Tuesday, March 11, 2008

Ocean Science 101: How our surf is made (Part 4: Breaking Waves)



Breaking Waves

This is where all the hard work of tracking a storm, watching a swell form, and then tracking it halfway across the world pays off. Breaking waves are the end result of all of this storm and swell activity...they are the meat and potatoes of surfing. In fact most surfers, in particular experienced surfers, know a lot about breaking waves, and know it on an almost instinctual level.

The size, shape, and power of a wave determine what and how we ride. You see it everywhere, from longboarding slow soft waves at Tourmaline or San-O, or driving down fast hollow sections at 56th Street on your fish, or big arcing cutbacks on your thruster as you speed through long sections at C-street. The characteristics of the breaking wave will set the rules on what type of board, gear, or surfing style is going to work the best.

Shoaling (waves hitting the beach)

Shoaling is the actual process of a swell moving from deeper water, turning into a wave, and eventually breaking along a surf spot.

Part of this process is actually driven by some of those characteristics of swell period. You may recall back is part #3 where I said that the energy in a swell actually extends downward in sort of a swell “column”…well shoaling starts to occur as the swell energy starts to reach the sea floor.
As the water depth decreases the energy of the swell is forced upward, which make the swell on the surface start to increase in height. As the energy moves into shallower water the swell starts to take the shape of a wave. It is important to note that the energy that is being squished into the sea floor is starting to reduce in speed thanks to friction. Eventually the water depth reaches a critical point where the wave can no longer support the top portion of water, which is moving faster than the base of the wave, and everything destabilizes. This is the point were the wave breaks and the crest of the wave spills forward in front of the base releasing a good portion of the waves energy.

Wave Shape

Wave shape is determined by a few different things. Water depth, swell-period, swell-height, and the shape & orientation of the beach can all play very important roles in wave shape.

Waves generally break in a water depth that is about half of the wave height. So an 8-foot wave would potentially break in about 4-feet of water. The actual math for determining water depth for a breaking wave is actually a lot more complex but the 1.75:1 or 2:1 ratio is a good general rule of thumb.

How the breaking portion of the wave is shaped is almost exclusively determined by water depth. Almost all waves that can throw decent barrels have the same characteristic…they all have a sea-floor that goes from very deep to very shallow in a short distance. In opposite of that is true as well…most longboard waves have a very long-gradual sea-floor shape where the wave slowly loses power before it finally spills over.

Now beyond water depth, the energy in a swell (both period and wave height) makes a difference in wave shape as well. The more energy in a swell the quicker it starts feeling the sea-floor and the more energy it has to spare creating the wave as the swell starts to shoal. A large long-period swell can throw some solid dumping waves on even the most gradual of sea-floor slopes. The converse is true as well a small short-period swell can make it almost all the way to dry sand before it has to break…and in some cases it may not break at all…just surge on to the beach in a sort of last gasp sputter.

Lastly beach shape and orientation are also important factors and can basically make or break a wave. In the case of both of these it is almost the imperfections that can cause a well shaped wave to form. Maybe it is a deep hole that has formed in the sandbar, or a jetty that piles up sand on one side but not the other, or a chunk of reef that is missing, or even a simple headland that has slowly eroded into place over the millennium…all of these “imperfections” in the sea floor allow a wave to break unevenly. The uneven breaking means that there is still an open rideable section ahead of the breaking section, and in cases where the sea floor is nice and steep the wave can be moving fast enough to create a hollow section. (if you are lucky, or if you live in Indo, which is sort of like being lucky all the time).

Constructive and Destructive interference (aka combo swells)

Some wave shape is determined by the types of swell directions that a beach is exposed to. When you have a beach with an open swell window you can get what is called constructive and deconstructive interference. This occurs when you have 2 swells coming in from different directions that hit the beach at the same time.

The overlapping nature of the swells, and the peak-to-trough-to-peak pattern, means that in some areas the swells will double up energy creating a taller breaking wave, and in other areas the swells will cancel each other out and there will be no waves at all.

This is the process that sets up those crossed-up, peaky days. These swells are actually more fun on a wider beach with a fairly consistent water depth…basically beaches that would normally close out on single swells like most beach breaks. Other breaks that have a more varied water depth can still be fun but generally don’t work as well as they could on a single pure swell.

Wave Refraction

Refraction occurs when a wave starts to feel the ocean floor. What happens is that the energy in the wave or swell starts to feel the sea-floor and begins to slow down thanks to friction. As the wave slows down it starts to change direction, generally pulling the waves energy toward shallower water.

Refraction can occur in all kinds of different water depths…long-period swells can feel the bottom at about 1000-feet, so you can start to see influences of refraction quite a ways offshore. Short-period swells don’t feel the bottom until much shallower water depths and can experience refraction in as little as 15-20’ of water.

Probably the most dramatic, and for surfers one of the most fun, examples of refraction is what occurs at point breaks.

At a point break you generally have a small headland that extends out into the deeper water along a coastline. Now most points are not sheer cliffs, they have a sea-floor that slopes away from the headland…usually this shallower area drops off pretty fast, particularly as you move along to the inside sections of the point…if you are very lucky there will be a deep bay protected by the point.

What happens is that the swell will hit along the top of the point and start to break as it hits the shallow water there. As the wave moves past the top of the point the energy that is still feeling the shallow water will continue to pull back towards the beach effectively extending the breaking portion of the wave…if the point is shaped properly, and the swell period is just right, it will wrap a ton of energy around the point and set up a very long ride. Next time you see Rincon on a big swell watch how the wave wraps in.

Wave Reflection

The last process that affects a breaking wave is called Reflection. This occurs when the swell energy hits a solid object, like a jetty or a cliff, and bounces off. Usually the wave will lose a lot of energy in the “bounce” but if the rest of the beach is shaped in the right way you can actually get the bounce energy to reflect back and add energy into a portion of the swell that did not hit the object.

Probably the best example of this is the Wedge in Newport Beach…hell the name alone should explain the process. Basically you get a big S swell that marches down the end of the North Newport Harbor Jetty. The swell instead of spreading out as it moves into the beach starts to pile up water and energy along the jetty. Eventually the swell moves out of the very deep water into extremely shallow water (or sometimes dry sand) and the energy piled up along the jetty bounces across the breaking wave, creating a large wedge shaped peak that throws some of the most retarded looking waves in SoCal. Other spots that are good, but lesser publicized, examples of reflection, are Surfside in HB and South Mission Jetty down in San Diego.


What this all means

I really believe that the more that you can learn about the ocean the better of a surfer and waterman that you become.

I don’t think you need to be an oceanographer, a meteorologist, or a surf-forecaster to have fun and score a few good waves (though it helps to be a surf forecaster)… but I think it is important to have at least a minimal understanding of the processes that go into creating surf.

Once you start to get some of the basic concepts down you will start to see how everything is connected, and eventually (at least in my experience) you will start to make better choices when it comes to surfing. You will find yourself getting more, quality surf, with smaller crowds, and better conditions…and really as a surfer can you ask for anything more.

Part 1: Overview – Types of Waves
Part 2: Wave Creation and Swell Generation
Part 3: Characteristics of swells

9 comments:

deckmanx said...

Adam, this is why I religiously read your blog. The info in here is great. You're straight up on your forecasts and get pretty detailed on all factors of the waves. I dig it.

Anonymous said...

adam, yeah this pretty much rules. thanks!

Yong Jung Shin said...

read the whole thing... Didn't know it would be so interesting ...

Anonymous said...

Thanks for the information. I always have trouble understanding wave mechanics because I feel that I am mentally retarded, but these articles helped me out bro, thanks.

Allomath said...

This needs a second read. Then a third. I am new to surfing, ergo clueless, and have to ask: Very high tides seem to make waves that roar in regardless of wind. Can't those be ridden? (You said tidal waves can't be surfed.)

Anonymous said...

Love the Blog and thanks for the lesson but could you possibly refrain from using the word " retarded " when describing unusual looking waves

Anonymous said...

wow, i read the whole thing, and understood!... the humor helps too hehe. i'm so glad i found your blog by chance!!

Anonymous said...

what is the difference between 'groundswell' and 'windswell' if all rideable waves are from a windswell?

Mike said...

Thanks - best teaching aid for a grom I've seen.