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Discussion Starter #1
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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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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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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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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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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Discussion Starter #7
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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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).

holes_2.jpg

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.

jump_2.jpg

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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Discussion Starter #10
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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Another hydro geek?

Turbulence, gnarl, whatever: I like it.

Anyhow, what you say is pretty much true. But for the most part viscosity is a constant for a given reach of stream. For instance, if you get a big slug of suspended clay from a flashflood on the Paria, the viscosity of the Colorado River increases (which as I understand will decrease turbulence and calm the raging waters).

What creates a hole is more the relationship between velocity and shear stress (i.e. resistance to flow). The boulder provides a big shear boundary, creating a low-pressure zone below. The water flowing upstream to fill that "hole" resists the downstream current on both sides. So you get two eddy lines (turbulent vortices on the shear boundary). As the current velocity increases, so does the shear stress, and likewise the extent of the turbulence, which tends to expand into the zone of lower pressure and velocity, until the entire hole becomes turbulent.

Given enough difference (shear) between the downstream current velocity and the upstream pull of the hole, this transition to chaos can occur even before water flows over the top of the boulder. It can also occur along blocky cliffs, forming eddies that can spin and flip a boat.

A fall is just that: a vertical drop, at a ledge, knickpoint, boulder fence, or faultline. Below most falls the channel slope decreases, so the fast-falling water rushes underneath slower water, creating a horizontal shear boundary (the killer wave) in addition to the lateral zones.

Placing a boat across a shear boundary can spin or flip it, since the water's pushing one part upstream (or up) and the other downstream (or down). We all know how this feels. To cross shear zones requires either pure momentum (blasting through) or adjusting one's balance (braces, etc.)

There are falls without holes, such as the stairsteps in hard bedrock where each drop comes down in an inclined slot, and pure gravity rules: there's so much fast water crashing down in such limited space that there aren't any low-pressure holes. (Not exactly navigable water.)

The physical factors are complicated, so much so that modelling turbulent flow takes a mega-computer and lots of time. But it helps to have a realistic sense of what's going on. Most experienced boaters develop that sense, even if they can't describe it in technical terms.

They just know– I love that.

yrs, Chip
 

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Any waterfall given enouogh water will become a hole. Even more and then it turns into a wave. I wonder what level Niagra Falls starts to green out at?
 

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The Impossible Dream

No bloody idea— but if it happens, let's run it.

There are some things worth dying for.

:twisted: :twisted: :twisted: :twisted: :twisted:

yrs, Chip
 

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More about falls

What you said about falls devolving into spatters and spray got me interested in looking at pix of falls. Where the flow is still coherent (i.e. green) when it hits bottom, it's got lots more energy than water that's wholly chaotic and turbulent (white). Part of that is that it's simply denser, and moving in the same direction. Where the fully turbulent water in rocky falls with lots of deflection is bouncing in many directions, with lots of air mixed in. That is it has dissipated some of the gravitational energy and acceleration. So the pools at the base of the really turbulent falls have lots of bubbles but look calmer than those below 'green' falls.

I'm not a kayaker and don't run sheer drops. So I wonder how these different sorts of falls feel, as far as difficulty and technique?

yrs, Chip
 

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Discussion Starter #15
Yeah, I have never run drops like this. My hunch would be that the hydraulic might be easier to manage at the bottom but the landing could be a lot harder too. Can anyone (with much better boating skillz than me) think of an example they have run?
 

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When you get those really turbulent falls that are all bubbly at the bottom the effect is like landing in a bunch of bubble wrap. It's so aerated that it's very cushy and I can think of a couple of drops I've ran where having that aerated water saved my butt, literally! When it's green and makes a big, meaty hole at the bottom it feels like jumping off the high dive, but there's also the expected battle out of the hydraulic to encounter.

It is very odd being in the bottom of a falls in an areated pool that is surging and bubbling and feeling like you're floating on air and have little to no purchase on your paddles blades. There also feels like there's nothing to catch an edge on...it's very surreal, and very awesome!
 

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Life in the hole

Thanks for the firsthand account.

During a semi-voluntary attempt on a mega-hole in Grand Canyon on a Jack's Pack Cat, the violent hydraulic ripped me off the boat (also the spare paddle I'd rigged with velcro straps and was using as a handrail). The Pack Cat shot straight up out of the thing like a surfboard in a big break— so Jack said.

I couldn't see it, being way down in the hole, which was snatching at my shorts and PFD and and hair and eyeballs, with a 'yak paddle gripped in either hand. The main sensation wasn't pressure, but rather swirling and pulling. Pretty weird, for sure.

I think the paddles helicoptered me out of the turbulent froth, and I shot to the surface a wave or two down, sucking air and repeating the mantra: breathe in the trough. Never did let go of the paddles, which made it hard to swim. But who wants to chase a f-ing paddle?

Anyhow, on other occasions I swam under steep waterfalls in clear water just to check things out. (Nothing huge— I'm not that brave.) And the water's force diverges and makes a radiating series of loops from the bottom up, in a flower shape or rosette, that shifts with the pulse of the falls. Pretty different to what happens at a lower angle with the current jetting into a hole.

Or so I imagine.

yrs, Chip
 
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