Why do waves come in sets - theory or reading?

NDC wrote:

I have just read the new-ish re-release of "Waves and Beaches - The Powerful Dynamics of Sea and Coast" by Willard Bascomb and Kim McCoy (published by Patagonia).
A fantastic book - learned as much in a few weeks reading as I have in decades of observation etc.

One thing the book explained with unusual superficiality was why waves arrive in sets.

I'm wondering if anyone knows the leading theories, and can either explain them or point me to good a good link or two

Here's what the book had to say
pp219-220: "Surf Beat: ... characteristics most evident to an observer on the shore is the variability in height of the breakers. A series of a dozen or so low waves will approach and break. Then there will be a group of several high waves - usually three or four in what is called a wave set - then another relatively quiescent period.

Sometime this variability is caused by the arrival of two swells (from two storms) of nearly the same period at the same time. When the crests of the two wave trains almost coincide, they reinforce each other and produce waves higher than either train. When the waves are almost completely out of phase and the crests in one train coincide with the troughs in the other, the resulting waves are small. as the phase relationship changes, a pattern emerges like that shown in figure 41."

https://ibb.co/FKzZzDZ

Anecdotally I had heard in the past that when waves from a single storm travel, they generally get more organised and cleaner, and the period increase as they travel further and further. Hence long period swells coming from sources further away. Part of this tendency to 'organise' I was told included the formations of sets and lulls. Which makes sense with my observation that the lag between sets is longer (the surf beat), and the difference between the average heigh of set waves compared to the lull waves becomes more exagerated in long period events.

Is this anecdotal know-how also a legitimate explanation ... in addition to the 'sometime' cause explained in the book? What are the other key causes? Any good reading, resources, links?

thanks any and all

NDC. I finished that book earlier this year, a great arduous read, you have to pause regularly to go back over what you’ve just read to understand the principles and store it away in the old grey matter. Undoubtedly the best book I’ve ever read about ocean dynamics. A must read for surfers who are interested.

Im not smart enough to tell on my own if people are just making stuff up or not when it comes to this. You can usually tell if it is good stuff based on the bibliography and citations.

Great question and I'll try and explain.

Waves arrive in sets due to the conservation of energy and momentum.

There's the group velocity of the set and also the solitary wave velocity which is 2x that of the group.

It's one of those magic phenomena's where to preserve energy, we see single waves propagating through the set, seemingly appearing at the back and then cycling through the peak of the set before fading at the front and then re-appearing at the back again.

This is why the single waves travel at 2x the speed of the group (set) and can be compared to how a racing peloton becomes more efficient due to the riders tucked in behind those in front see less drag and as a whole they move forward at a faster speed and with less work.

There are some cool GIFs here showing what I'm talking about.. https://en.wikipedia.org/wiki/Group_velocity

So once wind waves start escaping the storm they were generated by, they start organising themselves into groups and sets, as this is the most efficient way for them to travel through the medium, and then the further away from the source of the storm, the more drawn out these sets become as well. They become more lully but noticeable.

Dig that link Craig, awesome. Nice to see some calculus being used for good rather than evil :).

Does it the book talk about sand dynamics i.e formation of banks?

No worries at all NDC, super fascinating eh!

JaJake wrote:

Im not smart enough to tell on my own if people are just making stuff up or not when it comes to this. You can usually tell if it is good stuff based on the bibliography and citations.

JaJake. This latest copy is the third edition, 2020. It was first published by William Bascom in 1963, updated in 1979 and revised by Kim McCoy in the aforementioned year. The back cover says it all “As well as for students, surfers, sailors and the general public, this update is an essential handbook for climate scientists and ocean activists, providing clear explanations and detailed resources for the constant battle to preserve the ocean and the shore.” Bascom was the man. It’s a great read, no crap at all.

Going to put this book on the Christmas list.

MrBungle wrote:

Going to put this book on the Christmas list.

MrBungle. Wise wish from the white bearded one. It’s available mostly through Patagonia. A lot of the content is physics and maths but in a simplistic way that anyone can get the gist of the topic being explained/discussed. I found, after each topic or chapter, you just replayed it in your head and it kind of sticks, the best book on the ocean I’ve read, and i read a lot of books, often have four on the go, have never read a fiction book in my life, why, because it’s s fictitious, enjoy.