6.2 Wind-generated waves and swell

Wind-wave generation is a dynamic process driven by wind interacting with the ocean surface. As wind blows, it creates ripples that grow into larger waves. The size and shape of these waves depend on wind speed, duration, and the distance it travels over water.

Wave analysis and prediction are crucial for understanding ocean conditions. By studying how wind duration, fetch, and speed affect wave development, scientists can forecast wave characteristics. This knowledge is vital for maritime safety, coastal management, and understanding ocean energy transfer.

Wind-Wave Generation and Development

Process of wind-wave generation

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• Wind stress on water surface creates tiny ripples called capillary waves
• Capillary waves increase surface roughness allowing wind to transfer more energy
• Energy transfer from wind to waves amplifies wave height and wavelength
• Phillips mechanism initiates wave formation through pressure fluctuations
• Miles mechanism leads to exponential wave growth through wind-wave interactions

Factors in wave development

• Wind speed determines rate of energy transfer (stronger winds generate larger waves)
• Wind duration affects total energy input (longer-blowing winds create bigger waves)
• Fetch (distance wind blows over water) influences maximum wave size (longer fetch allows larger waves)
• Wave steepness (ratio of height to wavelength) increases as waves grow until breaking point
• Energy transfer efficiency varies with wave age (young waves grow faster than mature waves)

Concept of fully developed seas

• State where wave energy input balances energy dissipation through breaking
• Maximum wave size reached for given wind conditions (speed, fetch, duration)
• Equilibrium between wind input and wave breaking limits further growth
• Significant wave height represents average of highest one-third of waves
• Wave spectrum in fully developed seas shows energy distribution across frequencies

Wind waves vs swell

• Wind waves (sea) have irregular, choppy appearance with shorter periods and steeper profiles
• Wind waves generated by local winds, reflect current wind conditions
• Swell has more regular, smooth appearance with longer periods and less steep profiles
• Swell generated by distant storms, travels long distances with minimal energy loss
• Transition from wind waves to swell occurs through dispersion and sorting by wave speed
• Swell undergoes refraction (bending around coastlines) and diffraction (spreading around obstacles)

Wave Analysis and Prediction

Analyze the effects of wind duration, fetch, and wind speed on wave development

• Wind duration determines time available for wave growth
  1. Minimum time required for full wave development varies with wind speed
  2. Longer durations allow waves to reach maximum potential height
• Fetch effects wave characteristics
  • Fetch-limited conditions occur when wind duration exceeds time needed for full development
  • Longer fetch allows larger waves to develop (given sufficient wind speed and duration)
• Wind speed directly influences wave height and period
  • Beaufort scale relates wind speed to observed wave conditions
  • Higher wind speeds generate taller waves with longer periods
• Wave forecasting models (SMB method) use wind speed, fetch, and duration to predict wave characteristics
• Dimensionless parameters aid in wave analysis
  • Wave age compares wave speed to wind speed
  • Fetch-limited growth curves show relationship between fetch and wave development
• Energy transfer efficiency varies with wave state
  • Young waves (wave speed < wind speed) grow faster than mature waves
  • Fully developed seas have reduced energy transfer efficiency