Onderwerp: De zevende Golf

@Werner: Waarom zeg je dan niet gewoon dat alle golven anders zijn? Een getal noemen lijkt me allen maar verwarrend en als mensen het geloven zelfs gevaarlijk (na de 7e kunnen we overstag). Tellen heeft ook geen zin en een regelmaat ontdekken ook niet want die bestaat niet. Wat ik helemaal met je eens ben is dat je deze wetenschap kunt gebruiken om de veiligheid aan boord te vergroten.

Wave Trains, Groups, Sets and Consistency
In deep water, as waves move away from the fetch that generated them, they form a continuous chain of swells known as a wave train. Wave trains radiate outward in all direction from the fetch, with the largest waves moving in the same direction as the winds within in the fetch. Over distance and time, waves that are moving at nearly the same speed keep pace with one another and form a group. There can be anywhere from 3-15 or more waves in a group. An interesting thing occurs as the group travels. A group normally consists of smaller waves in the lead, larger waves in the middle, and smaller waves again at the rear of the pack. Waves in the back of the pack move forward and build in size, peaking somewhere in the middle of the pack. When they reach the front, they start getting smaller again, then disappear, only to reappear again at the back of the pack. It becomes impossible to track an individual wave within a group in deep water due to this phenomenon. What is actually happening is rooted in the physics of deep water wave propagation (and is not just limited to ocean waves, but also other naturally occurring waves both in and out of the water). Individual waves in the pack move at twice the speed of the group, but are bound to the group by the energy they share. So though the individual waves move faster than the group, when they reach the front of the pack they get pulled back. It's two steps forward and one step back. Ultimately they only propagate at the group speed (which is half the individual wave speed) in deep water (more on this in a minute).

Another explanation for the disappearing wave theory goes as follows: Groups of similarly paced waves eventually encounter other waves moving at close to the same speed and in the same direction. Minor differences in the speed of individual waves can give an observer the impression that waves in a group disappear. In fact, it is very difficult to track an individual wave in deep water because there are normally multiple wave trains interacting with one another. As one wave train moves 'into-phase' with another (that is, the two wave trains come close to matching each others speed for a short period of time), the height of those waves appears to increase. In essence, the waves move on top or through one another as one wave train overtakes another, giving them false height as they momentarily match speed. As they move out of phase (separate because of difference in speed) the wave height decreases back to normal.

Regardless of the explanation, as groups of waves move further away from their source in deep water (a thousand or more nmiles) the group becomes better defined, with waves of the exact same speed traveling together. If the waves in a wave train have sufficient energy (normally a period equal to or greater than 15 seconds), they can continue with little loss of size or energy, for thousands of miles or until they reach land and break. That's because at these periods (or wavelength) all the energy is traveling deep under the oceans surface, and there's little that can stop it (except for shallow water). A wave with a 14 second period reaches down into the ocean about 516 feet. A 17 second period wave at 761 ft, 20 second at 1053 ft and 25 secs to a whopping 1646 ft! So you can see how little impact an opposing wind wave with a 7 sec period reaching down only 129 ft would have on one of these long period deep water swells. Also, wave groups of different heights and periods can pass right through each other with no effect to each. In fact, other than shallow water or land, the only factor that causes significant decay is the surface tension of the ocean itself. The same tension that allows an insect to carefully walk on water has the long term cumulative ability to slowly sap the height out of a long period swell, but only over thousands of miles. Conversly, a short period swell is much more vulnerable to the affects of surface tension and opposing wind and waves becuase of it relative lack of energy.

Eventually the group starts to encounter shallower water. When this happens the group velocity and the individual wave velocity become the same. As mentioned before, a group can have anywhere from 3-8 or upwards of 15 waves in it. In shallow water, this group is known as a 'set'. If watching the surf, you will occasionally notice a group of waves break that are bigger than the normal background surf. These 'sets' appear as often as every few minutes to once every half hour or more, depending on how far they have traveled to reach your shore. The further the travel time, the better the organization. In shallow water, the in-phase/out of phase phenomenon is no longer applicable. From a surfers perspective, this is good. It would indeed be difficult to catch waves if they disappeared from under you.

Now it's time for a little math. The speed (in nautical miles per hour) of an individual deep water wave is about 3 times it's period (in seconds). That is, an individual wave with a 13 second period travels at 39 kts/hr. Contrary to what you might intuitively think, there is a linear relationship between wave period and wave speed. But because most deep water waves move in groups, the group speed is half that of an individual wave (within the group), or in this example about 19.5 kts/hr. As the wave moves into shallow water, the group speed and the individual wave speed become the same, so the individual wave starts traveling at the group speed, or 19.5 kts per hour. This wave speed formula is approximate, and actually wave speeds are a fraction different, but this is close enough for all but the most detailed surf forecaster.

With this knowledge we can begin to understand the nature of sets, and what governs their frequency at your beach. As mentioned earlier, the further away the swell source (fetch), the better the opportunity for the deepwater group to organize. Since groups are really a collection of individual waves moving at the same period, we can begin to understand why there is so much time between sets that travel a long distance. Consider a summer swell that reaches California and was generated 6000 nmiles away near New Zealand. Assume that two groups were generated at nearly the same time, one with a period of 17 secs, and another at 16.95 secs. That's right, only .05 secs difference. The 17 sec period group would take 226.24 hours to reach the coast, while the 16.95 sec group would take 226.91 hours, or a difference of .67 of an hour (40 minutes). Considering that 20-30 minutes is the standard 'wait-time' for southern hemi swells in California, you can see how this example is not too far off the mark. Each set is just a small fraction of a second shorter in period than it's predecessor. Now if the storm was a bit closer to the coast, say 4500 nmiles, then the travel time would be 169.68 hrs and 170.18 hrs respectively for the 17 and 16.95 sec period groups, or a difference of .5 of and hour or 30 minutes between sets. As the fetch moves closer to the coast, the wait time between sets progressively decreases. This holds true up to a point were the fetch gets too close to the shore, somewhere in the 1000 nmile mark.

Now not all beaches have access to a thousand or more nmiles of exposed ocean, and even on coasts that do have such access, storms may form close to shore. In these conditions, sets (of sorts) still occur, but on a much less organized scale. Because the individual waves haven't have time and distance to organize into well defined groups of waves moving at the same speed, the sets have fewer waves in them, typically from one to three. And the sets don't arrive a the coast arranged with the most energetic wave first, but rather a mix of short and longer period waves jumbled together. If fact, these sets really aren't sets at all in the classical sense, but are really just a wave or two that are larger than the average. This could be due to a combination of any number of factors coming together at the same time and place to make a grouping of larger than average waves. It could be that a just a few waves of nearly equal speed arrive at the coast at the same time but haven't traveled sufficient distance or had time to be joined by many more waves (like classical set theory), or that waves of different periods are overtaking one another just at the point where they are interacting with land (modified rogue wave theory), or that a real swell from far off is arriving but buried under locally generated wind waves creating the impression of a local set. Or two groundswells or windswells from different directions are interacting to produce peaks. All these conditions can and do occur, which often makes it difficult to know exactly what swell you are riding and where it came from.

Previously 'fetch' was defined as the amount/distance of ocean surface area affected by winds blowing in the same direction. One might presume that a storm sits relatively stationary over fixed area of the ocean, blowing up chop and waves equally in all directions. But, in reality, most large storms (other than hurricanes) typically follow the jet-stream, taking a course from west to east at anywhere from 20-30+ kts. In some instances, a storm's forward speed comes close to matching the speed of the waves it's generating. This allows progressively more chop (and energy) to start piling up on top of wind waves generated earlier in the storms lifecycle (providing the waves are traveling in the same direction the storms is heading). The wind waves aren’t given the chance to escape, but rather, build up to tremendous heights averaging 50 or more feet. Though a storm might have only 700 nmiles of proper fetch, if the storm travels 2,000 miles over several days heading in a constant direction at the right speed, the effective fetch area is 2000+ nmiles. That's the equivalent of 2000+ nmiles of core fetch over a multi-day period aimed at someone's beach! There are tables that predict wave heights and period based on wind speed and fetch, but by any standard, such a situation is sure to create huge swells. When this situation occurs, it's called 'traveling fetch'. Traveling fetch is as much bound by meteorology as it is by simple geography. There are only a few west facing locations in the world which are bordered by several thousand miles of open ocean and positioned near the tracks taken by winter storms. Such locations include the Northwest United States, Western Australia, Chile/Peru and to a lesser extent Southern Europe. Hurricanes can create traveling fetch too, but since their fetch area is typically much smaller, the effect is less pronounced. Because some level of traveling fetch occurs in most every storm, this explains why the largest waves come from the front of the storm, and smaller or no waves come from the sides and back of a storm.

But within the domain of traveling fetches, there is a category we call 'virtual fetch'. It is a special type of traveling fetch that is identified only by running the swell arrival calculations. When the fetch travels directly at a beach at just the right speed such that the swell it generates over multiple 12 hour intervals arrives a nearly the same time for a given frequency (period), then we call the fetch a 'virtual fetch'. That is if the 17 second period swell generated from a storm on say Saturday a 5 AM, 5 PM and Sunday 5 AM all is expected to arrive a location X on Tuesday at noon, then we say the storm produced virtual fetch.

But what does virtual fetch have to do with sets? Well not every swell generated from over a thousand miles away produces sets with 10 or more waves. It is really a factor of the internal organization of the storm that generates the swells and it's propensity to generate virtual fetch. The better the internal organization and more virtual fetch, the greater the potential to generate sets that contain a large number of waves. For swells generated over the North Pacific that impact the US west coast, it is not uncommon for a set to contain upwards of 15 waves. Such swells can be traced directly to the storms capacity for generating virtual fetch.

Lees dit stuk eens:
www.stormsurf.com/page2/tutorials/wavebasics.shtml

waverider en reinier, ik lees behoorlijk engels ,maar wellis nietis spelletje en ik heb de grootste is geloof ik voor 99 % van de forum leden niet ( meer) interessant dan wel te volgen. heeft ook niets meer met zeilen op zich te maken, de zoveelste golf is belangrijk voor de zeiler op het water, vooral als het een beetje heftig is.
dus als je bijna in je broek sch..t ga je geen golfjes meer tellen, laat staan waar ze weg komen.
wij ouderwetse zeilers zeggen dan gewoon zoiets van jongens dis een dikke, hou je vast en dan gaan we dat en dat doen.

zeil-dude, je snapt het dus niet, een golf VERPLAATST zich dus niet, maar gaat als het ware op en neer.

sietse-ten schreef :
Da's nieuw voor me. Een golf die zich niet verplaatst.
Dit al gelezen: nl.wikipedia.org/wiki/Oppervla...vloeistofdynamica%29 ?

@zeil_dude, in het engelse stuk dat ik gepost heb waardoor veel mensen zijn afgehaakt komt een flink gedeelte van de theorie en ook jouw vraag aan de orde.

ok . proefje, je neemt een afwas teiltje, vul het met tien cm diep water. laat in het midden een steen(tje) vallen, er ontstaan golven maar in het midden geen gat. dus de rimpeling blijft op zijn plaats, alleen de oppervlakte gaat op en neer.