hole vs waterfall? - Mountain Buzz
 



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Old 06-22-2007   #1
 
Redpaddle's Avatar
 
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hole vs waterfall?

Here's the question: when does a hole become a waterfall? When does a waterfall become a hole? Obviously something like Tunnel on Gore can be more of a huge frickin' hole than a falls (depending on the level) although there is a considerable drop. Conversely,there are falls (like obj) that are definitely not holes. What are the characteristics that differentiate a hole and a waterfall? And, perhaps most interestingly, how does this distinction affect the rating of a run?

Are there different paddling techniques for both?

Are there different boats that perform better for both/either?

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Old 06-22-2007   #2
 
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I think that they are two totally seperate things, but one can create another. For instance you have water falling a vertical distance over a rock, at the bottom there is bound to be some sort or recirculation or hole. The severity of the hole will depend on many things: the uniformity of the ledge from which the water is falling, the amount of water falling over the ledge, the depth of the bottom of the river below the falls, the uniformity of the bottom or lack thereof at the bottom of the falls, and how constricted walls or rocks in the pool at the bottom make the resulting hole. All of these factors and more can make a hole bad or not, it depends on how they all come together. I think that it can be said that there will be a recirculation at the bottom of almost every "falls" out there, it just depends on many factors as to what the hole might or might not look like.
I think the boats that are good at running falls are also the ones that run holes well, you want speed, stability and somehwhat rounded edges when landing off of huge drops, with that said, if you get stuck in a big hole, it is certainly easier to get out in a boat with edges (playboat) than in a boat that is shaped like a torpedo (creek boat).
I hope this helps and doesn't add to the confusion. I think a great resource on water hydrology is William Nealy's Kayak- it has great descriptions of what happens above and below the surface with great illustrations to boot.
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Old 06-22-2007   #3
 
Denver, Colorado
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Just to add to this:

A hole is created by water "falling" so to speak. You have water traveling along with a uniform streambed and then there is an abrupt drop in the stream bed...the water "falls" over this drop creating a hole. Obviously the drop needs to be big enough to effect the volume of water, and those two are in direct proportion to each other.

So, in a manner of speaking all holes are also falls, albeit often times incredibly small.

Waves, conversely, are created by an obstruction pushing the current up!

Thus we have that all falls create holes, and all holes are created by falls! They are NOT mutually exclusive, and in fact could be said that they are necessary of each other!
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Old 06-22-2007   #4
 
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check or egg

I think the real question is what came first the hole or the waterfall?
kind of a chicken or egg thing really
-p-
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Old 06-22-2007   #5
 
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The water came first, then the flow, then the almighty boater...
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Old 06-22-2007   #6
 
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Evan, do you even have moving water where you are? Shouldn't you be canoing up in the boundary waters? When you get back out here then we will talk about some waterfalls and holes.

P.S. you should hurry up cause every one out here has to do this silly thing called work and it is cramping my boating time.
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Old 06-22-2007   #7
 
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Kevin, the fact that both you and I are spending this much time talking about boating is pathetic. I, at least, have an excuse. You on the otherhand should have installed a kayak escalator on the narrows by now to run the gnar gnar over and over.

And I doubt anyone would ever be able to cramp your style *cough* hot pants *cough*
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Old 06-22-2007   #8
 
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Like I said I am down whenever, it is finding boating partners to boat with. I was supposed to go hit up Black Rock and lower CC today and three people bailed on me.
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Old 06-22-2007   #9
 
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Some Hydro Stuff

Maybe this is boring, but quite a few hydrology geeks are also boaters. Here are a few drawings by Janice Fong from California Rivers and Streams by Jeffrey F. Mount (U. Cal. Press, 1995).

Basically, gravity drives the water downhill while the bed, banks, boulders, etc. resist the flow. This leads to differences in speed, both vertically and across the channel. Where a boulder stops the flow, water has to rush in from downstream to fill the "hole" in the current. (We all know what eddies are.) See below for visuals (click to zoom).

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When the water flows over the top of the boulder, it forms a hydraulic drop as well– the water zooming over the boulder crashes into the water eddying upstream, and forms a boil, then a breaking wave. But at high current speeds, holes can be really turbulent without a big drop above: there might only be a few inches sliding over the boulder.

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All flow in a rocky channel is turbulent, but plain fast water is sub-critical (that's Fr <1, a calculation of energy). The water accelerating down a drop or falls is super-critical (Fr >1) which means it's gained more energy than it can dissipate. So when it meets anything that resists the flow, it explodes into spiraling vortices, shedding energy like mad.

Falls are hydraulic drops that are particularly steep, so the transition back to sub-critical flow is particularly violent.

So– almost every fall has a hole at the bottom, but a hole can form where there's not enough drop to really call it a fall.

Forgive me for going factual on y'all. But I live and breathe this stuff.

yrs, Chip
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Old 06-23-2007   #10
 
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Chip: thank you so much! I really liked that explanation in particular (not to badmouth the others!) I am an aquaculturist and we talk some about water flows within pipes, pumps etc. one of the ideas that has really entertained me is laminar vs turbulent flow.

here is my impression of what I have learned and extrapolated and feel free to correct me/ fill in the gaps:

the combination of inertia and the viscosity of water leads to a preference for water to keep a standard "flow path" that is to say, the majority of the flow or water pressure is oriented in one direction (unlike gasses).

but given the chance, water, as most things in the world, will readily revert to turbulent or "chaotic" flow if that initial viscosity is ruptured. then, the vectors are scattered in all directions. In pipes, I have been taught that this arises from side wall interactions at velocities above 5 feet per second (roughly).


so, in a river situation, my curiosity is what role does this viscosity play in rapids. maybe more simply, if the water can stick together, does it somehow share it's energetic momentum and change the nature of a drop? perhaps this has more to do with vectors than viscosity, but obviously the second depends upon the first.

This all came to mind from seeing waterfalls in Australia that were so big that at the bottom the water acted more like rain than anything else. So my thought was that waterfalls whose water is so dissipated by the time it gets to the base creates an entirely different hydraulic (if any) than one whose water is all following a common vector.

does this make my question more clear?

thanks for the help guys.

-E
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