Oceanography Chapter 8

Wave Reflection

Angle of reflection is the same as the angle at which the wave hits the barrier.

The Surf Zone

Breakers - Water particle orbital speed slows in the wave trough more than in the wave crest • Plungers - Lose energy more quickly - Form on narrow, steep beach slopes • Spillers - More common - Form over wide, flat beach slopes

Episodic Waves

• Abnormally high wave • Related to a combination of - Constructive wave interference - Changing water depth - Currents

Wave Motion

• Circular particle orbits • Particle motion decreases with depth • MotionstopsatD=L/2

Depth Profile of Internal Waves

• Color-coded density variation with depth in the Strait of Messina

Wave Interaction

• Constructive interference - Waves are in phase - Waves add constructively and increase wave height - May result in breaking of large waves • Destructive Interference - Waves are out of phase - Waves cancel each other to some extent, smaller wave heights

Anatomy of a Wave

• Crest • Trough • Wavelength (L) • Wave height (H) • Wave period (T) and frequency (f) • Wave steepness (S)

Two Types of Wind-generated Waves

• Deep-water waves - Depth > L/2 • Shallow-water waves - Depth < L/20

Wave Diffraction by an opening and Interference caused by multiple openings

• Diffraction can occur by an opening in a barrier and multiple openings can produce wave interference behind a barrier

Progressive Wind Waves

• Generated by storms • Storm centers - Sea - Waves with different L and T • Propagating away from the storm - Swell - Dispersion, speed depends on L or T

Forces Influencing Waves

• Generating force - Wind • Wind-generated waves - Earthquakes • tsunami • Restoring force - Surface tension • Capillary waves - Earth's gravity • Gravity wave

Stages of Tsunami Evolution

• Generation • Propagation • Inundation

Internal Waves

• Oscillating pycnocline • Displacement of pycnocline - By low-pressure storms - Currents moving over bottom topography

Wave Energy

• Potential energy - change in height of water surface • Kinetic energy - motion of water particles in their orbits

Standing Waves

• Reflected progressive waves • Nodes and antinodes • Seiches

Shallow-water Wave Refraction

• Refraction - Bending of waves caused by variations in wave speed • Concentrates energy on headlands • Energy is dispersed in bays

Wind-generated Gravity Waves

• Restoring force is gravity • Most common waves

Capillary Waves

• Restoring force is surface tension • Cat's paws, fast moving patches of capillary waves

Tsunami

• Seismic sea waves - Generated by earthquakes that produce vertical sea floor displacement • Shallow water waves - Long wavelength (100 - 200 km) - Long period (10 - 20 min) - Speed ~200 m/s (depends on water depth) • Wave height - Open ocean (may be as much as several meters but this is spread over a long wavelength) - Close to shore (may be over 30 m)

Shallow-water Wave Speed

• Speed is a function of water depth D • Shallow water waves are non-dispersive

Deep-water Wave Speed

• Speed is a function of a property of the wave, L or T

Wave Steepness

• Steepness = Height/Length • S=H/L • Smax =1/7orwhenthecrestangle approaches 120°

Satellite Image of Internal Waves

• Strait of Messina • Separates the island of Sicily from the Italian peninsula

Rip Currents

• Surf zone transport of water is both onto the beach and along the beach • Water accumulates in the surf zone until it can flow seaward • Return flow in a fast, concentrated current • Can stir up sand particles

Tsunami Alert System (Project DART)

• Surface buoy transmits data • Precise pressure recorded rests on the bottom

Wave Diffraction around a Barrier

• Wave energy encountering a barrier can bend around behind the barrier

C = celerity = wave speed

• speed = (wave length) / (wave period) • C = L / T • Once a wave is created: - The speed may change - The period remains the same • In deep water, wave speed depends on L • In shallow water, wave speed depends on water depth

Shallow-water Wave Motion

•Water particles move in an elliptical orbit • Motion extends to the bottom

Wave Height

-Controlson wind wave height: - Wind speed - Wind duration - Fetch

Shallow-water Waves

-Shallow-water waves travel in water whose depth is less than half the wavelength of the wave

Energy from Waves

-Tremendous amount of wave energy -Methods of harnessing wave energy - Vertical rise and fall of the waves can power pumps - Oscillation of the water surface can drive pumps - Vertical rise and fall of the waves can compress air, driving a turbine -Cost, location, lack of regularity, and environmental effects

Wind Speed and Wave Height

Significant wave height is equal to the average wave height of the highest 1/3 of the waves.