What is the relationship between refraction and giant waves in Nazaré?

The waves of Nazaré start because of a storm in the open sea. The winds cause a disturbance on the surface, transformed into energy. When the depth is great, the propagation of the waves (energy) is practically constant. As they approach the coast, therefore in shallower places, the wave length decreases, but its height increases.
When the wave speed is higher at the crest than at the pit, the wave breaks. In the case of the Nazaré wave, another factor is important. A promontory divides the beach in two. On the left side, the seabed is normal, that is, the depth gradient decreases steadily when approaching land. But, on the right side of the promontory, there is a canyon, a kind of underwater gorge, the deepest of the entire European coast. https://marsemfim.com.br/formacao-da-onda-de-nazare/Acesso em 11/02/2021

veraamorim said:

When the depth is great, the propagation of the waves (energy) is practically constant. As they approach the coast, therefore in shallower places, the wave length decreases, but its height increases.

That describes tsunami waves, not normal sea waves
Normal sea waves don't go to any great depth, but tsunami waves do, they are in the full water column from seabed to surface

The way in which the Nazerre wave is created is explained in https://www.surfertoday.com/surfing/the-mechanics-of-the-nazare-canyon-wave where the critical importance of the underwater canyon is discussed, as is how the sea floor profile causes the wave to refract. Other factors also contribute.

The incoming wave effectively splits into two. One part continues forward. The other part is traveling in deeper water so travels faster and is refracted and directed towards the first part.

When the two waves meet, their wave heights add producing a wave equal, in principle, to the sum of their individual heights when they are in perfect phase.



Any wave approaching a shoaling sea bottom will rise in height because a shallow water wave travels slower where the depth is less. The front encounters the shallow and slows while the back is still in deeper water and continues at the original speed, causing the water to pile up. A shallow water wave is one where the wave length is longer than the water depth.

This is precisely what happens with a tsunami and, in fact, tsunami is Japanese for "harbour wave" as the waves are only seen close to the shore. A tsunami is all but imperceptible in the deep ocean where it is traveling at, say, 500 km per hour, but is only 1 foot high. As its wave length is so long - say 500 km - when a complete wave passes by, a boat is lifted by 6 inches in 15 minutes, then falls by one foot in 30 minutes, then is lifted by 6 inches in 15 minutes, something completely imperceptible to those aboard. It is only when the wave slows on reaching a shoaling shore that its height builds. The standard tactic for a boat in harbour is to head out to the open sea if a tsunami is imminent.

Different sea floor profiles give different build heights which is why one area may be devastated but a nearby area is hardly affected.

davenn said:

Normal sea waves don't go to any great depth, but tsunami waves do, they are in the full water column from seabed to surface

Normal sea waves close to the shore behave exactly like tsunami waves. Both a 'wind driven wave approaching the beach' and a 'tsunami wave during its entire journey' are shallow water waves meaning their wave length is longer than the water depth and their speed is proportional to the depth.

The key factor governing the wave behaviour is the ratio between the wave length and the water depth.

Normal wind driven sea waves in the deep ocean are different as here they are deep water waves because their wavelength, of the order of hundreds of metres, is much less than the water depth of thousands of metres. It is commonly said that "they cannot feel the bottom".

I'd say that the difference between tsunami waves and the rest is due to the way they are formed. A tsunami is caused by a 'step function' in water volume where it is formed. There is suddenly a displacement of a lot of water, all of a sudden. If you analyse this into all the wavelengths, there is (virtually) no upper limit to wavelength; just that imposed by the total water depth and displaced volume.
Otoh, a regular ocean wave is formed by a steady process over a long distance and there will be a definite wave train with a wavelength set by the, wind speed etc. Energy is transferred from wind to waves in two stages, initially due to friction by forming tiny waves a;; over the water surface, Then there is a form of resonance when the wave speed is near the wind speed . (This is like a travelling wave amplifier tube which transfers electric beam energy into EM waves which are slowed down (via a helical line) to match the electron speeds.) This wave-making process on the ocean gives a certain maximum wavelength of wave to be generated which, in open ocean, 'doesn't feel the bottom' until approaching the shore.

The travelling wave effect could be responsible for water wave energy transferring back to wind when wind speed drops(?)

Freak waves can form where two or more wave fronts interfere with each other, forming unusual maxima. I suspect that, as with other interference effects, coherence is needed for high peaks, which would imply that multiple waves of similar wavelengths would be needed. Similar wind speeds would need to be involved to get this situation, so that would involve oceans of space.

There are reports of increasing numbers of freak waves (not sure about real evidence. Every so often a massive ship just disappears and one 'non magic' explanation could be such a resultant wave.