Topic 16 - Waves

🌊 Topic 16: Waves

Course: GEOL 2503 – Introduction to Oceanography

🌀 What is a Wave?

• Definition: Energy moving through a medium.

• In the ocean, the medium is water.

• The energy moves, but the water itself doesn’t travel — it moves in circular orbits.

⚙ How Waves Begin

Generating Force = the initial energy that forms waves.

• Wind – main cause of most ocean waves.

• Pebble in a pond.

• Earthquakes, meteorite impacts – cause large waves or tsunamis.

🌍 Restoring Force

The force that returns water to its undisturbed level.

• Surface tension → restores small waves (ripples, capillary waves).

• Gravity → restores larger waves (gravity waves).

🌬 Types of Waves

1. Surface waves: travel along the water surface.

2. Gravity waves: restored by gravity.

3. Wind waves: generated by wind.

4. Progressive waves: move (progress) through water.

📏 Wave Terminology

🔄 Wave Motion

• Water particles move in circular orbits.

• The energy moves forward, not the water itself.

• Orbit diameter decreases with depth → motion negligible below ½ wavelength.

🚀 Wave Speed (Celerity)

C=LTC = \frac{L}{T}C=TL​

Where:

• C = wave speed (celerity)

• L = wavelength

• T = period

Once formed, a wave’s period stays constant, even if speed changes.

🌊 Deep-Water Waves (DWW)

• Occur when water depth > ½ wavelength.

• Speed depends on wavelength.

• Causes wave dispersion — longer waves travel faster than shorter ones.

⛈ Storm Centers and Seas

• Most waves are progressive wind waves.

• Generated by wind, restored by gravity.

• Sea: area of active wave generation, with chaotic, irregular waves of various sizes.

• Swell: waves that have moved away from the storm — smooth, long-crested, and regular.

🌬 Wave Dispersion

• Long waves travel faster → sort out from shorter ones.

• Process called sorting or dispersion.

• Results in swell traveling far from the storm area.

📈 Factors Controlling Wave Height

1. Wind speed – stronger wind → higher waves.

2. Wind duration – how long wind blows.

3. Fetch – area over which the wind blows.

Fully developed sea: maximum wave height for given wind conditions.
Significant wave height: average height of the highest one-third of waves.

⚡ Wave Interference

• When waves from different sources meet:

  • Constructive interference: crests align → higher waves.

  • Destructive interference: crest meets trough → waves cancel.

• Combination of both leads to irregular wave patterns.

💀 Episodic (Rogue) Waves

• Caused by intersecting waves, changing depths, and currents.

• Extremely large and unpredictable — can sink ships.

• Amplitude can reach 5–15 m or more.

📏 Wave Steepness

S=HLS = \frac{H}{L}S=LH​

• Maximum stable steepness ≈ 1/7 (crest angle ≈ 120°).

• When exceeded → wave breaks.

🏖 Shallow-Water Waves (SWW)

• Form when depth < ½ wavelength.

• Orbits flatten (become elliptical).

• Speed depends on depth, not wavelength.

• Cause refraction and wave breaking near the shore.

↩ Wave Refraction

• Refraction = bending of waves as they approach shore at an angle.

• Part in shallow water slows down first → wave bends, becoming parallel to the shoreline.

• Causes wave energy concentration on headlands and dispersion in bays.

💥 Wave Breaking

• Occurs in surf zone (where waves slow, steepen, and collapse).

• Water moves forward when waves break.

Types of Breakers:

⬅➡ Longshore and Rip Currents

• Longshore current: movement of water along the coast, caused by waves hitting at an angle.

• Rip currents: water piled up on shore returns seaward in narrow, fast-moving channels.

🌊 Tsunamis (Seismic Sea Waves)

• “Tsunami” = Japanese for harbor wave.

• Not caused by tides.

• Generating forces: earthquakes, submarine volcanoes, landslides, or asteroid impacts.

Characteristics:

• Very long wavelengths (100–200 km)

• Long periods (20–30 min)

• Low wave heights (1–2 m) in deep ocean

• Behave as shallow-water waves

• Travel at ~600 km/hr (372 mph)

⚫ Internal Waves

• Occur at boundaries between water layers of different densities (e.g., thermocline).

• Move slower than surface waves.

🪞 Standing Waves

• Do not progress — oscillate up and down.

• Formed when waves are reflected back on themselves.

• Have nodes (no motion) and antinodes (maximum motion).

• Example: water sloshing in a bay or harbor.

✅ Summary: Types of Waves