Rapids – how to understand them
Rapids look impressive, and terrifying. They throw spray high into the air, creating rainbows and the sound of their movement can be almost deafening. The idea of travelling through them can be demoralising, but understanding the nature of rapids helps extreme sports people master them.
Rapids may look chaotic, but they are often highly predictable. The volume of water, the steepness of the river, the width of the channel, and the obstacles in the water, all have measurable effects. Experts can read a stretch of rapids, spotting the hazards and seeing the safest way through.
To begin with, knowing the amount of water flowing in a river is important because the river's speed increases as more water flows through it. Double the water means double the speed, so a mild rapid can become a raging torrent during a rainy season or after a severe storm. Water sports participants must also know the flow rate because water is heavy. It actually weighs 1,000 kilograms per cubic meter – which is 62 pounds per cubic foot – like being rolled on by an elephant! In rapids, such water exerts tremendous pressure on a boat, raft or canoe.
Three basic states of flowing water exist: laminar, turbulent, and chaotic. Laminar describes the smooth-flowing currents in an unobstructed river. Even these currents can be complicated, for their speeds vary. Surface water is slowed by wind, while deep currents are slowed by friction with the riverbed. Turbulence occurs when obstacles, such as rocks or a sudden narrowing of the river channel, obstruct the current's flow. Obstacles force too much water into too little space, so the water runs faster and laminar sheets break into individual ribbons of current. If water runs into a boulder, a turbulent zone emerges where the water and rock meet and while current runs faster around the boulder's edges, behind the rock, it will form an area of backward-flowing water called an eddy. Shear zones between the eddy and the fast water can be strong enough to keep a craft from reaching calm water.
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