Explain why do giant waves form off certain coasts?

Favorable geological conditions

The presence of certain underwater geological formations is a determining factor in the occurrence of giant waves. Typically, these are canyons, trenches, or sudden rises in the seabed. When the ocean floor quickly transitions from great depth to a shallower area, it causes water masses to abruptly slow down: the energy of the wave is then concentrated in a small area, making the wave grow larger and become very powerful. This feature is perfect for creating the monsters that surfers and enthusiasts call big wave spots, like Nazaré in Portugal. These specific underwater features thus play a key role in the sudden amplification of waves offshore from certain coastlines.

Influence of ocean currents and weather conditions

Giant waves often depend on what happens at the surface and underwater. Ocean currents play a significant role, as when a wave crosses a current flowing in the opposite direction, it suddenly slows down and can significantly amplify. The result: a wave that seemed ordinary quickly becomes huge. This is common in famous areas like off the coast of South Africa with the Agulhas Current or near Nazaré in Portugal. Alongside the currents, weather conditions enhance all of this: a strong storm, prolonged winds in the same direction, or a sudden drop in atmospheric pressure can rapidly increase wave size. When these conditions combine at the right moment, it quickly becomes colossal.

Amplifying effect of underwater reliefs

Underwater reliefs act as real launch ramps for giant waves. When a swell comes from the open sea and crosses an area where the seabed rises sharply, the energy of the wave is concentrated upwards, causing it to grow. Underwater plateaus or canyons often trigger this phenomenon: it's like when a crowd is pressed into a narrow corridor, forcing everyone to slow down, crowd together, and push upwards. As a result, the waves become steeper, gain power, and reach sometimes extreme amplitudes. In fact, it is this kind of particular relief, like the underwater canyon of Nazaré in Portugal, that explains the record waves surfed at these spots.

Wave-current interaction and concentration phenomenon

When a wave encounters a powerful ocean current moving in the opposite direction, it can quickly become spectacular. Their frontal opposition concentrates the energy of the wave, causing its height and steepness to rise rapidly. The phenomenon is somewhat akin to cars piling up in the same spot on a narrowed highway: it causes a jam, they squeeze together, and the waves then take on an impressive size. This mechanism, called energy concentration, explains why giant waves sometimes suddenly appear at specific locations at sea, where strong currents roam and collide. The most famous surfing spots in the world, like Nazaré in Portugal, owe part of their fame to this impressive interaction between waves and currents.

Impact of human activities and coastal development

Human activities, such as the construction of dikes, ports, or the dredging of the seabed, are not trivial for waves. By altering the underwater terrain or creating new structures, we completely change the way waves travel and accumulate near the coast. For example, the installation of a pier can concentrate wave energy in a specific area, making some waves much larger than expected. Even the massive extraction of sand from the ocean floor influences the path of waves and promotes extreme heights in places that were previously calm. Human interventions thus often have an invisible but very real effect on the size of the waves that end up on our shores.