MadSci Network: Earth Sciences

Re: How and where do whirlpools form in nature and how large large do they get?

Date: Wed Aug 4 01:09:55 1999
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Earth Sciences
ID: 930767056.Es

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

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 

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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