2.1 Wave Formation and Propagation
4 min read•august 7, 2024
Ocean waves are born from wind-water interactions, growing with wind speed, duration, and . As they travel, waves undergo and refraction, changing speed and direction based on wavelength and water depth.
Nearshore, waves experience 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)
- increases with wind speed until a fully developed sea state is reached, where the energy input from the wind balances the due to breaking and other processes
- The 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 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
- 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
- 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
- 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
Key Terms to Review (24)
Dispersion: Dispersion refers to the phenomenon where waves of different wavelengths travel at different speeds, causing them to spread out over time. In wave propagation, this leads to a separation of wave groups based on their frequencies, impacting how waves interact with each other and the environment. Understanding dispersion is crucial in predicting wave behavior, especially when waves encounter varying depths and currents in oceans or other bodies of water.
Energy Dissipation: Energy dissipation refers to the process through which energy, particularly mechanical energy, is transformed into heat or other forms of energy that are no longer usable for performing work. This concept is crucial in understanding how waves lose energy as they propagate through different mediums, impacting their height, speed, and overall behavior. As waves travel, they interact with the environment, resulting in energy loss due to friction, turbulence, and absorption by the medium.
Erosion: Erosion is the process by which natural forces such as water, wind, and ice wear away and transport soil, rock, and other surface materials from one location to another. This phenomenon is crucial for shaping coastlines and influencing wave dynamics, as it alters the landscape where waves interact with land, thereby affecting wave formation and propagation patterns.
Fetch: Fetch refers to the distance over water that the wind blows in a single direction, generating waves. This concept is crucial because it influences the size and energy of the waves formed, as longer fetches can produce larger and more powerful waves. The relationship between fetch and wave formation connects directly to how waves propagate and their characteristics, impacting various aspects of ocean dynamics and energy capture methods.
Gravity waves: Gravity waves are oscillations that occur in a fluid medium, such as water or air, where the restoring force is gravity. These waves are generated when disturbances, such as wind or seismic activity, create a displacement that gravity then acts to restore. They play a crucial role in energy transfer within oceans and atmospheres, significantly affecting wave formation and propagation.
Internal Waves: Internal waves are gravity waves that occur within a fluid medium, such as the ocean, where there is a variation in density, usually caused by differences in temperature or salinity. These waves propagate along interfaces between layers of different densities and can significantly influence ocean circulation, mixing processes, and the transport of energy and nutrients in the water column.
James Clerk Maxwell: James Clerk Maxwell was a Scottish physicist known for formulating the classical theory of electromagnetic radiation, which unified electricity, magnetism, and light as manifestations of the same phenomenon. His work laid the foundation for modern physics, especially in understanding wave behavior, including the formation and propagation of waves in various media.
Marine ecosystem impacts: Marine ecosystem impacts refer to the changes and effects that various human activities, including energy extraction and environmental alterations, have on marine life and habitats. These impacts can disrupt food webs, alter species distributions, and degrade vital ecosystems like coral reefs and estuaries, ultimately affecting biodiversity and ecosystem services that humans rely on.
Oscillating Water Column: An oscillating water column (OWC) is a type of wave energy converter that uses the movement of water within a column to generate energy. As waves enter the column, they create changes in water levels, causing air above the water to be compressed and expanded, which can drive a turbine to produce electricity. This mechanism connects with various aspects of wave energy technology and site assessments for effective energy capture.
Power Take-Off: Power take-off refers to the mechanism that converts the kinetic energy of waves or tides into usable mechanical energy for electricity generation or other applications. This system is crucial for harnessing renewable energy from ocean resources, allowing for efficient energy conversion and integration into power grids. The design and effectiveness of power take-off systems significantly influence the overall performance of wave and tidal energy devices, especially in terms of energy extraction and operational reliability.
Scouring: Scouring refers to the process of erosion caused by the action of waves and currents on seabeds or riverbeds, leading to the removal of sediment and material. This phenomenon is critical in understanding how waves form and propagate, as it can alter seabed topography and influence wave behavior, including wave height, energy distribution, and overall coastal dynamics.
Seabed Interactions: Seabed interactions refer to the complex physical and biological processes that occur between the seabed and water column, significantly affecting wave formation and propagation. These interactions can influence wave characteristics such as height, direction, and energy dissipation as waves move across varying seabed types. Understanding these interactions is crucial for predicting wave behavior and designing marine structures like tidal and wave energy devices.
Shoaling: Shoaling is the process whereby waves increase in height and decrease in wavelength as they approach shallow water. This occurs due to the interaction between the wave energy and the ocean floor, which causes the wave's speed to decrease and its shape to change, leading to a steeper profile. Understanding shoaling is essential for grasping how waves behave as they propagate and interact with coastal features.
Spectral Density: Spectral density is a measure that describes how the power or variance of a time series signal is distributed across different frequency components. This concept is crucial for understanding wave formation and propagation as it provides insight into the energy distribution of waves, helping to identify dominant frequencies that influence wave behavior and characteristics.
Surface Waves: Surface waves are a type of wave that travel along the interface between two different mediums, such as water and air, characterized by their circular motion and significant energy transfer at or near the surface. They play a crucial role in energy transmission, influencing both wave formation and propagation as well as the characteristics and parameters associated with wave behavior.
Swell: Swell refers to a series of mechanical waves that travel across the surface of a body of water, generated primarily by distant weather systems. Unlike choppy waves that occur locally due to wind, swells are characterized by their long wavelength and smoother, more uniform appearance as they propagate over long distances. Understanding swells is crucial as they influence wave energy resources, coastal processes, and navigation safety.
Walter Munk: Walter Munk was a prominent American oceanographer known for his pioneering work in the study of ocean waves and their behavior. He significantly contributed to the understanding of wave formation, propagation, and interaction with coastal processes, influencing the fields of oceanography and coastal engineering.
Wave breaking: Wave breaking occurs when a wave reaches a critical height and can no longer maintain its shape, resulting in the wave crest collapsing and spilling or crashing down. This phenomenon is a critical aspect of wave dynamics, as it influences energy dissipation, sediment transport, and coastal processes, making it essential to understand in the context of how waves form and propagate and their characteristics and parameters.
Wave diffraction: Wave diffraction refers to the phenomenon where waves bend around obstacles or spread out after passing through narrow openings. This behavior is a fundamental characteristic of waves, including water waves, and plays a crucial role in understanding how waves interact with their environment, particularly in terms of energy distribution and propagation patterns.
Wave frequency: Wave frequency refers to the number of wave cycles that pass a given point in a specific amount of time, typically measured in Hertz (Hz), where one Hertz equals one cycle per second. This concept is crucial for understanding how energy travels through water, as it influences wave behavior, energy transport, and the interaction between waves and coastal structures.
Wave height: Wave height is the vertical distance between the crest (the highest point) of a wave and the trough (the lowest point). This measurement is crucial for understanding wave dynamics, energy potential, and the design of systems that harness wave energy. It influences how waves propagate, interact with structures, and contribute to the overall energy available in oceanic systems.
Wave Refraction: Wave refraction is the bending of waves as they approach shallow water or encounter changes in the seabed topography, causing them to change direction. This phenomenon is crucial in understanding how waves propagate and interact with coastal features, influencing erosion, sediment transport, and the overall energy distribution along shorelines.
Wave Spectrum: The wave spectrum refers to the distribution of wave energy across various wavelengths and frequencies within a water body, illustrating the characteristics of waves in terms of their height, period, and direction. It provides valuable insights into how waves are generated and propagate, as well as the interactions between different wave components in an ocean or other large bodies of water.
Wind Shear: Wind shear refers to the change in wind speed and direction with height in the atmosphere. This phenomenon is significant in wave formation and propagation, as variations in wind speed can influence the generation of waves and their energy transfer across the water surface. Wind shear affects how energy is distributed among waves, ultimately impacting their size, direction, and stability as they propagate through the ocean.