2.1 Wave Formation and Propagation

Ocean waves are born from wind-water interactions, growing with wind speed, duration, and fetch. As they travel, waves undergo dispersion and refraction, changing speed and direction based on wavelength and water depth.

Nearshore, waves experience shoaling and breaking, dramatically altering their height and energy. These processes shape coastlines, drive currents, and influence sediment transport, making wave dynamics crucial for coastal engineering and energy extraction.

Wave Generation and Characteristics

Wind-Wave Interaction and Development

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• Wind blowing over the ocean surface generates waves through a combination of normal and tangential stresses at the air-water interface
• As wind continues to blow, waves grow in height and wavelength, with their growth dependent on wind speed, duration, and fetch (the distance over which the wind blows)
• Wave height increases with wind speed until a fully developed sea state is reached, where the energy input from the wind balances the energy dissipation due to breaking and other processes
• The wave spectrum describes the distribution of wave energy across different frequencies or wavelengths, with the peak of the spectrum indicating the dominant wave period or wavelength

Fetch and Swell Formation

• Fetch refers to the uninterrupted distance over which the wind blows in a constant direction, influencing wave growth and development
• Longer fetches allow waves to absorb more energy from the wind, resulting in larger and more organized waves (swell)
• Swell waves are long-period waves that have propagated away from their generation area and are no longer influenced by local wind conditions
• Swell waves are more regular and have a narrower directional spread compared to locally generated wind waves (sea)

Wave Propagation

Dispersion and Refraction

• Wave dispersion refers to the phenomenon where waves of different wavelengths travel at different speeds, with longer waves moving faster than shorter waves
• Dispersion causes wave groups to spread out as they propagate away from their generation area, leading to a more regular and organized wave field
• Wave refraction occurs when waves encounter changes in water depth or current, causing the wave crests to bend and align more parallel to the depth contours
• Refraction focuses wave energy on headlands and shoals while dispersing energy in bays and behind islands, influencing the spatial distribution of wave heights along the coast

Diffraction and its Effects

• Wave diffraction is the bending of waves around obstacles or through gaps, allowing wave energy to propagate into the lee of the obstacle
• Diffraction is most pronounced when the obstacle size is comparable to the wavelength, such as waves passing through a narrow harbor entrance or around a breakwater
• Diffraction helps to explain the presence of wave energy in areas that are sheltered from direct wave approach, such as in the shadow zone behind a headland or within a harbor
• The combined effects of refraction and diffraction can lead to complex wave patterns and energy focusing, particularly in coastal areas with irregular bathymetry or structures

Nearshore Wave Dynamics

Shoaling and Wave Height Changes

• As waves propagate from deep to shallow water, they undergo shoaling, a process where the wave height increases and the wavelength decreases due to the interaction with the seabed
• Shoaling is caused by the conservation of wave energy flux, which requires the wave height to increase as the group velocity decreases in shallower water
• The increase in wave height during shoaling depends on the initial wave steepness and the bottom slope, with steeper waves and gentler slopes leading to greater shoaling effects
• Shoaling plays a crucial role in determining the wave heights and energy levels reaching the shore, influencing coastal processes such as sediment transport and beach morphology

Wave Breaking and Energy Dissipation

• Wave breaking occurs when the wave steepness exceeds a critical threshold, typically in shallow water or due to wind forcing
• Breaking waves dissipate a significant portion of their energy through turbulence, air entrainment, and the generation of currents and sediment transport
• The type of wave breaking (spilling, plunging, or surging) depends on the wave steepness and the bottom slope, with plunging breakers being the most energetic and associated with high levels of energy dissipation
• The energy dissipated during wave breaking drives nearshore circulation patterns, such as rip currents and longshore currents, which play a vital role in sediment transport and coastal morphodynamics
• Breaking wave height is often used as a key parameter in coastal engineering design, as it determines the forces acting on coastal structures and the potential for overtopping and flooding