Explain why do waves crash on the shores?

Origin and formation of waves

Waves mainly come from the wind, quite simply. When the wind blows over the surface of the water, it transfers energy to the molecules, creating ripples and then troughs and bumps, gradually forming waves. The stronger the wind, the longer it blows, and the greater the distance it covers, the bigger the waves will be. The energy accumulated in these waves then travels over long distances, often hundreds of kilometers, before reaching the shore. It is worth noting that earthquakes, underwater landslides, or volcanic eruptions can also trigger waves, often enormous ones, such as tsunamis. But most of the time, our nice little beach waves simply come from the wind brushing over the water.

Influence of the underwater topography

The underwater relief plays a decisive role in how waves evolve. When deep bottoms suddenly rise towards shallower areas, the energy of the wave concentrates and it begins to grow, losing speed until it becomes unstable. This is called refraction: the waves come closer together, deform, and become taller, ready to break near the shore. Reefs, sandbanks, or underwater cliffs act as real ramps propelling the waves upward, giving them their final shape before crashing onto the beach.

Phenomenon of swell and breaking waves

The swell roughly corresponds to trains of waves that travel over long distances, without necessarily needing wind in the area. Imagine a pulse originating from a storm far out in the ocean: it propagates slowly toward the coast in the form of regular undulations (swells), without the transport of water but rather of energy. As it approaches the shore and the seabed rises, it is slowed down by friction with the ground. As a result, the back of the waves, remaining fast, catches up with the front, and their shape changes. They gain height, become steeper, until they reach a critical point where they break and collapse, forming the famous crashing waves that we see crash onto the beach.

Role of tides and ocean currents

Tides, primarily caused by the gravitational pull of the Moon and the Sun, greatly influence the impact of waves on coastlines. At high tide, the elevated sea level allows waves to strike directly and harder against the shore, resulting in greater erosion. At low tide, waves lose some of their energy by breaking further from the shore. Ocean currents also play a role: they direct or enhance the trajectory of waves, affecting their power as they reach beaches or cliffs. Some currents, like tidal currents near the shore, provide additional energy to the waves by pushing them toward the coast, increasing their strength and ability to carry sand or erode rock.

Importance of wind and storms

The wind plays a major role in the creation and evolution of waves. By blowing across the surface of the sea, it gradually transfers its energy to the water, creating ripples that, depending on its speed and duration, grow into real waves. The stronger the wind blows, the larger these waves become, sometimes reaching gigantic proportions during storms. During these intense moments, powerful gusts cause chaotic, unpredictable waves that are particularly destructive when they hit the shores. This is why you often observe impressive and turbulent waves during a storm, while a gentle breeze typically creates light and peaceful waves.