K1/C1 Tech
Paddling’s Black Holes When playing in whitewater really sucks (you down)
by Braden Frandrich, photos by Ryan Creary W
Whirlpools have always held a strange fascination as black-hole vortices into which things disappear and don’t return. Rumors of thirty-metre fishing boats disappearing in seconds, counter- rotation whirlpools in the southern hemisphere, and the elusive “floater” that never flushes down, have all been dispelled as local folklore and myth. Or have they?
To river-people, whirlpools have always had a mystic aura surrounding them. However, it is possible to understand these behemoths and the forces that influence them. Whirlpools are, believe it or not, con- sistent river features. Playing in whirlpools adds a new dimension to river running, building your understanding and making you feel more comfortable when acciden- tally dropping into them. Playing in whirlpools increases bracing and rolling abilities, and sometimes, in a really big one, increases your lung capacity.
WHIRLPOOLS More than you wanted to know Formation
Whirlpools generally occur along strong eddylines where two strong, opposite flow- ing currents collide. When the two currents
converge, the centrifugal force (force away from the centre of the circle) creates a low-pressure area in the centre of the cir- cle. Water wants to move from high-pres- sure areas to low pressure areas and this is what creates the centripetal force (force towards the centre of the circle) in the whirlpool establishing the spin-momentum of the water. Gravity affects the spin- momentum creating the downwards flowing tendency of the vortex, and accentuating the spin.
Progression
Once a whirlpool forms, its longevity depends on its spin-momentum and inter- action of the two opposing currents. Very strong opposing currents form a whirlpool which spins extremely fast, and due to the increased centrifugal and centripetal
forces, a very tight, deep whirlpool is the result. Slow moving, opposing currents form a very shallow, wide whirlpool without much downwards-sucking motion. Whirlpools move downstream along the eddy line. The reason is, downstream flow- ing current always moves faster than the upstream flowing back eddy. The whirlpool progresses downstream along the eddyline because of the difference in force between the two currents acting on the vortex.
Dissipation
Whirlpool dissipation is a result of a loss of spin-momentum and the two opposing currents no longer being in opposition. The friction of the water on itself causes the spinning forces to stop. Dissipation of the whirlpool occurs as whirlpools move laterally away from the eddyline into the downstream or eddy cur- rent, or as they move to the downstream end of the eddy where the eddy current is not strong.
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www.rapidmag.com
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