tend to be pushed out from under the Moon owing to friction with the bottom of the ocean. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 3

1. True or false? “Some places have only 1 high tide and 1 low tide each day.”

2. The “bulge” theory of tides does not accurately describe the motion of the tidal bulges over the surface of the Earth: they do not travel all the way around the Earth each day. Describe how the tidal bulges actually move over the surface of the Earth.

3. Why can’t the tidal bulges stay underneath (“keep up with”) the Moon? Give at least 1 reason.

Storms and Tides

Storms are low-pressure systems, which causes the sea surface to rise beneath them. Very large storms (like hurricanes) cause the sea surface to rise significantly (20 feet or more), and the resulting storm surge can flood the shoreline when they hit land, reaching miles inland. Recall that air pressure is caused by the weight of the air above pressing down. Outside the storm, air pressure is higher and therefore pushing down more strongly on the ocean surface. The ocean surface goes down beneath the high-pressure air, and rises up where the downward push is weakest, beneath the storm. (The water that is pushed down outside the storm has to go somewhere, so it rises in the place where the downward push is weakest, beneath the storm.) This is somewhat like a teeter-totter or see saw on a playground. If a heavy person sits on one end and a light person sits on the other, the end with the heavy person goes down and the end with the light person goes up. The downward pressure of the light person is simply not enough to keep it down.

When the storm comes ashore, the increase in sea level grows even higher in places where the storms’ winds push the mound of water into coast. As the water comes ashore, it often gets even higher, because the water runs out of space as the ocean gets shallower. In addition, as the water meets resistance from vegetation and buildings and simply friction with the ground, the water in front slows while more water continues to be pushed in from behind by the storm winds.

Since storm surge effectively raises sea level along the coast, storm surge will be worst (flood inland the farthest) if the storm comes ashore at high tide, especially during spring tide T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 4

conditions. This is when Superstorm Sandy hit New York in 2012, helping it do more damage. On the other hand, storm surge will be smallest (flood inland the least) if the storm comes ashore at low tide. In other words, high tides raise sea level and thus make storm surge higher and worse. Low tides push the ocean surface down so the make storm surge less high and less dangerous.

4. Do storms make high tides higher or lower (than they would otherwise be)?

5. Do storms make low tides higher or lower (than they would otherwise be)?

Tides and Currents

Tides play an important role in creating currents in many estuaries. As sea level rises towards high tide in the ocean, water flows into the estuary because sea level in the estuary is now lower than in the ocean: Water flows downhill, pulled down by gravity. When sea level sinks towards low tide in the ocean, water flows out of the estuary, because in the estuary sea level is now higher than in the ocean. Again, water is just flowing downhill.

By the way, the tide flowing into estuaries and towards land is called the flood tide. The flood tide is when sea level is rising. The tide flowing out of estuaries and way from land is the ebb tide. The ebb tide is when sea level is sinking.

In estuaries that are large and have narrow or shallow connections with the ocean, it can take a long time for enough water to flow in or out so that the level of the sea surface in the estuary is the same as the level of the sea surface in the ocean. As sea level changes more and more in the ocean, a bigger and bigger difference between sea level in the ocean and sea level in the estuary can develop. The bigger the difference in sea level, the faster water flows between the locations. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 5

A big difference is sea level means that a great weight of water is being held back and “wants” to go through. (The Earth’s gravitational pull is stronger on a lot of water than on a little water.) In short, the larger the tidal range, the greater the size of the estuary, and the smaller the connection with the ocean, the faster the water moves into or out of an estuary.

Tidal flows can be quite treacherous and are often an important cause of mixing in estuaries. Sailors must be aware of these flows, or their ship could get pushed into rocks or shallow water and run aground. In the days when all ships were powered by the wind via sails, ships could not leave port if the tidal currents were against them. There is an old saying: “time and tide wait for no man.” If members of a ship’s crew did not get back to the ship in time, the captain would have to decide whether to leave without them or to wait for them and the next tide causing water to flow in the right direction.

6. If sea level is rising in the ocean, does water flow into bays (estuaries) or out of them?

7. If sea level is falling in the ocean, does water flow into bays (estuaries) or out of them?

8. Do high tides in a bay (estuary) occur earlier or later than the high tides along the coast?

9. Do low tides in a bay (estuary) occur earlier or later than the low tides along the coast?

10. Is it safer for a ship to enter or leave an estuary near high tide or near low tide? (Hint: Think about how deep the water is. What could happen to a ship if the water is too shallow?) T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 6

Tides Quotes

Tides have had a huge impact on human culture and thought, because they have affected our ability to gather food from the ocean and transport ourselves and goods. People’s livelihoods have always depended upon the tides and their knowledge of the tides.

Here are a few selected quotes about tides. Perhaps they will speak to you in a different way now that you have learned so much about tides.

“When you get into a tight place and everything goes against you until it seems that you cannot hold on for a minute longer, never give up then, for that is just the place and time when the tide will turn.” - Harriet Beecher Stowe

Just like moons and suns, With certainty of tides, Just like hopes springing high, Still I'll rise. - Maya Angelou

There’s no point in fighting the tide. It ebbs. It flows. You ride it. - Karen Marie Moning

The lowest ebb is the turn of the tide. - Henry Wadsworth Longfellow

Time and tide wait for no man. - Geoffrey Chaucer

Time and tide will wait for no man, saith the adage. But all men have to wait for time and tide. - Charles Dickens

There is a tide in the affairs of men, Which taken at the flood, leads on to fortune. Omitted, all the voyage of their life is bound in shallows and in miseries. On such a full sea are we now afloat. And we must take the current when it serves, or lose our ventures. - William Shakespeare

A rising tide lifts all the boats. - A favorite saying of President John F. Kennedy

Tidal Power

In some places in the world, tides are used to generate electrical power. All you need to do is to build a dam that blocks the flow of the water into or out of an estuary. Then, once the water on one side is at its highest, let the water run through, just like hydroelectric power on a river. The flowing water spins turbines containing magnets (spinning a magnet produces electricity). This is T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 7

only economical in big estuaries with narrow entrances and large tidal ranges. Smaller dams are cheaper to build and maintain. Large tidal ranges cause more water to flow in and out, and more water means more electricity can be generated.

Tidal Bores

An interesting phenomenon that occurs in some estuaries, particularly the mouths of rivers, is a tidal bore. If something slows down the motion of the water into the estuary, then the water flowing in can build up into a single, large wave crest that moves up the river or narrow estuary. In a river, the water flowing down the river pushes against the tide. A narrow estuary can funnel all the water, causing it to build up more and more in the space the remains. Sometimes the bore travels for miles, and people have been known to surf such waves. By the way, a bore is a kind of wave in which the water is just surging forward. An everyday example of a bore is the film of water that rushes up the slope of the beach after a wave breaks along the shoreline.

Seiches: Very High and Very Low Tides in Estuaries

If the timing of the tides is related to how long it takes the tidal wave crest to move up or down an estuary like a bay or harbor, then the crest of the incoming tide can interfere (add) with the crest of earlier tides that reflected off the end or sides of the estuary. This causes the high tides in the estuary to be extremely high in some places where the tidal crests “add.” The same thing can happen with low tides. The low tides in the estuary can be extremely low where tidal troughs “add.” Large changes in sea level due to the shape of an estuary and the timing of disturbances (like tides) are known as seiches. Harbor and port designers need to take this possibility into account, otherwise the ships could easily run into the bottom at low tide. The Bay of Fundy in Canada is famous for having the highest and lowest tides in the world owing to the seiche effect.

11. True or false? “Tides can be used to generate electricity.”

12. What is a tidal bore?

13. What is a seiche? T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 8

Tides and Ocean Life

Tide pooling is going to the coast to look at organisms (animals and algae) that live in tide pools. Tide pools are small depressions or “bowls” in the shoreline where water is caught when the tide goes down.

The region that is covered by water at high tide and exposed to the air at low tide is called the intertidal zone. The intertidal zone is one of the harshest environments where ocean organisms try to live. The area is exposed to the air and submerged underwater daily by the tides. The area can be drenched by freshwater during storms (causing the organisms to bloat and interfering with their bodies’ chemistry) and tide pools become increasingly salty (causing dehydration) as water evaporates due to heat from the Sun. If the tide is low during the day, the Sun itself bakes and dehydrates organisms, and if the tide is low at night, they freeze. (Remember: ocean water does not get as warm or cold as the land, in part owing to its high heat capacity.)

Many organisms “close up” during low tide, and wait for high tide when they can feed and breathe. (They have gills, not lungs, so they can only extract oxygen from water.) Most tide pool animals capture small plankton that are drifting in the water, though some graze algae off the rocks or hunt other animals who live there.

14. What are tide pools?

15. What is tide pooling?

16. How can the water in tide pools become fresher (less salty) than ocean water?

17. How can the water in tide pools become saltier than ocean water?

18. Where does it get warmer during the day, in tide pools (on land) or in the ocean?

19. Where does it get cooler during the night, in tide pools (on land) or in the ocean? T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 9

20. What do most of the animals living in the intertidal zone (e.g., tide pools) eat?

High tide is also a rough time to be in the intertidal zone. The coast is pounded by waves, which not only smash the organisms, but try to pick them up and throw them into rocks or the beach. To avoid this, organisms have adaptations that help them quickly dig down and hide beneath sandy sediments, or hold securely onto large rocks, things like body parts that act like suction cups and glues that work in water. The waves also cause large rocks to roll over the organisms, or pick up and fling small sediments at them. (A grain of sand is no big deal to you or me, but it is a large boulder to a tiny tide pool animal.)

In addition to the physical conditions (waves, temperature, salinity, and so on), biological conditions are also tough. Land predators attack during low tide and ocean predators like sea stars and whelks (predatory snails) attack at high tide. Moreover, there is a constant battle for the best real estate, places that are covered by water most of the time, like tide pools.

Shells and exoskeletons (skeletons on the outside) are common adaptations to life in this environment. They not only offer protection from predators, but also from pounding by waves and sediments, drying out in the Sun, cold winds, salty tide-pool water, and more.

Since conditions are so tough in the intertidal zone, you might wonder why so many ocean animals want to live at the coast in places like tide pools. The key reason is lots of food. Remember: There is more life near the coast. The most valuable property is the part of the coastline that is always underwater, but just a little bit. No one wants to live higher up on the shore and sometimes be exposed to the air, but you might choose to do so because there is less competition for the space, and you still get the big benefit of the coast (lots of food), at least at high tide.

Good Tide Pooling Behavior

When you go tide pooling, be careful where you step, and if you lift up a rock to look underneath, put it back the way you found it. The rock offers the organisms on the bottom protection from waves, the Sun, predators, and so on.

Do not pull on animals like sea stars that are stuck to the rocks. You can damage their suction cups, and break the material that glues them to the rocks.

Do not take any shells or rocks home. The organisms that live there might need them for protection. For example, hermit crabs use discarded shells as homes, and sea anemones put pieces of shells and rocks on their bodies, using them to create something like a shell for protection. If everyone who visited a tide-pool took home just 1 or 2 shells, the tide pools would quickly be completely wiped out. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 10

21. What strike (hit) tide-pool animals and algae with great force (physical impact) each day?

22. True or false? “Most of the animals in the tide pools are stuck to the rocks and remain in the same place at both high tide and low tide.”

23. What adaptation / body part / structure do most tide pool animals have to help protect them from the harsh conditions in the intertidal zone?

24. How should you behave when you visit the tide pools? What can you do? What might harm the environment?