From a distance, the shape of a coastline doesn’t seem to change much. However, as we move closer, it becomes clear that ocean waves shape and reshape this habitat, bringing new debris from the ocean and removing old material from the shoreline.
This is the work of constructive and destructive waves.
The continuous ebb and flow of the coastline is essential to the unique life it brings with it. By comparing constructive vs. destructive waves, we can better understand how this process works and why it’s a key part of the ocean’s ecosystem.
Main Differences Between Constructive vs Destructive Waves
- Constructive waves build coastlines, while destructive waves erode them.
- Destructive waves have higher energy, resulting in crushing waves as opposed to rolling waves.
- Constructive waves have longer wavelengths and lower frequencies than destructive waves.
- Destructive waves form in storms, while constructive waves are related to calm weather.
- Constructive waves are associated with a short fetch, as opposed to the long fetch of destructive waves.
- Constructive waves are found on gentry-sloping coastlines, while destructive waves break on coasts with a steep gradient.
Credit: Ggia (Left) & trf57 (Right)
Constructive waves, also known as building waves or beach-building waves, play a significant role in shaping the coastline. Constructive waves carry a large amount of energy over a long distance, which results in a wave with a gentle slope as it approaches the shore.
Because of their low crest and long wavelength, the waves break gently on the shore.
Constructive waves deposit more sediment onto the land through their swash than they remove in their backwash, thus building up and constructing the coastline.
According to the definition of a constructive wave provided by A Dictionary of Earth Sciences, “constructive waves are associated with low-energy conditions and a gentle offshore gradient.”
In short, the characteristics of a constructive wave include:
- Long wavelength
- Low frequency
- Low amplitude
- Gentle break
- Strong swash
- Slow rolling waves
Constructive waves are found in calm conditions.
Waves are formed when the wind blows over the ocean’s surface, transferring its energy to the water and forming ripples over its surface.
Over a distance, this energy increases and forms waves. The stronger the wind and the longer the distance it blows over (the fetch), the larger the waves.
Constructive waves are formed when the wind blows gently to moderately over a small fetch.
As the waves approach the shore and the sea bed becomes shallower, the bottom of the wave begins to slow due to friction. This creates a rolling effect (known as the shoaling effect) that raises the wave’s peak.
The steeper the gradient and the larger the wave, the higher the peak will become.
As constructive waves have a small amplitude (low wave height), they remain small and roll onto the beach instead of crashing.
This creates a wave with a swash that’s stronger than its backwash.
Impact on the Coastal Ecosystem
Beach-building waves are important for exactly what gives them their name – building beaches.
Other than the recreational time and tourism opportunities they offer us, coastal beaches are essential to the larger coastal ecosystem.
Not only do beaches and sandy shorelines provide support for numerous branches of life, but they form an important protective barrier between the ocean and the fauna on land.
As constructive waves deposit sediment, this barrier is increased, thus reducing erosion during rough weather.
This is similar to how barrier islands protect the land. Similarly, constructive waves build up tombolos and splits (ridges of sand that extend from land).
These create unique ecosystems, protecting the young and providing breeding grounds for an abundance of organisms.
Destructive waves, also known as erosional waves, behave in a nearly opposite way to constructive waves. Destructive waves form in rough conditions, generally during a storm when wind speeds are high and blow for long periods of time.
Destructive waves are large and form tall peaks which break onto the shore.
These larger wave types of waves have stronger backwash than swash, which results in the removal of sediment from coastlines.
In short, the characteristics of destructive waves include:
- Short Wavelength
- High frequency
- High Amplitude
- High Energy and Errosive Break
- Strong Backwash
- Steep Crests and Troughs
As with how most waves form, destructive waves gain their energy primarily from wind, far out at sea.
With strong winds comes more energy, and when this wind is allowed to blow for a long time over a far fetch, it creates large waves with a higher frequency and short wavelength.
When these waves approach a shallowing shoreline, the shoaling effect occurs, but at a larger scale than with constructive waves.
Because destructive waves have more energy and higher amplitude, they form higher peaks.
Steep coastlines are also associated with destructive waves, which, along with their height, cause the wave to become top-heavy and eventually crash onto the beach.
Due to the high amount of backwash created during this crash, more sediment is removed than deposited.
Impact on the Coastal Ecosystem
The most evident impact of destructive waves on the coastline is erosion.
Removing sand and rocks from the shoreline can drastically reshape the coast, often disturbing its inhabitants.
Due to the rapidly changing environment and powerful waves, habitats are quickly lost, often displacing animals and vegetation. One such example of this is the erosion of sand dunes that harbor sensitive life.
When destructive waves combine with high tides and heavy rainfall, coastal flooding can become an issue.
This can greatly affect the lives of humans, as well as other animals that live along the coast.
Over time, destructive waves can form impressive landforms.
Sea cliffs, sea stacks, and similar rocky coastal ledges can contribute a large sound of their shaping to destructive waves.
Although most of the impacts of destructive waves come across as negative, they are an important part of the natural ecological cycle.
Destructive waves help redistribute sediment back into the ocean and, with it, the nutrients that the land has to offer.
The sediment is then deposited elsewhere.
You can think of this as turning the solid in your garden. The more the ocean’s sediment is mixed, the better the nutrient availability.
Along with this sediment redistribution comes the reshaping of coastal areas, which is essential to balance and maintain a healthy coastline.
For example, destructive waves can remove sand and leave rocky shores, which supports life that the sandy beach otherwise couldn’t.
Furthermore, destructive waves bring most of our wave-loving recreational activities into the ocean, as well as the largest waves ever surfed.
Bodyboarders and surfers prefer destructive waves, which are taller, more powerful, and more frequent.
How do climate change and rising sea levels influence constructive and destructive waves?
Rising sea levels can increase the coastal erosion that occurs with destructive waves. Furthermore, changing weather patterns alter the timing, and energy of these waves, drastically altering their effects.
What are some global examples of coastal areas affected by constructive and destructive waves?
Popular beach destinations, such as the Gold Coast in Australia, Waikiki Beach in Hawaii, and the Mediterranean coastline of southern Europe, are examples of constructive waves. Regions exposed to strong storms, such as those in the U.S. during hurricane seasons, and areas highly susceptible to erosion, such as parts of England, experience many destructive waves.
How can coastal communities adapt to the effects of constructive and destructive waves?
Implementing sustainable coastal development practices, restoring and protecting natural coastal buffers like dunes and wetlands, constructing well-designed coastal defenses, and developing coastal management plans are essential to maintaining a healthy relationship with the coastline.
How do constructive and destructive waves affect coastal tourism?
Constructive waves bring white sandy beaches, long sandbars that create lagoons, and calm waters for tourists to bathe in. On the other hand, destructive waves remove all these pleasures but bring with them crushing rides for extreme watersports lovers.