… cavitating

Bets, I heard Niagara Falls is head-cutting pretty fast. Really? Through solid rock?

— JJ Mahoney

Yes indeed, Mahoney dear. Historically, Niagara Falls “eroded backwards” almost 4 feet per year. When humans started diverting some of the flow to generate electricity, we reduced the erosion rate to 2 feet per year. This is still impressive considering it’s cutting RIGHT THROUGH ROCK.

Engineers “to the rescue”

Back in the 60’s some folks got worried about the falls’ natural migration. Maybe they had a HUGE HOTEL or HYDROELECTRIC PLANT that wouldn’t do quite so well if their waterfall were nine blocks up the street. The United States Army Corps of Engineers decided to take a peek:

Wowza! The Corps just up and diverted flow to the Canadian side of the river. Engineers dodged tourists to study the dry river bed and determined that cracks in the bed allow water to soak down behind the rock face, building hydrostatic pressure that could push the rock outward and hasten decay.

The Army Corps of Engineers then “repaired” Niagara Falls. Seriously. Workers installed cables, sealed cracks, and/or drilled holes to release pressure, depending on which account you read.

And did it work?

Not really. Even the Army Corps can’t eliminate ordinary erosive forces, and it turns out waterfalls have one EXTRA-ordinary erosive force – cavitation.

The Niagara River flows so quickly over the falls that bubbles form inside the water.

Bubbling water, hmm…

How is that different than boiling water?

It isn’t. Normally, water boils when it’s super hot. (I can identify — when something “makes my blood boil,” I usually feel pretty hot under the collar.)

But Niagara Falls makes cold water boil.

The technical part:

Liquid water turns into water vapor at very specific combinations of pressure and temperature: under lower pressure, water will vaporize at a lower temperature.

For example, at sea level, where the entire earth’s atmosphere weighs on a pot of water, it will boil at 212°F. In Denver, one mile LESS  of atmosphere pushes down on the pot. Water boils there at 202°F. [My Mom says this is why coffee tastes better in San Francisco — she’s pretty AND always thinking!]

When liquid is MOVING… then a whole other variable comes into play: speed.

Free-falling water’s energy has three components: kinetic (from speed), potential (from elevation), and pressure. As the water falls, it loses potential energy, i.e., height above sea level. In a channel, that energy would be transferred into friction work against the bed and banks which is why rivers generally don’t accelerate as they go (unless the total amount of water flowing into the channel changes). But in empty space, the potential energy is transformed into kinetic energy and the water speeds up. Energy can be neither created nor destroyed, so the water’s pressure must decrease to balance the increased kinetic energy.

Depending on temperature, water can drop to its boiling pressure when it reaches a velocity of 40 feet per second.

Upstream of the falls, during regular flows, Niagara River travels at 2-3 feet per second — pretty typical of most streams. During “bankfull” spring runoff, velocities are usually around 5-6 feet per second. Over Niagara Falls, water speeds reach 100 feet per second (68 mph!).

Ouch

The bubbles themselves – officially called “cavities” since if you think about it they ARE actually voids inside the water – don’t hurt anything. But they pop when the water slows, and the collapse emits a shock wave strong enough to etch rock.

Have you ever felt your blood run cold AND roil within your veins? Ack, why do we do this to ourselves? Of course, sometimes, there’s…

Cavitation as a Weapon

Two kinds of shrimps have developed special claws that snap so quickly and precisely as to direct low pressure bubbles onto passing fish. Killing them. I think I dated one of those guys once.

What to make of all this OR… “Potential Life Lessons”

Option 1 — If you go THAT fast, you WILL be powerful in a whole new way. Eventually you’ll experience a sort of internally-caused “suction” that will vaporize your energy until the next time you slow a bit and then, POP, a little shock wave will dissolve whatever it hits. Is this good or bad? It’s just the way the world is. Maybe you can dissolve untrue thoughts or institutions of evil. At times you will erode your own foundation and… horrors, MOVE a little! That’s okay too. No river or human lives a completely static life in one location. Thank goodness.

Option 2 — Definitely avoid external “suctions” that drain your energy! They strike fast, so if you see a smiling pistol shrimp headed your way, run.

Option 3 — Beware the Army Corps of Engineers? Nah — even entire fields of study can evolve, albeit slowly. Please remember that some of your best friends are engineers, and write again soon! XO,

Yohoney

2 thoughts on “… cavitating

  1. hatt

    internally-caused suction is now my favorite phrase. it sounds so much more refined than saying something sucks. I love this and you! my beloved dad took us all to check out the engineering at Niagara when I was about 5, he was so thrilled by it. I will never forget that experience! walking in tunnels under all that powerful water, blew me away.

    Reply
  2. kayce

    This is SO interesting. It causes even more to bubble up inside of me… Hmmmm. I wonder what shall explode (or implode) next? I’m all for movement, but that shrimp guy is a little creepy 😉 My other reading today was about the slow pressure of dripping water and the ability to cut through stone. Coincidence? xo

    Reply

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