Friday, November 12, 2010

Wave Theory and Principles Interpretted. Part II

div>In Part I, I just spoke a bit about a few terminology clarifications, how wind duration and fetch affect the quality of the waves and also a few tidbits on wave power and speed.

Here's a beauty hack at the lip by Pierre Morneau from Montreal, at Sandbanks last fall. Great shot by Nic Chapleau. Pierre rips.

Part II is all about wave transformation, or, how a wave changes when it encounters features in the lake, sea or ocean, things like the bottom, the shoreline and man-made structure. This is the magic that makes a place like Sandbanks work for us! But, I do stop short at wave breaking, which I will leave to a bonus Episode III - joy for all!!

So, to continue with the lecture; As waves begin to enter shallower water, they begin to ‘feel bottom’, and start to transform. Their motion and progress becomes compromised by the bathymetry (sea floor topography) and shape of the shoreline.

Deep-water waves (within small amplitude wave theory) are basically comprised of water particles making circular motions. The diameter of the rotation is largest at the surface and gets smaller as you go down from the surface, to the point there is no movement at a depth of roughly half the wavelength. The next diagram shows these orbits in both shallow and deep-water, but it is perhaps better understood looking at Fig9.5 in this Google Book, called Essentials of Oceanography by Tom Garrison (I didn't want to copy it in here due to copyright infringement, but its a good little diagram).

So, when I talk about waves "feeling" bottom, it means that the bottom is interfering with the orbit of the water particles and hence the orbits become non-circular, and more horizontally stretched as ellipses. Again, keep in mind this is all theoretical and not a whole lot in real-life will look much like this due to so many other factors.

Here's one of my faves of me from last year, by Ilan Artzy. I like the spray on this bottom turn. Not a huge wave or anything like that, but a fun one nonetheless.

Two things happen when waves begin to interact with the bottom: 1) Refraction & 2) Shoaling.

1) Refraction. Wave refraction is a change in direction of the wave due to a change in speed. Picture in your mind a wave coming side-onshore (like in the photo below, from the southeast). As the wave front/crest starts to enter shallow water, it will begin to slow down. Now imagine this happening along the whole wave crest. The wave crest will start bending since the part of the wave nearest shore will have wrapped/bent more than the part of the wave further offshore (because it has slowed down the most).

Ultimately, waves tend towards shore such that the wave crests will ultimately strike parallel to the shoreline (or in different terms, the wave ray will tend towards perpendicular to the shoreline).

The end result is that waves bend to become almost perfectly onshore by the time they hit the actual shoreline. See image. For those Sandbanks regulars – this is very important to us!!! Without refraction the wave action at Sandbanks would be pretty damn lame – it is the refraction that turns on-shore conditions into our pseudo-'side-shore' conditions.

And here is a diagram of a wave wrapping around an island. Suffice to say that sailing in lee of the island would suck for a couple of reasons: No wind & messy waves!

2) Shoaling. Another thing happens when waves start to touch bottom. As the bottom begins to interfere with the circular motion of water particles under wave action, the shape of the waves begins to change and this is called shoaling. The shallower the water gets, the more the waves shoal, or bunch-up and steepen (jack up) until ultimately they break. This is a pretty intuitive transformation that most anyone who has watched a wave will recognize and understand: As waves approach shore, they will appear to jack up and get bigger and ultimately break in one form or another. The bigger the wave, the further out it will break. It is pretty obvious to most people that the bigger waves at Sandbanks are further out from the beach, or further upwind near West Point, because they haven't shoaled as much, or for as long, and thus have more of their original energy left.

Here is a wiki on the topic of wave shoaling, with a really neat little graphic that shows the process and impact of shoaling on wave height and wavelength (By Régis Lachaume). See below (its a 2Mb .GIF, so I hope it uploads for you).
(ahh, sorry, looks like animated .gifs don't work in Blogger blogs...)

Another couple of things can happen to waves as they travel through water. They can interact with features they encounter. The refraction and shoaling mentioned above fit this category, with respect to natural shorelines and lake-bottom. Another couple examples of wave transformation are: 3) Reflection and 4) Diffraction.

3) Reflection. If you have ever had the pleasure of sailing in front of the ‘PUC dock’ in Kingston, you’ll know what this is all about. In fact, this is what makes Emily St (Richardson Beach) in Kingston much messier than locations further upwind. When waves strike an object that is unable to absorb its energy (virtually all things), some amount of the wave energy is reflected, or bounces back off that object. This can happen straight on or at an angle to the object. When a wave hits something like a concrete vertical breakwall head on, a considerable portion of the incident wave energy can be reflected back from whence the wave came. This creates a nasty mess of standing waves, or waves that appear to be stationary and pop out of no where. You don’t want to be sailing anywhere near there unless you are experienced in rodeo. All surfaces do reflect some energy, even a sand beach. The least wave-reflective shorelines in nature are likely extremely long, gently-sloped sand beaches and vegetated shorelines such as coastal wetlands. Anything man made or hard tends to reflect energy in a nasty way. So, a big two thumbs down for shitty breakwall design!!

4) Diffraction. Diffraction is the bending of waves around an obstacle in their path, or through an opening. I won’t talk much about this but it is a pretty neat phenomenon. One of our most beloved bloggers Giampaolo (Maui Surf Report) mentions the odd surf session scored in Kahului Harbour. Since I have never actually seen the spot, I looked it up on Google Maps (in fact, I used a NOAA free website called 'Geogarage Marine', which can overlay available marine bathymetry charts under Google Maps aerials - really neat!) . Generally, as waves enter a harbor gap like that, they start to spread out radially from the entrance, resulting in a down-size in wave and wave power as the energy disperses through the harbour. Think of it this way: You have a harbor gap that is about 200ft wide that allows energy to pass through and then dissipate throughout a harbor that is about 1000ft wide or more. I imagine when huge waves are pounding the North Shore of Maui and closing out everything, maybe that harbor still works for surfers since the waves go from deep to shallow fairly quick and the big stuff doesn’t make it through, plus you may have smaller waves getting in, wrapping nicely. Just a guess.

Alright, so the next few photos are just some gratuitous shots from Sandbanks, and don't have a whole lot to do with anything except for waves!!

Bottom turn by Pierre (photo by Nic):

... and heres another shot from last fall. Nothing spectacular about the action, but just looking past that to the bomb in the back that no one is riding!!


Nord_Roi said...

Good Stuff Fish!!!

For the double up size I was talking about, do you think it all about the spead of the wave? Do you think more the wave got a long interval the more spead it carry, more vertical it can get? We saw the same stuff in Hatteras this fall when Igor Hit. When the wave pass to 2.5m at 14 sec..that was way over to a pure windswell at 2.4 at 6 sec. Energy I guess. I know they mesure the wave by the back in Hawaii, but the mesurement was taken by windguru.

Nicolas said...

And there I was, thinking you were just another windsurfer (with a rocket science degree). Excellent stuff teach. Now, will this improve my technique? I would think so.