|MadSci Network: Earth Sciences|
Hmmm...I couldn't find a whole lot either after hunting for details on the Norwegian maelstrom. The biggest I have any visual evidence of (river rafting video) are some 20-30 feet in diameter and occur on large volume rivers. These form due to powerful currents moving by an obstruction and creating an eddy behind that obstruction. As is usual in eddies, the current in them moves back upstream and thus a zone of highly unstable water (often called an eddy fence) is formed between the main current going downstream and the eddy current going upstream. Such eddy fences contain both whirlpools and boils (rising water...essentially the opposite of a whirlpool in which the water goes down). One could, I suppose, consider most eddies as whirlpools of sorts, in which case there are some very large ones along the sides of major rivers. These may be 50 yards long or more, but generally are much slower currents and have little to none of the downward motion that one might usually associate with the word "whirlpool". The larger and more powerful the currents, the larger and more powerful the whirlpools possible are. To find the "biggest" might take some sleuthing around the major tidal bores (Bay of Fundy comes to mind). I have never read of any especially gigantic whirlpools associated with these tidal currents, however. One would probably have to locate local experts in Nova Scotia to get good information on such eddies and whirlpools as may exist there. I did once see a satellite photo of the Gulf Stream that had very large votices spinning to its sides. The Gulf Stream, of course, is one of the most massive if not the most massive current on the planet, but whether you wish to consider these vortices as whirlpools in the classic sense is debatable. They were several miles across. Unfortunately, I can not locate the source now; I believe it was a "Scientific American" or "American Scientist" article. If we go to gaseous fluids, then the Great Red Spot on Jupiter may well be the largest rotating body of fluid in the solar system that bears some resemblence to a whirlpool (although perhaps a cyclonic storm is a better analogy). This is, admittedly, streching the definition of whirlpool. Also, the mechanism driving the rotation is probably not opposed currents but more like that driving a hurricane. If you wish to *really* stretch the definition of whirlpool to be any rotating fluid, one might then be able to consider spiral galaxies as a form of extremely tenuous, but nonetheless rotating system of gas and dust. But since your message came in with the .Es (Earth Science) ending, I presume you want a more down-to-earth answer. So, back to rivers and tidal currents, and I'll pass along at least the little I know on the rest of your questions: a. How big to they get? See above... b. How small? I have seen whirlpools no more than an inch across after I dip my oar in the water (I also run rivers). They are little vortices that are shed from the blade tip and whirl off for a few seconds. Look at the sides of your canoe paddle and you will probably see some form as you pull the paddle through the water. c. How do they form? Almost any current that has a non-shoreline boundary can form one. In the case of a river current flowing by an obstruction, the current sets up a Bernoulli force and drags water from the eddy away with it. However, this water must be replaced by other water from somewhere, which turns out to be the downstream end of the eddy. Thus the eddy current flows upstream, and creates the highly unstable eddy fence where it "rubs" against the main current. Put something between your hands while they are held flat; now move one hand away from you, and one hand towards you. The object will tend to rotate. If your hands represent the main and eddy currents, and the object the unstable water of the eddy fence, you can readily see how the latter can be set into rotation. d. Where can you see them? Any river will have some; the bigger and faster the river current, the more likely. I believe the Fraser River in Canada has some "bodacious" whirlpools in it Tidal bores may as well, but I have no direct evidence of that. In Tennessee, you might try the Ocoee River; it has some reasonably fast whitewater. e. Forces involved? Hmm. Powerful whirlpools on big whitewater rivers they pontoon at least partially underwater or submerge a smaller raft. The force must be then at least a few tons to counter the buoyancy force of the submerged air. The potential force involved (this is a gross overestimate, most likely) is that of the moving current of the river, which can be huge. Even a little 1000 cubic feet/second river is running 31 tons of water by any point every second. Having even a small portion of that "land" on you is pretty potent. f. How far might a person go in one? This is a little hard to answer. One can certainly be pulled down several feet at least. I suspect the depth is highly dependent on the river's depth at the point in question. Once you are "down", river currents may move you laterally quite a distance (50 feet or more) even in a moderate rapid, before you resurface. I've directly seen people yanked down from a raft and bounced rather rudely along the river bottom for 50 feet in a class III rapid with perhaps 1500 cubic feet/second flowing. g. River Hydraulics formation? These are a vertical equivalent to an eddy. That is, water flows over an obstruction and plunges downward after doing so. The water going down drags water from its downstream side along with it. All this descending water goes to the bottom, and is turned downstream only by the resistance of the riverbed. But something must replace the water that was dragged down, and it comes from downstream of the obstruction. Since the only replacement current is down along the river bottom, some of theat rises back to the surface and then goes back upstream to get dragged down again. What this leads to is a zone (called a hole, or stopper, or keeper, or hydraulic) into which water flows from all directions on the surface, and only exits vertically downward. Hard to get out of, as doing so means opposing a current no matter how you try to leave along the surface (which is usually where river runners prefer to remain). h. How strong are river hydraulics? They can and do rip boats apart. I watched one video of a raft caught in a major hydraulic that lost all its gear and oars, and had the steel frame bent like a pretzel and then torn off. Most of this damage is due to the upstream current pushing the boat under the water falling over the obstruction. Water is heavy and it is quite easy to get hundreds if not thousands of pounds of force applied to the boat when the falling water hits it. Another video segment showed a river wide hole on the Yangtze river in China where it goes through a major gorge, while the river was in flood. This was an especially bad one because not only did the river drop over a ledge, but the water almost immediately crashed up against one shore (river made a sharp turn here) and fell back in onto itself. The backcrashing wave was easily 15-20 feet high. (Since this particular Mad Scientist turns out to be a swiftwater rescue instructor, let me strongly urge you to take a class that discusses these things in more detail. Every river runner should.). i. implied question about weirs...Manmade weirs are so symmetrical that there are no natural outlets from the hydraulic. That is, they tend to be river wide, with absolutely no breaks in the upstream current holding you in them. Almost all natural hydraulics do have ends that you can attempt to get out of, or breaks where at least some water flows out along the surface. Weirs are thus highly deceptive; they often look rather mild and innocent. But that current flowing back up is more than enough (usually a few mph, say 4 to 8) to hold swimmers (even Olympic swimmers can't mangage much more than a moderate to fast walking pace) and boats.
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