23/11/2010
Waves are one of the most visible and dynamic forces shaping our planet's coastlines. They are a constant reminder of the immense power of the ocean, a spectacle that has captivated humans for millennia. But what exactly are waves, and how do they come to be? This article will delve into the fundamental aspects of wave formation, the factors that govern their size and behaviour, and introduce some of the key terminology used to describe these oceanic phenomena. We'll also touch upon how these powerful forces contribute to the shaping of coastal landscapes.
The Genesis of Waves: More Than Just a Ripple
While we often associate waves with the vast expanse of the open ocean, their origins can be traced back to much simpler beginnings. In essence, waves are oscillating movements observed in liquids, most commonly in seawater. The fundamental motion of water molecules within a wave is circular. While the surface motion appears to be up and down, the wave itself propagates forward. The primary driver behind the formation of most ocean waves is wind. As wind blows over the surface of the sea, it exerts friction on the water's surface molecules. This friction can gradually build up to form ripples. With continued wind action, these ripples can increase in size, developing into larger waves. The distance over which the wind blows (known as fetch), the duration for which it blows, and the speed of the wind all play crucial roles in determining the eventual size of the waves generated.
Factors Influencing Wave Size: The Trifecta of Power
The size and power of a wave are not arbitrary; they are the direct result of a combination of key factors. Understanding these elements is key to comprehending the erosive and depositional forces that waves exert on coastlines.
The three primary factors influencing wave size are:
- Fetch: This refers to the distance of open ocean over which the wind has been blowing uninterruptedly. A longer fetch allows the wind more time to transfer energy to the water, resulting in larger waves.
- Wind Duration: The length of time the wind blows over the water is critical. The longer the wind blows, the more energy it can impart to the water, leading to bigger waves.
- Wind Speed: Naturally, a stronger wind will transfer more energy to the water surface than a weaker wind, thus generating larger waves.
It is important to note that these factors are interconnected. A strong wind blowing for a long duration over a vast expanse of ocean will produce the most powerful and largest waves.
Wave Anatomy: Understanding the Terminology
To effectively discuss and understand waves, it's essential to be familiar with the correct terminology used to describe their characteristics. These terms help define how waves behave and the work they do.
| Term | Description |
|---|---|
| Wave Crest | The highest point of a wave. |
| Trough | The lowest point of a wave, situated between two crests. |
| Wavelength | The horizontal distance between two successive crests or two successive troughs. |
| Wave Height | The vertical distance between the crest and the trough. |
| Frequency | The average number of waves passing a given point per minute. |
Constructive vs. Destructive Waves: Shaping the Shoreline
Waves can be broadly categorised into two main types based on their characteristics and the impact they have on the coast: constructive and destructive waves. The distinction is crucial for understanding coastal geomorphology.
| Feature | Constructive Waves | Destructive Waves |
|---|---|---|
| Wave Height | Lower (typically < 1 metre) | Higher (typically > 1 metre) |
| Wavelength | Longer | Shorter |
| Frequency | Lower (9-11 waves per minute) | Higher (11-15 waves per minute) |
| Shape | Gentle sloping front | Steep front |
| Type of Break | Spilling break | Plunging break |
| Swash vs. Backwash | Stronger swash, weaker backwash | Weaker swash, stronger backwash |
| Energy | Low energy | High energy |
| Effect on Coast | Deposition, builds up beaches | Erosion, removes material from beaches |
| Origin | Often from distant storms | Often from local, powerful storms |
Constructive waves have a strong swash that carries material up the beach and a weak backwash that removes only a small amount of material. This results in the building up of beaches. Destructive waves, on the other hand, have a powerful backwash that erodes the beach, carrying material away from the shore. They are often associated with storm events close to the coast.
Waves Approaching the Shore: A Transformation
As waves move from deep water towards the coast and enter shallower water, they undergo significant changes. These transformations are a direct consequence of their interaction with the seabed.
| Change | Description |
|---|---|
| Wave Speed | Decreases due to friction with the seabed. |
| Wavelength | Decreases as the waves bunch up. |
| Wave Height | Increases, as the energy is compressed into a smaller volume of water. |
| Frequency | Increases. |
| Wave Shape | Becomes more elliptical, and the crests become steeper. |
| Breaking | Eventually, the wave becomes too steep to support itself and breaks. |
The process of wave refraction also occurs as waves approach an irregular coastline. This is the bending of wave crests so they become more parallel to the shore. It happens because different parts of the wave crest enter shallow water at different times. Friction with the seabed slows down the part of the wave that is in shallower water, while the part in deeper water continues at a faster speed. This causes the wave to bend, and the wave energy tends to concentrate on headlands and dissipate in bays.
Coastal Processes: Erosion, Transportation, and Deposition
The energy of waves is a primary force responsible for shaping coastal landscapes through a cycle of erosion, transportation, and deposition.
Coastal Erosion
Erosion is the breakdown and removal of rock and soil. At the coast, this is primarily carried out by wave action. There are several key processes:
- Hydraulic Action: The sheer force of the water hitting the cliff, compressing air in cracks, and causing them to widen.
- Abrasion: Sediment carried by the waves grinds against the cliff face, wearing it away like sandpaper.
- Attrition: Sediment particles carried by waves collide with each other, becoming smaller and more rounded over time.
- Solution (Corrosion): The dissolving of soluble rocks, such as limestone, by the slightly acidic seawater.
Coastal Transportation
Once material has been eroded, it is transported along the coast. The primary method for this is longshore drift. This process occurs when waves approach the shore at an angle. The swash carries material up the beach at this angle, while the backwash, which is always perpendicular to the shore, pulls material straight back down. This zig-zag movement gradually moves material along the coastline.
Other forms of transportation include:
- Suspension: Fine, light material carried within the water column.
- Saltation: Larger particles bouncing or skipping along the seabed.
- Traction: The rolling or dragging of the largest, heaviest material along the seabed.
Coastal Deposition
Deposition occurs when the energy of the transporting medium (waves or currents) decreases, causing it to drop the material it is carrying. This often happens in sheltered areas like bays or in the lee of headlands. Features such as beaches, spits, and bars are all landforms created by deposition.
Swanage: A Coastal Case Study
Swanage, located in Dorset on the South Coast of England, provides an excellent example of these coastal processes at work. Its location in a sheltered bay with a sandy beach makes it a popular destination. The geology of the Swanage coastline is varied, with different rock types of varying resistance to erosion. This geological variation, combined with the forces of erosion, transportation, and deposition, has created a diverse coastal landscape featuring features like headlands and bays.
The pebbles often found on South Coast beaches, like those near Swanage, suggest a region with significant wave energy capable of moving larger sediment. Sandy beaches, conversely, are more commonly found in bays where wave energy is dissipated due to shelter and wave refraction, creating low-energy environments conducive to the deposition of finer sand.
Frequently Asked Questions
Q1: What is the main force that creates ocean waves?
The main force that creates most ocean waves is wind.
Q2: What are the three main factors that determine the size of waves?
The three main factors are fetch (distance of open ocean), wind duration, and wind speed.
Q3: What is the difference between constructive and destructive waves?
Constructive waves have a stronger swash than backwash, depositing material and building beaches. Destructive waves have a stronger backwash than swash, eroding beaches and removing material.
Q4: How does wave refraction affect the coastline?
Wave refraction bends wave crests to become more parallel to the shore, concentrating wave energy on headlands (leading to erosion) and dissipating it in bays (leading to deposition).
Q5: Name three processes of coastal erosion.
Three processes are hydraulic action, abrasion, and solution.
Q6: What is longshore drift?
Longshore drift is the movement of sediment along the coast in a zig-zag pattern caused by the angle of waves approaching the shore.
In conclusion, waves are a fundamental and powerful element of our natural world. Their formation, driven by the wind, and their subsequent behaviour as they approach the shore, dictate the processes of erosion, transportation, and deposition that continuously sculpt our coastlines.
If you want to read more articles similar to The Power of the Sea: Understanding Waves, you can visit the Automotive category.