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Waves and Water Dynamics

Chapter 8 Lecture

Alan P. Trujillo Harold V. Thurman

Essentials of Oceanography

Twelfth Edition

Most waves are wind-driven.

Most waves are generated by storms.

Waves transmit energy across the ocean surface.

Deep water and surf zone waves have different characteristics.

Tsunami are special fast, long waves generated by seismic events.

Chapter Overview

Disturbing force causes waves to form.

Wind blowing across ocean surface

Interface of fluids with different densities

Air – ocean interface

Ocean waves

Air – air interface

Atmospheric waves

Water – water interface

Internal waves

Wave Generation

Associated with pycnocline

Larger than surface waves

Caused by tides, turbidity currents, winds, ships

Possible hazard for submarines

Internal Waves

Waves transmit energy

Cyclic motion of particles in ocean

Particles may move

Up and down

Back and forth

Around and around

Wave Movement

Progressive waves oscillate uniformly and progress without breaking

Longitudinal

Transverse

Orbital

Progressive Waves

Also called push-pull waves

Compress and decompress as they travel, like a coiled spring

Energy transmitted through solids, liquids, or gases via longitudinal particle movement

Longitudinal Waves

Also called side-to-side waves

Energy travels at right angles to direction of moving particles.

Generally only transmit through solids, not liquids

Transverse Waves

Also called interface waves

Waves on ocean surface

Orbital Waves

Crest

Trough

Still water level

Zero energy level

Wave height (H)

Wavelength (L)

Still water level

Wave Terminology

Wave steepness = H/L

If wave steepness > 1/7, wave breaks

Wave period (T) = time for one wavelength to pass fixed point

Wave frequency = inverse of period or 1/T

Orbital Wave Characteristics

Diameter of orbital motion decreases with depth of water.

Wave base = ½ L

Hardly any motion below wave base due to wave activity

Orbital Wave Characteristics

Wave particles move in a circle.

Waveform travels forward.

Wave energy advances.

Circular Orbital Motion

Wave base – depth where orbital movement of water particles stops

If water depth is greater than wave base (>½L), wave is a deep water wave.

Deep Water Waves

Lack of orbital motion at depth useful for floating runways and other structures

Deep Water Waves

All wind-generated waves in open ocean

Wave speed = wavelength (L)/period (T)

Speed called celerity (C)

Deep Water Waves

Speed of Deep Water Waves

Water depth (d) is less than 1/20 L

Water “feels” seafloor

C (meters/sec) = 3.13 √ d(meters) or

C (feet/sec) = 5.67 √d (feet)

Shallow-Water Waves

Wind-generated waves in shallow nearshore areas

Tsunami

Tides

Particle motion in flat elliptical orbit

Shallow-Water Waves

Characteristics of both deep- and shallow-water waves

Celerity depends on both water depth and wavelength

Transitional Waves

Capillary Waves

Wind generates stress on sea surface

V-shaped troughs

Wavelengths less than 1.74 cm (0.7 in)

Gravity Waves

Increasing wave energy

Pointed crests, rounded troughs

Wavelengths greater than 1.74 cm

Wind-Generated Wave Development

Capillary Waves

Ripples

Wind generates initial stress on sea surface

Gravity Waves

More energy transferred to ocean

Trochoidal waveform as crests become pointed

Wind-Generated Wave Development

Sea

Where wind-driven waves are generated

Also called sea area

Sea

Wind speed

Wind duration

Fetch – distance over which wind blows

Factors Affecting Wave Energy

Directly related to wave energy

Wave heights usually less than 2 meters (6.6 feet)

Breakers called whitecaps form when wave reaches critical steepness.

Beaufort Wind Scale describes appearance of sea surface.

Wave Height

TOPEX/Poseidon satellite Wave Heights

Beaufort Wind Scale

USS Ramapo (1933): 152-meters (500 feet) long ship caught in Pacific typhoon

Waves 34 meters (112 feet) high

Previously thought waves could not exceed 60 feet

Maximum Wave Height

USS Ramapo undamaged

Other craft not as lucky, e.g. aircraft carrier Bennington (1945)

Ships damaged or disappear annually due to high storm waves

Wave Damage

Fully developed sea

Equilibrium condition

Waves can grow no further

Swell

Uniform, symmetrical waves that travel outward from storm area

Long crests

Transport energy long distances

Wave Energy

Fully Developed Sea

Longer wavelength waves travel faster and outdistance other waves.

Wave train – a group of waves with similar characteristics

Wave dispersion – sorting of waves by wavelengths

Decay distance – distance over which waves change from choppy sea to uniform swell

Wave train speed is ½ speed of individual wave.

Swells

Wave Train Movement

Collision of two or more wave systems

Constructive interference

In-phase wave trains with about the same wavelengths

Destructive interference

Out-of-phase wave trains with about the same wavelengths

Wave Interference Patterns

Mixed interference

Two swells with different wavelengths and different wave heights

Wave Interference Patterns

Massive, spontaneous, solitary ocean waves

Reach abnormal heights, enormous destructive power

Luxury liner Michelangelo damaged in 1966

Basis of The Perfect Storm

Rogue Waves

Difficult to forecast

Occur more near weather fronts and downwind of islands

Strong ocean currents amplify opposing swells

Rogue Waves

Surf zone – zone of breaking waves near shore

Shoaling water – water becoming gradually more shallow

When deep water waves encounter shoaling water less than ½ their wavelength, they become transitional waves.

Waves in Surf Zone

As a deep-water wave becomes a shallow-water wave:

Wave speed decreases

Wavelength decreases

Wave height increases

Wave steepness (height/wavelength) increases

When steepness > 1/7, wave breaks

Waves Approaching Shore

Surf as swell from distant storms

Waves break close to shore

Uniform breakers

Surf generated by local winds

Choppy, high energy, unstable water

Shallow water waves

Breakers in Surf Zone

Spilling

Plunging

Surging

Three Types of Breakers

Gently sloping sea floor

Wave energy expended over longer distance

Water slides down front slope of wave

Spilling Breakers

Moderately steep sea floor

Wave energy expended over shorter distance

Best for board surfers

Curling wave crest

Plunging Breakers

Steepest sea floor

Energy spread over shortest distance

Best for body surfing

Waves break on the shore

Surging Breakers

Like riding a gravity-operated water sled

Balance of gravity and buoyancy

Skilled surfers position board on wave front

Can achieve speeds up to 40 km/hour (25 miles/hour)

Surfing

Waves rarely approach shore at a perfect 90-degree angle.

As waves approach shore, they bend so wave crests are nearly parallel to shore.

Wave speed is proportional to the depth of water (shallow-water wave).

Different segments of the wave crest travel at different speeds.

Wave Refraction

Wave energy unevenly distributed on shore

Orthogonal lines or wave rays – drawn perpendicular to wave crests

More energy released on headlands

Energy more dissipated in bays

Wave Refraction

Gradually erodes headlands

Sediment accumulates in bays

Wave Refraction

Waves and wave energy bounced back from barrier

Reflected wave can interfere with next incoming wave.

With constructive interference, can create dangerous plunging breakers

Wave Refraction

Two waves with same wavelength moving in opposite directions

Water particles move vertically and horizontally.

Water sloshes back and forth.

Standing Waves

Nodes have no vertical movement

Antinodes are alternating crests and troughs.

Standing Waves

Seismic sea waves

Originate from sudden sea floor topography changes

Earthquakes – most common cause

Underwater landslides

Underwater volcano collapse

Underwater volcanic eruption

Meteorite impact – splash waves

Tsunami

Long wavelengths (> 200 km or 125 miles)

Behaves as a shallow-water wave

Encompasses entire water column, regardless of ocean depth

Can pass undetected under boats in open ocean

Speed proportional to water depth

Very fast in open ocean

Tsunami Characteristics

Tsunami vs. Wind-Generated Waves

Tsunami Generation and Propagation

Sea level can rise up to 40 meters (131 feet) when a tsunami reaches shore.

Tsunami Destruction

Most occur in Pacific Ocean

More earthquakes and volcanic eruptions

Damaging to coastal areas

Loss of human lives

Example: Hilo, Hawaii in 1946

$25 million damage

159 deaths

Tsunami

Krakatau – 1883

Indonesian volcanic eruption

Scotch Cap, Alaska/Hilo, Hawaii – 1946

Magnitude 7.3 earthquake in Aleutian Trench

Papua New Guinea – 1998

Pacific Ring of Fire magnitude 7.1 earthquake

Historical Tsunami

Historical Large Tsunami

December 26, 2004

Magnitude 9.2 earthquake off coast of Sumatra

1200 km seafloor displaced between two tectonic plates

Deadliest tsunami in history

Coastal villages completely wiped out

Indian Ocean Tsunami

Detected by Jason-1 satellite

Traveled more than 5000 km (3000 mi)

Wavelength about 500 km (300 mi)

230,000–300,000 people in 11 countries killed

Lack of warning system in Indian Ocean

Indian Ocean Tsunami

Jason-1 Satellite Detection of Indian Ocean Tsunami

March 11, 2011 – Tohoku Earthquake

Magnitude 9.0 earthquake in Japan Trench

Felt throughout Pacific basin

Most expensive tsunami in history

Initial surge 15 meters (49 ft)

Topped harbor-protecting tsunami walls

Amplified by local topography

Japan Tsunami

Killed 19,508 people

Disrupted power at Fukushima Daiichi nuclear power plant

Reactors exploded

Radioactivity problem initiated

Japan Tsunami

Pacific Tsunami Warning Center (PTWC) – Honolulu, HI

Uses seismic wave recordings to forecast tsunami

Deep Ocean Assessment and Reporting of Tsunami (DART)

System of buoys

Detects pulse of tsunami passing

Tsunami Warning System

Tsunami Watch – issued when potential for tsunami exists

Tsunami Warning – unusual wave activity verified

Evacuate people

Move ships from harbors

Tsunami Watches and Warnings

Lots of energy associated with waves

Mostly with large storm waves

How to protect power plants

How to produce power consistently

Environmental issues

Building power plants close to shore

Interfering with life and sediment movement

Waves as Source of Energy

Wave Power Plant

First commercial wave power plant began operating in 2000.

LIMPET 500 – Land Installed Marine Powered Energy Transformer

Coast of Scotland

500 kilowatts of power under peak operating capacity

Wave Power Plant

Portugal – 2008

Ocean Power Delivery

First wave farm

About 50 wave power development projects globally

Wave Farms

Global Wave Energy Resources

End of Chapter 8 Waves and Water Dynamics