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Old 06-23-2007   #11
Chip's Avatar
SE, Wyoming
Paddling Since: 1986
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Posts: 1,098
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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Old 06-23-2007   #12
Join Date: May 2006
Posts: 169
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?

if i'd known it was gonna be that kinda party...
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Old 06-23-2007   #13
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SE, Wyoming
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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.

yrs, Chip
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Old 06-27-2007   #14
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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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Old 06-27-2007   #15
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Newport, Oregon
Paddling Since: 1989
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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?
"Paddle silently, boof loudly"
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Old 06-27-2007   #16
Denver, Colorado
Paddling Since: 2004
Join Date: Apr 2005
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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's very surreal, and very awesome!
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Old 06-27-2007   #17
Chip's Avatar
SE, Wyoming
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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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