Waves & TidesCh.3, Part 3

Teahupoo, Tahitti

Objectives

1. Diagram the parts of a wave

2. Understand how waves are formed and what determines their size and shape

3. Contrast basic tidal patterns

Mike Parsons @ JawsLet’s watch . . .

Jaws, Maui

Make a list

• List all the words that come to your mind when you think of the word “wave.”

How does it all begin?

Closer to home than you think… Shipwrecks, East Cape of Baja

Wind and Storms• But how do storms form?• Remember what high and low pressure are?• The earth warms and cools and the air is always moving north

and south to try and equalize pressure. • How do high and low pressure form?

High & Low Pressure

• High Pressure:– Air cools, molecules close

together, and it sinks towards surface

– Fair weather, light winds

• Low Pressure:– Air warms, molecules scatter,

air rises

– Weather is cloudy, strong winds

Weather Front• Cold weather front

– Cold air rushes in from the high to the low.

• What does this create?

Wind and Friction!• Wind moves air molecules

--> air molecules push on surface of water --> water molecules move in same direction

• Ripples --> chop --> to wind waves

• Rough, raw energy in 35-40 ft wind waves

Raw Energy• The size of waves depends on:

– Speed of wind

– Length of time wind blows

– Fetch• Span of open water over which

wind blows

• Typical winter storm:– 45-55 knot winds blowing over

600-1000 miles for 36 hrs.

• Raw wind waves escape storm and move into calm waters– Wind stops blowing or waves

travel faster than storm

Raw Energy!

Raw Energy --> Ground Swell• Chop dissipates • Clean swell energy

results• Further away the

storm --> cleaner the swell– South swells in

summertime from NZ are 6000 miles away from Nor Cal!

Parts of a Wave• Crest:

– Highest point

• Trough:– Lowest point

• Wavelength:– Distance between crests

• Period:– Average time interval

(seconds) between successive crests or troughs of a wave

Traveling Swell• Water particles do NOT move along with a wave, but move in

circles– Creates momentum

• Energy is transferred through the water• What do you notice about the arrows in the diagram on the left?

Freight Train Diggin’ Deep• Waves with 15+ sec period

maintain energy (6000 plus mi). Why?

• The energy runs deep. How deep?– 14 s --> 514 ft deep – 17 s --> 761 ft deep– 20 s --> 1053 ft deep– 25 s --> 1646 ft deep!!!

• So… a 7 s period swell reaches down 129 ft --> won’t affect serious groundswell– Neither do other swells.

Energy just goes right though each other

Conveyor Belt• Waves with similar momentums synchronize with

each other and form wave groups, a.k.a wave trains• Each wave moves forward from back to front of group

How Fast Do Waves Travel?• Knots to MPH

– (1 knot = 1.15 mph)

• Wave speed depends on . . .– Period– wavespeed = period (3)

• What’s the speed of a wave with a period of 13 s?– 39 kts/hr– How about 20 s?– 60 kts/hr!

• Most waves move in a group, so the group speed is 1/2 of individual wave speed. Impossibles, Bali

Group vs. Individual Speed

• Individual waves travel at twice the group speed

Waves entering shallow water• Wave feels ocean bottom --> friction --> 3 things happen:

1. Wave speed decreases2. Wavelength decreases3. Wave height increases

• What’s this called?

Pipeline, HI

Shoaling

• When the depth reaches 1/2 of the wavelength, wave rises

• Where is friction greatest on wave? Top or bottom?

• What part of the wave is moving faster?– Top!

• What happens when the top is moving significantly faster than the bottom of wave?

Breaking Waves• Wave pitches!• Not all waves “pitch.”

3 Main Types of Breakers

1. Surging:• Steep slope beach; wave

rolls instead of breaks right onto beach

2. Plunging:• Moderate slope; wave curls;

surfing!

3. Spilling:• Gentle slope; break far from

shore over long distance; crest spills down face of wave

Manos, Northern Indonesia

Plunging waves are my favorite . . .

Only a surfer knows the feeling

Why So Many Types of Waves?

Bathymetry and Shape of ocean floor

• Bathymetry: – Water depth based on sea level– This is based on topography of ocean floor

Fly through

The answer lies in the bottom• A steep slope from deep to

shallow water result in a minimal loss of wave energy prior to breaking.

• A change in depth!

Mavericks, HMB, CA

Jaws• At Jaws depth changes from

120 ft to 30 feet almost instantly

• Energy focuses on tip of shallow reef --> wave slows down, stands up and breaks

• Walls on either side in deep water moves ahead

• Longer the period, the more energy, bigger the wave

• What is the term we use to describe this “focusing” based on depth?

• Refraction—more on this in a minute

Teahupoo, TahitiThe Heaviest Wave in the World

What is responsible for the mutant shape of this wave?

The Reef

• Thousands of years of freshwater runoff carved a pass through reef

• Teahupoo sits a km out to sea at southern end of pass• No continental shelf. The first thing that Antarctic

swells hit after 10, 000 km journey is this waist-deep, razor sharp reef.

The Horseshoe-shaped Reef• Teahupo'o's reef is shaped like your left arm if you're looking

at your watch.

• The inside of your arm forms the lagoon while swells approach your upper arm from the left side

Below Sea Level?

• Because it so deep behind reef and so shallow on top of reef, Teahupoo breaks “below” sea level

• What lies beneath?

Some video footage

Do you want to see a longer video on Teahupoo“Solid?” (30 min)

Refraction vs. Diffraction

Refraction Diffraction

Refraction• the direction of the wave is changed due to changes in

water depth• Remember, part of wave closer to shore-> moves slower

than part in deep water (it’s “feeling” the bottom)• Part of wave in deep water catches up and becomes

parallel to shore

Diffraction

• Any change in the direction or intensity of a wave after passing an obstacle

Diffraction annimation

Ft. Point, SF

• Diffraction and refraction?

• Check out google earth for a birds eye view.

• NW swell must diffract into Bay, and then again bend the corner toward the Fort.

Refraction or Diffraction or Both?

• How many places can you find it happening?

Back to bottom types: OB

• OB is sand bottom with constantly shifting bars

• Ocean bottom shaped by waves, storms, tides and currents

• Billons of gallons of water slosh in and out of the Golden Gate each tidal shift

Sloat, 11-22-07

Rivera, 11-5-05

The Bar of Bars: The Potato Patch

• Read Doc Article for HW• Rivers bring sediment into the

bay from Sierras• Rushes out through channel

under bridge -- no chance for sediment to settle

• Passes Pt. Bonita and Pt. Lobos, slows down, fans out and settles

• North Bar, aka Four Fathom Bank, has the shallowest section called the Potato Patch (23ft)

Tsunamis

• Produced by earthquakes, landslides, or other disturbances on sea floor

• Wavelengths up to 150 miles

• Travel up to 435 mph

Impact From 2004 Tsunami

• 9.1 earthquake

• How many deaths?

• 225,000

• Before and after satellite pictures

Tides

• What causes the tides?

• The gravity of the sun and moon and the rotation of earth, moon and sun

• Moon’s gravity strongest on side of earth closest to moon

Tides: Bulges• Why the bulge on the side

opposite of the moon?• Centrifugal force

– Caused by earth and moon “system” rotating around common center of mass

• You have two bulges (high tide)

• What’s happening at the other two places?

• Low tide!

Tides: Spinning

• Because the earth spins, the place where high and low tides occur are constantly changing– Notice the red flag– About every 6 hours

Tides: Spring and Neap• When are tidal bulges the largest?

• When sun and moon are in line, acting together—spring tides

Tides: Types• Semidiurnal: two high, two low

• Mixed semidiurnal: successive high tides of different height

• Diurnal: one high and one low

Tides Vary• Based on continents, islands and bottom topography• Continents block westward passage of tidal bulges

Tidal Bores• A high wave caused by

an extreme incoming tidal flow

• Shape of estuary must be shallow and uniform

• Only occur in about 100 rivers in the world

Hangzhou, ChinaQiantang River

Can reach up to 29 ft and 25mph!!

Surfing a tidal bore?

Surfing the Amazon