The waves increase in size before reaching the coast due to the phenomenon of swell approaching shallow areas, forcing them to compress and rise, thus increasing their height before breaking.
In the open sea, waves advance carrying energy, but the water itself moves little. When they reach the coast, the seabed rises, the depth decreases, and everything gets disturbed: the circular movement of the water is compressed upwards, resulting in an increase in wave height. Essentially, the wave slows down when it hits the bottom and stands up. Thus, as the depth decreases, its amplitude increases, until it breaks on the shore. The shallower it is, the larger and taller the waves grow before crashing onto the coast.
The shape and slope of the seafloor play a significant role in wave amplification. When a wave approaches an area where the bottom rises quickly (such as near an underwater cliff or a steep slope), it gets compressed: its energy concentrates vertically, and it becomes taller. The same happens when the coastal relief is complex with bays, rocky points, or reefs: these obstacles force the waves to slow down abruptly, storing the energy that then accumulates in height. In this way, a shoreline with twisted and rugged relief often produces larger waves than a flat beach where nothing slows their progression.
Refraction is when a wave gradually changes direction as it approaches the coast. This primarily happens because the speed of waves varies with depth: when the front of a wave reaches shallow water, it slows down, while in deeper areas it continues to move quickly. As a result, the wave starts to "pivot," directing its energy towards specific areas of the shore. This concentration of energy causes a local increase in wave amplitude as it approaches the shore, creating areas where the waves break more strongly. This is also why we often see waves arriving almost parallel to the coast, even if they were coming from a different direction offshore.
When waves break towards the shore, their trajectory is rarely parallel to the beach. They often arrive at a slight angle, which causes a phenomenon called refraction. This situation causes the wave lines to gradually curve towards the shallower areas near the coast and concentrate their energy in a more restricted zone. As a result, you end up with energy that was initially dispersed out at sea, but gathered together as it reaches the shoreline, leading to a notable increase in wave height. It's exactly as if several small waves decided to join forces to create a larger wave near the shore, sometimes creating specific points where the sea appears particularly turbulent.
The littoral currents, these water movements parallel to the coast, significantly impact the wave height when they reach the shore. When a wave arrives in an area where these currents flow in the opposite direction, the wave slows down, compresses, and its amplitude increases noticeably (accumulation effect). Conversely, when the current follows the same direction as the waves, they tend to stretch and lose height. As a result, in certain places, you may observe particularly powerful and impressive waves, while just a few meters away, their height will be much lower. These local variations related to the currents make the coasts more or less suitable for certain activities, such as surfing or swimming, depending on the arrangement of the currents.
The highest wave ever officially recorded measured about 30 meters in height and was observed in Alaska in 1958 following a landslide caused by an earthquake.
Experienced surfers use the principle of wave refraction to identify the best spots where waves will be amplified and form perfect tubes.
Even hundreds of kilometers from the coast, a wave can carry energy over a vast distance without significantly displacing large quantities of horizontal water. The surface water mainly describes a vertical circular motion.
Some specific reefs or seabeds can cause a very localized amplification of waves, creating areas known for their exceptional size, such as Nazaré in Portugal.
This abrupt variation may be due to the shape of the seabed: reefs, sandbanks, submarine trenches, or the effect of refraction can concentrate wave energy at specific locations, thereby causing a significant difference in amplitude over a short distance.
Yes, tides strongly influence the size and power of waves. At low tide, when the seabed is shallow, waves often break further out to sea and lose energy as they approach the shore. Conversely, at high tide, the increased depth allows waves to get closer to the shore before breaking, potentially increasing their amplitude.
Yes, strongly. Weather conditions such as strong winds, offshore storms, or depressions directly influence wave height as they approach the coast, increasing their energy and amplitude.
Wave refraction is the phenomenon where waves change direction in response to a variation in underwater depth. This concentrates the energy of the waves on certain parts of the coastline, potentially creating dangerous conditions in specific locations.
The beaches where the seabed rises sharply towards the shore cause a rapid increase in energy and thus a marked amplification of the waves. The shape and slope of the underwater terrain play an important role in the intensity of the waves.
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