Friday, November 12, 2010

Wave Theory and Principles Interpretted. Part I

Following on my undergraduate education, I studied coastal engineering in more depth for my Masters of Science degree. This is part 1 of a 2 part blog post on some of the basics of waves.

A lot of good useful information for us windsurfers comes from “Small Amplitude Wave Theory”, which is one of the most simple and most widely applicable mathematic principals for quantifying the movement of waves. There are many other theories as well, each having their place. I studied this stuff for a few years, way back in my university days and have forgotten most of it, aside from the basic principles that apply to our beloved Lake Ontario. Despite immersing myself in Coastal Engineering, somehow I still ended up living on a lake, rather than say an ocean, where arguably I should be residing. But, I digress…

Here's a shot of Guy Trudeau at Mac's last year, taken by Nic Chapleau. Looks like a sweet wave to me!


Here is a link to the wiki on the topic. http://en.wikipedia.org/wiki/Water_waves

So, without getting into all the crazy math containing far too many greek symbols, which I have long since forgotten anyway, here are some of the more basic wave principles and observations I took away from all this education, as they relate to Lake Ontario and Sandbanks.

What is 'Swell'? First off, some terminology: Lake waves, by definition, are not “swell”. Lake waves are more accurately referred to as “locally-generated seas”, or in other words, waves produced by the action of the wind itself, locally. Some refer to this as “wind swell”, which kinda makes sense, but there is actually no true "swell" on the Great Lakes. Swell, by definition, is wave action created by distant wind events. By travelling long distances and moving away from the storm, the waves become much more organized with longer periods, wavelength and speed. In particular, storms send their higher energy, lower frequency waves further abroad, and this is true ‘swell’. Despite the Great Lakes being pretty dang big pools, they are generally too small for true ‘swell’, and generally too small for tides as well.

Consider this to be the first problem for us windsurfers. In the Great Lakes, it means that waves and wind will generally travel in the same direction = on-shore conditions. In a place like the north shore of Maui, waves are often direct onshore, while the wind is sideshore - a much better recipe for fun.

Fetch. The wave period, height and degree of wave organization is also a function of fetch (distance over which the waves can develop) and duration of the storm. The longer a storm blows, the more organized and longer the waves become (tending towards a "fully developed sea"). At Sandbanks, if a really good storm whips up, we usually get 5-7 second waves. Heaven forbid it should blow for a day or two straight, we may even get up to 8s waves, in which case let the ejaculation begin.This is the ‘peak’ wave period, or the period of the wave spectrum with the most energy. The peak wave period can be either wind duration- or fetch-limited. In the case of Sandbanks, the limitation rapidly becomes fetch – we’ve got 200km at best. Anyway – this has two impacts: First, the waves are generally a lot messier than ocean swell since there is a fairly broad ‘wave spectrum’, in that there are waves with all sorts of different frequencies (periods) and directions out there, interacting with both positive and negative interference. Second, the biggest and best waves are still relatively small and limited in wavelength, meaning the waves are spaced fairly close together.

Getting beat down. Speculation: I believe that a good indicator of what we feel when we get shmucked by a good wave and have our gear and gitch extracted from our clenched bodies, is wave ‘power’. Wave power is a function of wave period and height squared. So don’t think you can dominate the pesky 7 second waves at Sandbanks, then cruise on in to Ho’okipa and own the place. Typical true ocean swell there is probably more like 10-15s, and up to 15-25s on the big gnarly days. That’s why sailors like us from eastern Canada get pummeled and really feel it when we go to Hawaii. It truly is bigger and much bad-er!

Bigger waves + longer period = considerably more power, which in general means more “F$%^ you up” potential.

Here's a shot of me with a weak bottom turn at Mac's last year, by Nic Chapleau. Despite looking like a nice head-high wave, they are pretty gutless. Damn fun though!


Wave Speed. In general, the longer the wave period, and hence wavelength (these are related), the faster the waves travel. While waves do slow down as they ‘touch bottom’ and diffract (wrap) and shoal (‘jack up and break’), to be discussed in Part II, this trend still holds true. In Sandbanks, on the good days, one of the bigger shortcomings to the otherwise stellar front-side riding is the speed of the waves – they are simply too slow, making it fairly easy to outrun the waves on bottom turns, making it a lengthy and challenging approach back towards the wave to shmack the lip. This has a great potential to result in backwinding. While at Sandbanks, the psuedo-‘side-shore’ conditions (wave vs wind direction) are partly to blame, but so is the slow speed of the waves.

Here's a wee shot of JFLemay at Mac's, by Nic Chapleau:

Anyway, that is Part I of my blog-length dissertation on some basics of wave terminology and physics as it applies to real-world observations. Some of it straight out of the literature (interpreted by me), and some of it based on real-world observation in alignment with principles I have learned. Part II gets a little more 'interesting' and talks about wave wrapping and breaking...

Here's a shot by Ilan Artzy from the beach. This is all totally onshore...



Please let me know by posting a comment if this type of blog post is at all interesting or just too damn dry and boring. I tried to interject the science with the odd photo of some of the more decent waves from Sandbanks. Most photos courtesy of Nic Chapleau (unless otherwise noted), and the text-book images are all from the USACE Coastal Engineering Manual –Part 2. I had to buy an older version of that manual back in Uni for well over $100 so I don’t feel bad at all about snipping pieces out of the new on-line version. That was a lot of frickin' beer money I will have you know... Let’s hope the US military doesn’t come and send a drone over my house.

Hey there! If you like what you read, please support my interest in blogging by posting a comment or clicking links!

15 comments:

Brian S said...

Fish,
Interesting stuff. Regarding Great Lakes WS'ers getting pummeled in the Ocean, that's probably true, but fighting the short periods can still make you a man!

Fish said...

True enough - punching through the shorebreak at Outlet Beach is killer! It's a constant onslaught!

Nord_Roi said...

Good reading!
BTW, why a 1m at 12s is about a 1m face, when it pass to 2m at 12 sec.

Here an example of 2m at 17sec from last year:

http://i22.photobucket.com/albums/b337/Nord_roi/Halloween1.jpg

Nord_Roi said...

oups...erase some stuff by mistake..why when it pass to 2m at 12 sec the wave face double...and it seems almost exponential after that..

Fish said...

I'm a little fuzzy on your question, but I think I understand. I'll try to touch base on wave breaking in one of the next posts. I'll have to do a bit of research on that.

It probably has to do with a couple of things, which I can only take a stab at here in the comments.

First, wave height is a vertical dimension, measured trough to crest. A wave face is an 'angled' dimension which changes, just as height does, as a wave jacks up and then breaks. You have to think of a triangle held in the vertical plane - the diagonal being the wave face. With pythagorus theorum, the ratio of the diagonals will not be double for a jump from 1 to 2m, it may in fact be negligble. However, we are also talking about a curved surface, and that might account for some of the difference.

Secondly, the actual wave height increases as a wave shoals and 'jacks up'. That's why when a Hawaiian says its 4-5ft, you are sailing in logo high waves that seem more like 8-10ft. Whether they know it or not, intentional or not, I think they refer to the offshore 'deepwater' wave height (which is probably what the forecasts talk about). What we play in, in the break, is shoaled and at breaking, which can generally be 50-100% higher than the deepwater wave height. So a 1m wave might break at 2m, and a 2m wave might break at 4m (actually probably a bit less).

I'll try to put some more thought into it and do a discussion on breaking in the next post...

Thanks for the interest!

Nicolas said...

Educational AND funny all at once. Also nice to have a local aspect of your little science lesson, which BTW is not dry at all. Lookin' forward to part 2, and actually windsurfing in one of these waves!

Nic (mc2) Chap

Fish said...

Thanks Nic!

Oh - Steeve, I messed up. Waves at breaking are commonly up to 50% higher than the deepwater height, not up to 100%. It all depends on the conditions, and things like current, winds, etc can impact that ratio.

Jflemay said...

great article

jflemay

drzone said...

Great posting.
Are you going to talk about the diffraction effect, Mac's obviously but where else we should be looking for this to have side shore conditions in Ontario?

Fish said...

Yep - sure will John, but it is actually refraction that is making the magic. I'll have the other post up next week I think.

Think of diffraction as light shining through a prism, splitting visible light up into its spectrum. Refraction is bending due to shoreline effects...

PC said...

Waves are always an interesting topic.

I have one observation though. I always read that swell comes from conditions that occurred far away, as reprised in the wikipedia article.

However I always found that there is a strong correlation between the local swell and the current conditions at hand (strong wind), at least at a few miles offshore in the ocean. Surely the current wind must contribute locally somehow... Said differently, you'll never find a zero swell in a strong wind, unless it just picked up.

And yes there is almost no square jumping on lakes inland, however big. I gave up chasing that kind of sailing when I moved from oceanside to inland Canada...

Fun article!..

Fish said...

Hey PC - Thanks for reading!

We've found the spots and they are good. That is what the article is getting at. While the waves and wind will never be perfectly square (perpendicular) in a lake (in theory they can be, we just haven't found that), they are damn close, and the ramps at Sandbanks let you do whatever you want and are willing to do. You can even bear off on the waves and hit'em on a slight downwind, in case you are keen on planing or double forwards - I think Guy Trudeau was trying to pysch himself up to try the doubles. The only limitations there are what you are willing to try. I'll talk more about that in Part 2.

Secondly, your point about the correlation. That is what I refer to in my first bullet on Swell terminology. Locally generated Wind swell' and the conditions at that time correlate perfectly, since it is the location conditions that makes the waves happen. In other words, the wind and waves travel in the same direction. But, you lose that correlation when you talk about real swell, that arrives from distant storms, in which case the local winds that happen to be blowing might not correlate with the waves that are coming in. This is what makes Hawaii so awesome. Winter storms in the north Pacific push swell down to the North shore, and local winds are most commonly easterlies, ripping between Haleakala and the west Maui mountains.

Dan D said...

I must have missed part 1 before, but had a good read tonight. I once heard that a quick moving low pressure system can create "lake swell" as it sucks up a bit of the water and then drags it across the lake. Have you heard of this before?
It must be hard to prove because generally a quick moving low generally comes with wind, so any (if not all) waves are from the wind rather than the low.

Fish said...

Hey Dan - no, I have never heard of that. I think I can grasp the theory of what you are saying, but wonder if it ever happens. My initial thoughts though would be that the 'sucking up' influence of the low (in absence of any wind) would have a hard time overcoming the effects of gravity. Interesting...

Thanks for reading!

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