Wave heights

[AndrewB]

I\'m with Mainlysteam on this.

With respect, Aramas, your denial of a correlation between wind speed and wave height comes close to arguing that wave heights are completely unpredictable. There is a wealth of empirical research on the causes of wave height, good enough to build probabilistic models that enable a number of organisations to make useful forecasts of probable wave height for oceans and other large bodies of water, including all of those you mention. The models I've seen allow for a number of factors but always the pre-eminent one is the current wind strength. This is not to deny the significance of cross-swell from old or distant weather systems, specially if they are particularly powerful. However, this is an ocean phenomenon. In the shallow, relatively sheltered waters where most of us sail, like the English Channel, it is common observation that sea state will respond very quickly to the current local conditions.

The original question asking how, on average, wave height is affected by current may not have a simple answer - it is not the same in all circumstances - but it must have an answer.

 

[peterb]

The Met Office publish a book, "Meteorology for Mariners", which is one of the best guides to marine weather I've found anywhere. It includes a nomogram for estimating significant wave height. Factors taken into account include wind speed, fetch and duration.

One of the things pointed out is that wave height increases with both fetch and duration, but that there is little further increase beyond a fetch of 500 nm or a duration of 24 hours.

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[johndf]

Re: I\'m with Mainlysteam on this.

In deep water not affected by currents, there are well known predictions for wave height dependent on wind speed, fetch and duration. These are usually given in the form of graphs which make it possible to read of 'significant wave height' for a given combination of these variables. As soon as you add in shallower water, irregular bottom and tidal stream, everything gets much more complicated. The latter three are not that unusual. You'll find all three just 2 miles West of St Helier. With a bit of swell (caused by wind several hundred miles away or more), on the flood the sea will be more or less flat off Noirmont, but once the ebb sets in, particularly on a spring tide the waves can be pretty daunting. High and steep.

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[MainlySteam]

I think you are getting terribly confused between wave types - seas, swells, breakers, etc. For example, every sensible person knows breakers do not travel away from beaches, however, despite your assertion, waves do as soon as there is enough fetch for them to form.

You should also be careful about assuming what other peoples knowledge and experiences are when you make statements such as "Try doing a couple of years of oceanography..", you may appear a fool if it turns out that the person to whom it is directed has done more than that.

No one is saying all of the other factors are not relevant, but for the sailor on the water the present wind is what is the predominant dictator of the seas he encounters. Also, no one is saying that wave wind graphs are accurate from the moment the wind reaches a particular speed - we all know that these are dependant upon quite long periods of time.

You mention Cook Strait with which I am intimately familiar as those are my home waters. There are many influences on conditions there including current, upwellings (current and bottom topography induced), remote weather (swells), change in weather (historic driver of seas and swells experienced in the present), coastal topography. If, for the sake of example, we just take a tidal rip whose generation is dependant on current in a no swell situation, then without wind there will be no seas, when wind appears the seas will rise very quickly and those seas are generated by the wind, not by the current. The current is a factor only, the seas, of a different nature, will occur even if the current was to be able to be suddenly stopped in some kind of experiment.

Again you say that your view is especially true in bottlenecks such as Cook Strait, and others, where two ocean masses meet. You are are incorrect insofar as Cook Strait is concerned (and I suspect you are with your other examples, as among them there is no case "... where oceanic masses meet" - Cook Strait is the only case in your examples where such masses do meet). The conditions in Cook Strait are predominantly driven by the topography of the North and South Islands not by the ocean masses each side. There are local influences which also effect particular areas of the Straits, for example where there are upwellings, tidal restriction and wind acceleration off headlands. The prevailing wind is of Northerly aspect and the other wind is of Southerly aspect from the passages of depressions and anticyclones. These winds are accelerated through the Strait by the mountains of the South Island and the high hills of the North Island (in both cases they reach almost right to the Strait. When a depression or anticyclone passes in a latitude that would normally mean more easterly or westerly winds, these winds do not occur in the vicinity of the Strait, and it is quite normal to sea on weather maps quite severe bending of the isobars in alignment with the Strait when the pressure gradient is large.

Large southerly swells travel through the Strait when there have been extended periods of southerly weather (but never large northerly ones) but even when this is so, the seas experienced are predominantly those as a result of the current wind. It is well known to locals that upon a decrease in a northerly wind the seas moderate very quickly (within hours) and in the southerly case the same applies even though large swells may remain for several days (there is almost always a 1-2 m southerly swell through the Strait) but these are of little concern to a small vessel.

To continue this lengthy epistle, the southerly swells roll straight into the entrance to Wellington Harbour and to all intents and purposes makes it a bar entrance to large ships, but as smaller vessels are more affected by seas than swells they do not see it so. In the ultimate the swells crest and break through shoaling and constriction and there is a true bar entrance for everyone (and at that stage unsafe for everyone as the waves are steep and huge).

That entrance situation is local and driven mostly by factors other than the wind (coastal topography, shallowing, swells generated outside of the entrance) in the entrance and so is a case where present wind is not the predominant factor. But generally the wind experienced at the moment is what predominantly dictates sea conditions currently experienced and that will be experienced in the short term.

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 15/08/2003 00:01 (server time).</FONT></P>