Oceanography Lecture Notes Outline

Seiches, tsunami, and tides

I. Contents - Topics Covered

Seiches

Storm Surges

Tsunami !

Tides

Tides Affect on Marine Organisms

Tidal Power

II. What is a Seiche?

A. Defined

1. A seiche is a rhythmic rocking back-and-forth of a body

of water within a confining basin

· Confined basins include harbors, bays, and inlets

· Seiche wave periods range from minutes to

more than a day

2. Each confined body of water has a specific resonating

frequency dependent on two factors

· Size or shape of the basin

· The amount of water in the basin

3. Energy can be harmonically added to a seiche, thereby

increasing its amplitude

· The process of swinging ever so higher on a swing set is a similar phenomena

4. A seiche in the form a wave that rises and falls at the

ends of a basin, but with only a back-and-forth motion

in the middle of the basin is called a standing wave

· Standing waves oscillate vertically, with little to no forward movement

· Lake Geneva in Switzerland is an example

B. How Are Seiches Generated?

1. Seiche waves are generated and set into motion by a

disturbing force acting on the basin water

·Persistent wind that suddenly stops

·Ocean swell (surf beat)

·Storm surge

·Landslide

·Tsunami

·Tidal bore

C. Coastal Damage by Seiches is Rare 1. Only when associated with large storm surge

and spring tides

2. The rareness is attributed to the low wave height of

seiches (centimeters to 3 meters max)

III. Storm Surge

A. Defined

1. An abrupt bulge of water driven ashore by a storm system

· Hurricanes

· Winter frontal systems

2. A storm surge can be up to 1 meter high in deep water

3. A storm surge can add up to 10 meters in water height

when it crams up against a shoreline

4. Technically not a wave

· It has a crest

· It does not have a trough

· It has no period or wavelength

5. Storm surges are short-lived (typically hours)

a. The time it takes for a storm center to pass

B. How is Storm Surge Generated?

1. Created underneath an atmospheric center of very

low pressure

· Intense storm systems like hurricanes

· Low pressure causes ocean surface to bulge

· Bulge moves with low pressure center

2. When the storm surge meets the shoreline it piles up

very rapidly

· Acts like an intense, very high incoming tide

· Can add up to 10 meters of water above normal

sea level

· Storm surges are typically accompanied by storm-generated wind waves and swell

C. Coastal Damage by Storm Surge Can Be Catastrophic

1. Combination of storm surge, large surf, and high tides

spells disaster low lying coastal areas

2. Storm surge waters can batter low lying coastal areas

much like a small tsunami

3. Storm surges up to 40 feet high have been extremely

deadly in various shorelines of the world that are low

lying and have high population densities

· Bangladesh

· The Netherlands

· Florida and the Gulf Coast of U.S.A.

IV. Tsunami!

A. Defined

1. Tsunami are very long wavelength shallow water

progressive (gravity) waves caused by the rapid

displacement of ocean water

· Tsunami is mistakenly called a tidal wave

· Seismic sea waves are tsunami

· Not all tsunamis are seismic sea waves

B. The Nature of Tsunami

1. Tsunami are shallow water waves because they always

travel in water depths shallower than ½ their wavelength

· Tsunami wavelengths are up to 200 kilometers

5. Tsunami waves travel very fast

· Calculated by the shallow water wave equation:

C = √gd

where C is speed, g is acceleration due to gravity, and d is water depth (typically 15,000 feet in the Pacific)

· up to 500 miles per hour

· Can cross the Pacific Ocean basin in 10 hours

4. Tsunami waves in open ocean only 1-2 meter in height

· Ocean vessels on the high seas wouldn’t notice one

5. Tsunami resemble a swiftly rising tide (a tidal bore) rather

than a breaking wave when they make shore

· Picture a super gigantic “mush burger” wave

· Unlike a normal sea wave, the tsunami wave keeps driving onshore for minutes

C. How are Tsunami Generated

1. Tsunami are generated by several water disturbing forces

which acts to displace surface water

· Earthquake/Faulting (sea bottom displacement)

· Shoreline or underwater landslide event

· Volcanic eruption

· Bolide impact

2. Seismic sea waves are generated when the ocean bottom is rapidly raised, or lowered, along an underwater fault zone

during a large earthquake, i.e. large fault rupture

· Up-lifted sea bottom causes an initial “bump” in the ocean surface

Ø Crest of tsunami wave forms first

· Down-dropped sea bottom causes an initial “dimple” in the ocean surface

Ø Trough of tsunami wave forms first

3. After a tsunami is generated, the wave typically disperses

into multiple crests

· The first wave often is not the largest

· Because of the long wavelength, the crests may be separated by 10s of minutes or even hours.

D. Tsunami Are Classified Into Two Categories

1. Based on their proximity to origin

· Local

· Far traveled

2. Local tsunamis primarily affect a small area and are usually caused by landslides (often underwater) which are triggered by earthquakes or volcanic eruptions.

· These are sometimes very severe and occur with

virtually no warning.

3. The largest local tsunami on record occurred in Lituya Bay, Alaska due to a landslide.

· From damage to trees, it is estimated to have reached more than 1500 feet up the mountainside

· A wave about 150 feet high swept down the bay and out to sea

· Four of six people aboard three boats anchored in the bay survived.

4. Tsunami can hit coastlines that are thousands of kilometers from the point of tsunami generation

5. They are free gravity waves like ocean swell

6. They do lose energy the further they travel

7. The Pacific basin is notorious for abundant far-traveled tsunami

8. Tsunami can be predicted after an earthquake

D. When Tsunami Meet the Shoreline

1. Fast-moving tsunami waves change radically when they

encounter the shoreline very

· Slows down

· Wavelength shortens dramatically

· Tremendous increase in wave height

· First wave encountered may be either the trough or the crest

Ø Trough – Appears like a Super low low tide

Ø Crest - Looks like a humungous tidal bore

2. Low-lying areas along coastlines are at serious risk when a

tsunami hits

· A very rapid onslaught of sea water rushes onshore

· The driving surge pushes inland as Sea level

3. Examples of devastating far-traveled tsunami events

· Hilo, Hawaii – 1946

· Alaska, 1964

· Japan – 1703, 1960

· Lisbon, Portugal – 1755

· Indonesia – 1883

· Flores Island - 1992

· New Guinea – 1998

E. Tsunami Prediction and Warning Network

F. Important Tsunami Safety Tips

1. If you are in a coastal community less than 50 feet above

sea level, and you feel a severe earthquake (one that

makes it almost impossible to stand up, which is

causing substantial damage to buildings, or is opening

cracks in the ground), RUN for the highest point you

can reach within minutes.

· Once you see the wave, you cannot outrun it. If all else fails, some people have survived by climbing trees.

2. Even if you have felt no earthquake, or only a mild one, a

sudden recession of water is always a danger sign.

· Run away from the water to high ground.

3. Remember - more severe waves can follow for hours.

· Do not return to low-lying areas for 24 hours.

4. Ships at anchor should weigh anchor and head to sea.

5. Ships at dock should also, if there is a warning due to a distant earthquake.

· However, if at dock during a severe earthquake, it is questionable whether the best choice is to jump ashore and run inland, or to try to ride it out aboard (loose mooring lines if possible.)

6. Tsunami warnings for distant sites are still inexact.

· They can warn that a tsunami might occur, and approximately when, but the danger at a particular location depends on topography, the particular characteristics of the wave, and other factors

· This results in many false alarms, leading people to disregard alarms when they occur

V. Tides

A. Tides Defined

1. Tides are the regular rise and fall of sea level that occurs

either once a day (every 24.8 hours) or twice a day

(every 12.4 hours).

2. Tides are waves with very long periods (24.8 or 12.4

hours) and wavelengths (thousands of km)

3. Tides are shallow-water waves (that is, their speed is

slowed by friction with the ocean bottom) even in the deepest parts of the ocean.

B. The Equilibrium Theory

1. Tides are caused by the combination of several forces:

· Gravitational attraction between the Earth and moon

· Gravitational attraction between the Earth and sun

· Centripetal "forces" that result from the moon's orbiting around the Earth and the Earth's orbiting around the sun.

· Another important factor is the land blocking the free motion of the tide around the earth

2. The moon is the strongest gravitational force

· Being closest, it is responsible for most of the tide

3. The Earth and Moon are both orbiting around the

center of gravity of the Earth-moon pair

· A point of rotation within, but not at Earth’s center

· See Figure 11.13 in the text

4. At all points of the Earth's surface, there are two forces

acting to produce lunar tides:

· The gravity of the moon, and

· A centrifugal force that acts parallel to the Earth-moon axis, outward or away from the moon.

5. Gravity (tidal forces) becomes weaker with distance

· The gravitational attraction of the moon is strongest for the side of the Earth closest to the moon

· The gravitational attraction of the moon is weakest for the opposite side

· On the other hand, the centrifugal force is the same everywhere

6. Because there is a slight excess of gravity on the side of

the Earth nearer the moon, the ocean "bulges" toward

the moon on that side

7. Because there is a slight deficiency of gravity on the side

of the Earth which faces away from the moon, the ocean

"bulges" away from the moon on that side, also

8. The Earth rotates beneath the bulges

· We would expect 2 high and 2 low tides each day

· This in fact occurs in most places

Ø This is termed a semidiurnal tide

· However, some places have only 1 high and 1 low tide a day

Ø This is termed a diurnal tide

9. The Earth-Sun system acts like the Earth-Moon system,

except that the tide generated is smaller

· The observed tide is the sum of lunar and solar tides

10. The relative positions of the Earth, sun, and moon

change during a month

· When the Earth, sun, and moon are aligned (new and full lunar phases), the tidal forces reinforce each other and there are unusually large tides

Ø Termed spring tides

· When the Earth, sun, and moon form a right angle (first and third quarter lunar phase), the lunar and solar tides partly cancel out, so tides are smaller than average tides

Ø Termed neap tides

11. This model of tides (the equilibrium model) is not useful

for actually predicting tides

· Does not take into account many important factors

Ø Size of the ocean basin

Ø Shape of the ocean basin

Ø Bottom topography of ocean basin

Ø Northern or Southern Hemisphere

12. A more sophisticated model was created that could

accurately predict the tides – the dynamic model of tides

C. The Dynamic Theory of the Tides

1. The dynamic theory of tides describes the tides in terms

of a very large number (> 400) of factors that influence

them

2. The dynamic model of tides considers the fact that the

tide is trapped within each ocean basin

· Acts like a standing wave which rotates around a center point called a node

· In the case of tides, the node is called an amphidromic point.

3. In the northern hemisphere, tides rotate counterclockwise

due to the Coriolis effect

4. In the southern hemisphere, tides rotate clockwise

5. Some ocean basins, due to their shape, have more than

one amphidromic point

· There are about 12 in the world's oceans

D. There are Three Major Types of Tides:

1. Diurnal tides have 1 high and 1 low per day

· They are found in Australia, Antarctica, and the Gulf of Mexico

2. Semidiurnal tides have 2 equal highs and lows each day

· They are found in the Atlantic and Indian Oceans

3. Mixed tides have two unequal highs and lows each day.

· They are found in the Pacific Ocean.

VI. Tides Affect on Marine Life

A. Tides Have Important Effects on Marine Organisms

1. Tidal currents are often the strongest currents in coastal

areas

2. Important to migration and reproduction of animals

· For example, larvae may rely on such currents to move them toward or away from the coast

· Another is fish like the grunion

3. Intertidal organisms are strongly influenced by the

periodic advance and retreat of the ocean

· They are often arranged in patterns (intertidal zonation) which depend on the amount of time the area of beach is exposed to the air.

4. Tides affect navigation of ships

· Depth of bottom

· Tidal Currents

5. Tides affect on surfing conditions

· Surfers rely on tides for choosing when to surf

· Some locals are best at low tide

· Some spots are best at high tide

VII. Tidal Power

A. Humans Have Harnessed the Power of Tides

1. Electrical energy generation

· Much like a regular hydroelectric dam system

· Better because it can work both ways

· An inexhaustible source of energy

· Only practical at certain coastal localities

3. Flood control purposes

· Regulate tidal bores and currents

· Safer conditions for river mouths and inlets

· Also useful for storm surge events

VIII. Vocabulary Terms