At a coasts wave normally arrive in groups (sets) and we wondered why this is the case. Some internet research did reveal some clues but no convincing answer.
I want to know why waves gather in groups (sets) and what physical background is responsible for this.
I list some «facts» I found as a starting point:
It's said that:
- sets occur when different swells (originating from different storms) travel into the same direction and merge together: if a wave's crest coincides with another crest a bigger wave results, if the wave's peak hits a trough then a cancelling effect will make the sea go flat. Source: The Stormrider Guide: Europe The Continent. -> Does that mean if we have a single source of wave energy (1 swell) we will never see any sets? And does this also mean that in between sets we see the result of two waves having eleminated themselves?
- The same book also states that faster waves and slower waves group somehow. Faster waves with longer wavelengths overtake smaller waves and the longer the waves travel the more distinct is the grouping. But why do they travel in groups. It makes sense to me that the faster go up front but why do they organize? I read something about the groupiness factor but I don't really understand what this means. And all this still makes no set, right? It just explains, why we can measure distinct waves after the wave has travelled for some time?
- I also read that one can imagine that a set is also kind of a part of a bigger wave. The first set wave being kind of small, increasing in height to the wave in the middle and then decreasing again until the last and smallest wave of the set comes in. Is this a reason why the set seems to be hold together? And if yes, why?
- In this article, they say that the last waves in a set travel faster than the ones in front and that they increase in height until the get slower again (and are in front now). Does anyone has an explanation for this?
Although individual waves move at twice the speed of the group, they are bound to it by the energy they all share.
How can this be explained. Why can't a wave break out of a group and travelling a bit faster.
- And in general why are there groups in the first place. As written in the same article:
Wave measurements usually show a tendency for large waves to group together--often referred to by scientists as "groupiness."
Does that mean we can see the groups already or is it just group after group without any space between? Because if the pattern looks like this:
|group| ------- |group| ------- |group|
Where does the space in between result from? Also, if the first pattern is correct, this would directly contradict the statement from before, that sets result from two or more interfering swells...
I would be glad if somebody could explain this a bit more into detail, with keeping it understandable. :)
Thanks in advance.