Wave Dynamics -- Surfer's Almanac: Swell and Wave Formation

Swell – Swells is long, oceanic surface waves, consistently formed by tropical storms and wind systems. A few fractions of a second difference in a wave, on the other side of the ocean, creates a totally different set of waves. Swell waves are often mixed with local chop “noise” waves.

Swell Size -- Swell waves are measured in “swell size” which is 33% of the largest wave in a set (trough to crest).

Fetch – The fetch is just the “un-wave” part of water. The larger the flat water (fetch) and the stronger the wind, the larger the wave will be. Shores that have inslands in front of them have a very short fetch (blocked), so smaller waves. The swell is generated by wind blowing consistently over a fetch.

Local Wind – too intense of a local wind will make the swell waves too choppy and unsurfable. Ideal is a slight offshore wind, that blows into the face of the wave, scooping them out even more!

Wave Formation – Imagine a pond and throw in a rock; ripples form with measurable size, distance, and speed. Now substitude ocean for pond and huge tropical storm for rock; that’s how waves are made.

Wind to Wave: Waves on a pond create little ripple indentations. Additionally, on the open ocean, small (or large) hurricanes create small wave indentations that form with other waves to form chop, then the chop forms with more chop to form, again, more wind waves, produce a full wave!

We want Swell Waves – Wind waves dissipate the farther away from their souce. Yes, they do! They will eventually peter out to water surface tension….unless they generated enough energy from the center of the storm or wind zone to create a swell. The swell’s energy can then carry that wind wave the entire circumference of the globe without energy or wave-size loss! Good deal!

Three Swell Factors:Okay, so Swell waves form with the combination of

1. large fetch (pointing towards your beach

2. Wind velocity (strong)

3. Duration (long time of wind blowing on the fetch

Ground Swell -- Considering those factors, basically storms create a lot of chop and swell. Eventually the swell “clears the chop” and becomes smoothed chop-free waves. NZ storms great swell that can travel 6000 miles, such magnificent swell is dubbed, “Ground Swell”.

Waves have longest periods first, then waves from the same swell days later will have shorter periods. For example, ground swell starts off Sydney’s coast and builds to hit SoCal. The swell has 24sec periods the first day with huge waves. Next day the waves are smaller, but closer together with a 20sec period; third day has 16sec periods, etc. The waves break closer and closer together! Remember a tropical storm is simply an awesome boulder chucked in a pond (the ocean) that creates gnarly sets!

Wave Sets – A wave set is simply a wave group (that deep water traveling wave train of 3-15 waves) that has hit shallow water.

Number of Waves in a set = 3-15 (for long distance fetches)

Number of waves in set from a local (1000- miiles) source = 1-3 (less organized because less time for the wave groups to form)

If there’s a large distant storm and a local one, you could get intermittent 1-3 wave sets every 5-10 minutes and then a huge 3-15 wave set every 40-minutes, for example. There’s LOTs of stuff always going on in the ocean.

A “shallow-water wave group” IS a wave set! They appear as often as every few minutes to every half hour depending how long the waves traveled to reach the shore. The longer the waves travel, the better the organization, and greater consistency. So, if a storm caused a deep water wave group to travel 5000 miles to hit your california shore, the wave sets would be incredibly organized and reliable because of the distance they covered (that “refining time”). Obviously in shallow water, individual waves of wave groups don’t phase out/phase in, so you can actually surf them and they won’t dissappear (sometimes they do) but a true wave set are waves that all break, if they don’t it’s just chop, likely.

Wave Groups – As waves span out from the fetch that created them, the large waves move in the direction of the wind (the wind helps them, instead of goes against those “with the wind” waves). Then after awhile, the waves that are moving in the same pace in the same direction, form groups of about (3-15+ waves). The further away from the original fetch and deep water source (like thousands of miles away), the more defined the wave group gets.

Set Frequency.

Okay, so a huge storm breaks out in New Zealand. It takes a wave group to travel 6000 miles from that souther hemisphere to California and approx imately 200-300 hours depending on the wave period. But the interesting part is exponential growth of time. For example, say an 18sec and 17.95sec period wave group starts at that souther hem. Site (6000 miles away). Therefore, the further away the deep water source (storm for example) the more temporally spread out the sets will be. If the source is from a fetch only 1000 miles away, the .05 second difference only pans out to being a few minutes, instead of 40 with the 6000 mile distance covered.

Individual wave size in the wave group.

What’s VERY cool is in the physics of deep water wave propogation, in a wave group (say of 10 waves), all the waves change places, move forward, etc, but the middle waves are always the largest and the end and front waves always the smallest! Any wave that moves to the middle of the wave group gets big!. Another theory behind this is multiple wave trains overlap and it appears like some waves grow but it’s just mingling with the other wave train, a less eloquent theory than the physics, assembly-line middle waves always biggest ideology.

Individual wave movement and Speed in a group. Individual waves (within the group) move twice as fast as the group! However, when those 2x-faster moving individual waves reach the front of the wave group, they get pulled back, so it’s 2 steps forward, 1 step back with the wave group and individual wave dynamic. Determing the speed of an individual wave is simple (3 x (period))

3 x Period = Indiiviual wave speed

So if a period is 20 seconds, the individual wave is moving 60 knots/hr!

However when the wave group becomes a wave set (and hits shallow water) the wave set moves at the speed of the group (half that of the 2x as fast individual waves) so that wave set would be 30 knots per hour.

Traveling Fetch – Sometimes the storm can be moving in the direction of the chop so the waves for, say, 2000 miles beome well-formed sets, but they’re constantly being overtaken and regenerated by waves from the storm immediately behind it, making big waves that accumulate onto each other. There are only a few places open to thousands of open ocean positioned near the tracks of winter storms: NW USA, Chile/Peru, W Australia, and S.Europe. A “traveling fetch” phenomenon to some degree occurs with every fetch, but the waves in front are always bigger than the waves after the storm because of the traveling fetch “wave booster” phenomenon. The “virtual fetch” is just sets with 12 hour intervals, a “pseudo fetch”, but the traveling fetch produces massive waves. Now when you look at the totally gnarly conditions for any Maverick's surf contest, a director of the contest (Spansler, apparently, uses government satellites -- a good use of those orbitting bits of technology -- to predict these wave condtions) says, “We search for a big storm way off in the North Pacific, out past the international date line" (NYT). “That’s the best cradle for a Mavericks swell. Waves start their lives as wind. The longer and stronger gales blow in the same direction, the larger and more powerful the seas that result” (NYT). The technology used to wire into surf conditions to make Mavericks great is so high-tech. Mavericks competition coordinators use those government satellites as well as a Quickstat and Jason-1 satellite matrix to get water undalations, and surface data up to 3.3 cm over the ocean! It's like a lot of high-tech advancements in science and technology are plugged-in and utlized for this surf contest! So pre-condition for any Maverick's surf contest is that the wave's origin must be a storm PAST the international date line. Can you imagine that fetch! To put this in perspective, Hawaii is BEFORE the international date line. So the CLOSEST source of a storm to meet the Maverick's surf contest preconditions would be New Zealand. Even if it's that "close", you're still looking at a killer 5000-6000 mile fetch (NZ is 6000 miles from the US, but it's slightly south of due west of the International date line along the same latitudes as the U.S.). Can you imagine that! This surf contest is ENGINEERED to produce EPIC surf! Sweet!

Measuring Waves: Height and Period – A typical report is 6 ft @ 17 seconds, which means the wave heigh (trough to crest) is 6 ft for 33% of the largest waves and they occur every 17 seconds.

Height – trough (front wave barrel bottom) to crest (top of unbroken wave. This is for the largest 1/3 (33%) of all waves.

Period – he amount of time it takes for two wave crests to past a specific point. There are two forms of measuring data presented as “Significant Seas” and “Swell data. For surfing, Swell data is the most reliable because, “significant seas” don’t even exist from a surfing (shore only perspective), it’s for boats, we want surf, so “Swell data” it is.

Significant Seas – The Square root of the sum of the squares of all wave energy. A 5 ft swell from S and a 3 ft swell is 6 ft sees, but this isn’t practical for surfing. And will always be an overstatement of wavesize. Surfers ride one wave and if two swells of 3 ft each approach that’s the wave height. Sig. Seas would overstate wave height as 4 or 5 ft. But at sea in a boat, Sig. Seas are, obviously, very important. The largest wave in any given day will likely be the Sig. Sea height. In other words, if Sig. Seas says height is 8 ft. and swell data says 5 ft. you won’t see a 9 ft. wave and if you see an 8 ft wave, catch it! That’s the wave of the day!

Swell Window -- This is just the range a beach has for receiving waves. If a land mass is immediately north and directly south, it would have a swell window of 90 degres to 180 degrees or only 90 degrees. CA has a swell window (where islands don’t block the surf) of about 180 degrees sometimes.

Great Circles – bSwells travel away from their source on Great Cicle pathes or routes. If wind is blowing parrallel to your Great Circle path (connecting to your beach), then you could have a swell because that means no chop would break up swells traveling along the fetch of your Great Circle path. In short, Wind bloing parallel to a Great Circle path parrallel to your beach is a GOOD thing for good swell size. Additionally, remember that great circle paths curv and are not always perpendicular to the beach. For example, believe it or not. Southern Australian is on the great circle path for south california even though their in opposite Northern and Southern Hemispheres! Theoretically a storm near India could follow the great circle path to southern california if wind kept blowing parallel to the path, but this is very unlikely (because it’s probably 8000 miles), but totally possible.

Directional Spectrum – In ocean reality, swell waves may come from a variety of different directions. There may be 5ft @20 secs from the south and 2 ft @10 seconds from the north ,for exampl.e A directional spectrum depicts the period, direction, and density of all waves present at a fixed space and set period of time. IT’s produced by buoys with height, period, and direction sensors installed. In short, if you want to know the heigh, direction, and period of all waves at a point from all direction (from all swells), the directional spectrum does that. Sweet!

Spectral density just shows the relative wave energy present at all frequency periods for a fixed space and fixed time interval.

Storm and Fetch – Most storms (other than hurricanes) follow the jet stream west to east at about 20-30 knots.

Period Length effects Wave Depth and Strength

A wave train that has significant period (like 15 seconds or more) has enough momentum that it will lose little or zero energy, size, or speed, while traveling thousands of miles until reaching land and breaking! The reason for this is a wave with a period (wavelength) of 15+ seconds means that all fo the energy is travelling DEEP under the ocean surface (the wave crest is literally the tip of the iceberg of the entire wave. The actual wave you see on the surface is about 5% of the massive deep water wave! Example: A 14 second period reaches down into the ocean 516 ft. and a 20-second period reaches down 1053ft! Opposing wind or other things (other than shallow water) would have a VERY little impact on a 1053ft underwater wave! Even an opposing wind wave wit ha 7-second period (reaching down 129 ft) wouldn’t even effect a 20-second period 1053ft wave! Actually even different period (or same period) lenghted wave trains don’t effect each other and neither can wind. The only thing that impacts wave trains other than shallow water, where they break, is surface tension (the stuff bugs use to float on water). But still it’s tonly the short-period wave trains that are susceptible to surface tension decaying the wave.