SWIFTWATER DYNAMICS AND HAZARDS:
CHARACTERISTICS OF SWIFTWATER
Hydrology is the study of water flow and the natural forces associated with its movement. Three characteristics of swiftwater are: It is powerful. It is relentless. It is predictable. Water is relentless because, unlike ocean waves that break and give the swimmer a breather, river waves offer no break. Once the pressure is applied to the body, it stays until the victim is freed. Water is also predictable to a trained and experienced eye. It is this predictability that gives rescuers and edge.
RIVER ORIENTATION
Upstream/Downstream. Everything on the river is either upstream or downstream; not "north of here" or "south of there".
River Right/River Left (as you are looking downstream). Everything is on river right or river left; no "west bank" or "east shore".
HYDRAULIC EFFECTS
Helical Flow. The circular, rolling flow of water near the banks which forces water midstream, caused by friction between water flow and bank composition.
Helical Flow
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Spiral shape of flow
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Moves along the banks of both sides
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Carries objects out from bank towards laminar flow
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When helical flow meets laminar flow, helical "dives
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down" and returns to shore along the river bottom
Laminar Flow
Layered flows of water which are slower on the bottom and faster towards the surface (the surface is slightly slower because of air resistance); faster layers are usually found midstream and outside of curves.
Eddy
The horizontal reversal of water flow where the pressure of the current along an obstacle (such as a rock) causes the water behind the obstacle to reverse and flow upstream.
Eddy Fence
An obvious line in the river where the current moves in the opposite direction at each side. It can range from a gentle surface line to a wall of water.
Downstream V
A hydraulic effect in the form of a V pointing downstream. It is caused by convergence of downstream water flow in the channel of least resistance. The largest series of V's pointing downstream indicates the location of the main channel. (It may not be midstream.)
Upstream V
A hydraulic effect in the form of V's pointing upstream. It is caused by downstream water flow around an obstacle. Objects just below the surface that cause the V can be hazardous.
Haystack, Standing Wave
A rhythmic series of waves caused by large volumes of water flowing over a fixed object. First wave is largest, each successive wave is smaller and represents a dissipation of energy.
Hole, Stopper, Keeper
A vertical reversal of water flow where the pressure of the current falling over a gradient (dam) causes the channel water at the base of the gradient to be forced downward into a loop‑style reversal and back to the surface. At this point, part of the water continues downstream and part reverses back upstream to the base of the gradient. This reverse tends to be hazardous because it can cause an object to be recirculated (stopped or kept) in the hydraulically churning white water, which consists of 40% to 60% air.
RULE OF THUMB! A FROWNING HOLE tends to be a
keeper by recirculating in on itself. A SMILING HOLE
tends to flush objects out due to the current on both sides.
CATEGORIES OF SWIFTWATER
Class I. Moving water containing a few riffles and small waves. It has few or no obstructions.
Class II. Easy rapids with waves up to 3 feet and wide, clear channels that are obvious without scouting. Some maneuvering is required (most common).
Class III. Rapids with high, irregular waves often capable of swamping an open canoe. Narrow passages that often require complex maneuvering. It may require scouting from the shore. Rescue can be difficult.
Class IV. Long, difficult rapids. Requires precise maneuvering in very turbulent waters. Scouting is necessary and rescue is difficult.
Class V. Long, very violent rapids. Must be scouted. Rescue is extremely difficult.
Class VI. FOR EXPERTS ONLY! Nearly impossible to navigate. Rescue is next to impossible. A swimmer in the water without a PFD and a helmet WILL NOT SURVIVE!
RIVER DYNAMICS
Velocity
The speed of water movement. Velocity is measured in feet per second. An easy way to estimate the velocity of the water is to first measure out 100 feet of river. Next, observe the time it takes an object to float the 100 feet.
If it takes 5 seconds, velocity is 20 ft./sec. = 13.60 mph
If it takes 10 seconds, velocity is 10 ft./sec. = 6.80 mph
If it takes 20 seconds, velocity is 5 ft./sec. = 3.40 mph
Volume
If the river is 200 feet wide and 4 feet deep, and the velocity is 2 ft./sec., then the CFS = 1600 ft./sec. (200 x 4 x 2 = CFS)
Forces of water
Water forces obey the "square" law. If the water speed doubles, the force increases four times.
Example: If the river is flowing 1600 CFS through a channel 100 feet wide by 4 feet deep, then the water is moving at 4 ft./sec.
Example: If the velocity of the water at 5 ft./sec. = 3.4 mph, then the force on the legs is 16.8 lbs. The force on the body is 33.6 lbs.
Example: If the velocity of the water at 10 ft./sec. = 6.8 mph, then the force on the legs is 67.2 lbs. and the force on the body is 134 lbs.
Example: If the velocity of the water at 20 ft./sec. = 13.6 mph, then the force on the legs is 269 lbs. and the force on the body is 538 lbs.
RULE OF THUMB! Stay out of current that is over your knees.
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