Wave reflection is a process where waves, upon encountering a barrier, bounce back into the medium from which they originated.
The interaction is governed by the law of reflection, which states that the angle of the incoming wave (incident wave) equals the angle of the outgoing wave (reflected wave).
It applies to all types of waves, including water, light, and sound waves.
In the water environment, you can confirm the phenomenon either in a laboratory tank or on coastal shores.
Wave reflection also often changes and influences surf breaks and how swells behave when they reach the shores.
The Mechanics of Wave Reflection
When a wave encounters a vertical or steep surface - such as a seawall, cliff, or breakwater - it cannot pass through.
That's quite simple to visualize, right?
Instead, the energy of the wave is redirected back into the water, with part of its energy lost due to dissipation.
So, for reflection to occur, the barrier must provide resistance, preventing the wave's forward motion instead of absorbing the energy and canceling the forward velocity of the incoming wave.
If waves are traveling from 6 o'clock toward noon and encounter a barrier at a 45-degree angle, they will be reflected toward 3 o'clock.
"If the wave train is regular in period, a pattern of standing waves may be set up in which the orbits of the waves approaching the cliff and those reflected by it modify each other in such a way that there is only vertical water motion against the cliff and only horizontal motion at a distance out of one-quarter wavelength," notes William Bascom, author of "Waves and Beaches."
"Regular wave trains are rare in nature, and this unusual circumstance is called clapotis."
In this case, specific points along the wave (nodes) remain stationary, while others (antinodes) experience maximum vertical motion.
These standing waves are rare in natural settings but can be observed in controlled conditions, like a wave simulation tank.
Behavior on the Continental Shelf
Reflection plays a crucial role in shallow-water environments, such as the continental shelf.
Here, the interaction between waves and the seafloor or coastal structures becomes more complex.
Submerged barrier features like coral reefs or sandbars can cause partial reflection. Curiously, Polynesian navigators famously used wave reflections from reefs to identify landmasses during their voyages.
Also, waves reflect more efficiently on coastlines with steep beaches and cliffs, often producing dramatic effects.
For example, incoming and reflected waves can collide, creating explosive spray or a zippering effect as the impact point moves laterally.
One of the most common examples among surfers is the backwash.
Lastly, it's important to note that when it comes to energy redistribution, while some wave energy is reflected, part of it may dissipate as heat, turbulence, or friction against the shoreline, affecting the overall wave dynamics.
Reflection can also have unintended consequences.
For example, waves bouncing off seawalls may erode sand at their base, destabilizing the structure and damaging the surrounding beach.
This is one of the most common mistakes coastal planners make when addressing the need to protect the shoreline from the power of the ocean.
Differences Between Reflection, Diffraction, and Refraction
Reflection is just one of three key processes that modify waves as they interact with obstacles.
But how does it differ from diffraction and refraction?
Diffraction occurs when waves pass through a gap or around the edges of an obstacle, spreading out into the area beyond.
For instance, waves bending around the end of a breakwater and spreading into a harbor demonstrate diffraction.
Unlike reflection, diffraction redistributes wave energy rather than reversing its direction.
Then, refraction is the bending of waves as they travel into areas of different depths.
Shallow water slows the wave, causing its crest to bend toward the shallower area.
This is why waves tend to align nearly parallel to shorelines as they approach.
Reflection, by contrast, maintains the wave's original orientation relative to the barrier.
Real Examples of Wave Reflection
The wave reflection effect can be observed at many points.
For instance, you can see waves reflecting off harbor walls, causing oscillations and strong currents, delaying ship operations, and posing risks to vessels.
On steep coasts like those in Maui, Hawaii, reflected waves can often be seen traveling offshore, mingling with incoming waves to create complex patterns.
In San Sebastian, Spain, surfers extend their rides at Bahia de La Concha by taking advantage of the swell that bounces off the seawall into the bay.
Another unusual wave reflection pattern one can witness could be square waves, also known as cross seas, a checkered wave pattern that can be observed from higher viewing points.
As we've seen above, while designed to protect coastlines, breakwaters can also reflect wave energy, creating calmer zones behind them but potentially increasing erosion nearby.
Words by Luís MP | Founder of SurferToday.com
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