Waves

Waves

I. Wave Types

Wind-Generated Waves

  1. Cause: Surface wind blowing over water.

  2. Characteristics:

    • Height, length, and period depend on wind speed, duration, and fetch (distance wind blows).

    • Short-lived vs. fully developed seas.

  3. Examples:

    • Capillary waves: Tiny ripples, restored by surface tension.

    • Gravity waves: Larger waves where gravity restores the surface.

Swell

  1. Cause: Wind-generated waves that have moved away from the wind source.

  2. Characteristics:

    • Long wavelength, regular pattern, travel long distances.

    • Low steepness, more uniform than wind waves.

Tidal Waves / Tides

  1. Cause: Gravitational pull of moon and sun on oceans.

  2. Characteristics:

    • Large-scale, slow oscillations.

    • Produce high tide and low tide cycles.

  3. Types:

    • Diurnal: One high and one low tide per day.

    • Semidiurnal: Two high and two low tides per day.

Tsunamis

  1. Cause: Seismic activity – underwater earthquakes, landslides, or volcanoes.

  2. Characteristics:

    • Very long wavelength (100–500 km).

    • Travel extremely fast (up to 800 km/h in deep ocean).

    • Low amplitude in deep water, large waves near coast.

Rogue Waves

  1. Cause: Nonlinear wave interactions, constructive interference.

  2. Characteristics:

    • Unexpected, extremely high waves.

    • Dangerous for ships.

Internal Waves

  1. Cause: Density differences between layers in stratified water (thermocline).

  2. Characteristics:

    • Occur below the surface.

    • Can be very large in amplitude.

II. Wave Interaction with coasts

Key Processes

Wave Shoaling: Change in wave height, speed, and wavelength as waves move into shallower water.

Effects:

  • Waves slow down due to friction with the seabed.

  • Wavelength shortens, and wave height increases.

  • Sets the stage for wave breaking near the shore.

Surf Zone: Area where waves break as they approach the shore.

Processes:

  • Spilling breakers: Gentle slope → energy dissipates gradually.

  • Plunging breakers: Steep slope → concentrated energy → strong erosion.

  • Surging breakers: Very steep shore → waves surge up → maximum impact at coast.

Effects:

  • High energy can erode cliffs, beaches, and coastal structures.

  • Drives swash and backwash, redistributing sand along the beach.

Refraction: Bending of waves as they enter shallow water at oblique angles.
Effects:

  • Concentrates energy on headlands → erosion.

  • Spreads energy in bays → promotes deposition.

Diffraction: Bending of waves around obstacles (islands, jetties) or through gaps.
Effects:

  • Creates shadow zones of low wave energy behind obstacles.

  • Reduces erosion in sheltered areas.

Reflection: Waves bounce back from steep cliffs or seawalls.

Effects:

  • Can produce standing waves or wave interference patterns.

  • May increase erosion at cliff bases.

Longshore Transport (Littoral Drift): Movement of sand and sediment along the coast due to waves hitting at an angle.
Effects:

  • Forms spits, barrier islands, and beaches.

  • Can cause erosion in one area and deposition in another.

Tidal and Storm Surges: Extreme water levels due to tides or storms.

Effects:

  • Amplifies wave energy at coast.

  • Causes flooding, erosion, and sediment overwash.

Effects on Coastal Landforms

  1. Erosion – Cliffs, headlands, rocky shores.

  2. Deposition – Beaches, spits, barrier islands.

  3. Sediment Sorting – Finer particles transported farther; coarse materials stay nearshore.

  4. Formation of Coastal Features – Coves, arches, stacks, sandbars, tombolos.

III. Wave Properties

Wavelength (λ)

  • Distance between two consecutive crests or troughs.
    Determines the speed and energy of the wave.

Wave Height (H)

  • Vertical distance from trough to crest.
    Influenced by wind strength, duration, and fetch.

Wave Period (T)

  • Time taken for two successive crests to pass a fixed point.
    Longer period → more energetic waves.

Wave Frequency (f)

  • Number of waves passing a point per unit time.
    f = 1 / T.

Wave Speed ©

  • Distance a wave crest travels per unit time.
    c = wavelength ÷ period (c = λ / T).

Wave Steepness (S)

  • Ratio of wave height to wavelength (S = H / λ).
    Waves break when steepness > 1/7.

Crest and Trough

  • Crest: Highest point of a wave.
    Trough: Lowest point of a wave.

Orbital Motion

  • Water particles move in circular orbits beneath the wave.
    Orbit size decreases with depth; negligible below wave base (~½ wavelength).

Wave Energy

  • Energy is proportional to wave height squared (E ∝ H²).
    Energy transported horizontally through the wave motion, not water mass

Wave Base

  • Maximum depth at which a wave’s orbital motion affects water.
    Approximately half the wavelength.