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Rapids
Three factors, separately or in combination, can create rapids: gradient,
constriction, and obstruction.
The gradient
of a river is the rate at which it loses elevation
along its course. This loss determines the river's slope, and to a large
extent its rate of flow. Shallow gradients produce gentle, slow rivers while
steep gradients are associated with raging torrents.
Constrictions
can form a rapid when a river's flow is forced into a narrower channel. This
pressure causes the water to flow more rapidly (hence the name) and to react
differently to riverbed events (rocks, drops, etc.)
Lastly, there is obstruction.
A boulder or ledge in the middle of a river or near the side can obstruct the
flow of the river, and can also create a "drop" (over the boulder)
and "hydraulics" or "holes" where the river flows back on
itself--perhaps back under the drop--often with fearful results for those
caught in its grasp. (Holes, or hydraulics, are so-called because their foamy,
aerated water provides less buoyancy and can feel like an actual hole in the
river surface.) If the flow passes next to the obstruction, an eddy
may form behind the obstruction; although eddies are typically sheltered areas
where boaters can stop to rest, scout or leave the main current, they may be
swirling and whirlpool-like. As with hydraulics (which are, essentially,
eddies turned at a 90-degree angle), the power of eddies increases with the
flow rate.
Classification of whitewater
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Main article:
The most widely used grading system is the
, where whitewater (either an individual rapid, or the
entire river) is classed in six categories from class I (the easiest and
safest) to class VI (the most difficult and most dangerous). The grade
reflects both the technical difficulty and the danger associated with a rapid,
with grade I referring to flat or slow moving water with few hazards, and
grade VI referring to the hardest rapids which are very dangerous even for
expert paddlers, and are rarely run. Grade-VI rapids are sometimes downgraded
to grade-V or V+ if they have been run successfully. Harder rapids (for
example a grade-V rapid on a mainly grade-III river) are often portaged,
a French term for carrying. A portaged rapid is where the boater lands and
carries the boat around the hazard.
A rapid's grade is not fixed, since it may vary greatly depending on the
water depth and speed of flow. Although some rapids may be easier at high
flows because features are covered or "washed-out," high water
usually makes rapids more difficult and dangerous. At flood stage, even rapids
which are usually easy can contain lethal and unpredictable hazards. (Briefly
adapted from the American version[1]
of the International Scale of River Difficulty[2].)
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Class 1: no rapids, smooth flowing water
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Class
2: some rough water--the line is easy to see and pursue
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Class
3: whitewater but very safe for larger rafts; kayakers and
canoeists should have good rolling or self-rescue skills
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Class
4: whitewater for experienced paddlers only; the route
through the rapids may require quick maneuvering
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Class
5: whitewater for advanced paddlers; scouting the rapid may
be required, and there may be hidden hazards which require precise
maneuvering
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Class
6: impassable or exploratory; for teams of experts, taking
all safety precautions
Features found in whitewater
On any given rapid there can be a multitude of different features which
arise from the interplay between the shape of the riverbed and the velocity of
the water in the stream.
Strainers,
Strainers are formed when an object blocks the passage of larger objects
but allows the flow of water to continue - like a big strainer.
These objects can be very dangerous, because the force of the water will pin
an object or body against the strainer and then pile up, pushing it down under
water. Strainers are formed by many different objects, like storm grates over
tunnels, trees that have fallen into a river ("log jam"), bushes by
the side of the river that are flooded during high water, or rebar from broken
concrete structures in the water. In an emergency it is often best to try and
climb on top of a strainer so as not to be pinned against the object under the
water. If you are in a river, swimming aggressively away from the strainer and
into the main channel is your best bet. If you cannot avoid the strainer, you
should swim hard towards it and try to get as much of your body up and over
it.
Sweepers,
Sweepers are trees fallen or heavily leaning over the river, still rooted
on the shore and not fully submerged. Its trunk and branches may form an
obstruction in the river like strainers. Since it is an obstruction from
above, it often does not contribute to whitewater features. But it should be
mentioned here because in fast water sweepers can pose a serious hazard to
paddlers.
Hydraulics,
Hydraulics,
often referred to as "holes" or "souse-holes", are formed
when water pours over the top of a submerged object, causing the water
downstream to flow back over the top of the water which rushes over the
submerged object. Hydraulics can be dangerous: a boater may become stuck in
the recirculating water. In high volume water, hydraulics will aerate the
water, possibly to the point where it may even loose the capacity to carry any
water crafts.
Some of the most dangerous types of hydraulics are formed by lowhead dams (weirs),
underwater ledges, and similar types of obstruction. In lowhead dams, the
hydraulic has a very symmetrical character - there's no weak point - and where
the sides of the hydraulic are often blocked by a man-made wall, making it
impossible to slip off the side of the hydraulic. Lowhead dams are insidiously
dangerous because their danger cannot be easily recognized by people who have
not studied whitewater.
Waves,
Waves
are formed in a similar nature to hydraulics and are sometimes also considered
hydraulics as well. Waves are noted by the large smooth face on the water
rushing down. Sometimes a particularly large wave will also be followed by a
"wave train", a long series of waves. These standing waves can be
smooth or, particularly the larger ones, can be breaking waves (also called
"whitecaps" or "haystacks").
Because of the rough and random pattern of a riverbed, waves are often not
perpendicular to the river's current. This makes them challenging for boaters
since a strong sideways or diagonal wave can throw the craft off.
In fluid mechanics, waves are classified as laminar, but the whitewater
world has also included waves with turbulence ("breaking waves")
under the general heading of waves.
Pillows,
Pillows are formed when a large flow of water runs into a large
obstruction, causing water to boil up against the face of the obstruction.
Pillows can be dangerous because sometimes the object that forms the pillow is
undercut and so paddlers can be swept underwater - possibly to be entrapped.
Eddies,
Eddies
are formed, like hydraulics, on the downstream face of an obstruction. Unlike
hydraulics, eddies swirl on the horizontal surface of the water. Typically,
they are calm spots where the downward movement of water is partially or fully
arrested - a nice place to rest or to make one's way upstream. However, in
very powerful water, eddies can have powerful, swirling currents which can
flip boats and from which escape can be very difficult.
Whitewater craft,
there are many different types of whitewater craft that people use to make
their way down a rapid, preferably with finesse and control. Here is a short
list of them.
Whitewater kayaks,
differ from sea kayaks and recreational kayaks in that they are specialized to
deal with moving water better. They are often shorter and more maneuverable
then sea kayaks and are specially designed to deal with water flowing up onto
their decks. Most whitewater kayaks are made of plastics these days, although
some paddlers (especially racers and "squirt boaters") use kayaks
made of fiberglass composites. Whitewater kayaks are fairly stable in
turbulent water, once the paddler is skillful with them; but once flipped
upside-down, the skilled paddler can roll them back upright. This essential
skill of whitewater kayaking is called the "Eskimo
Roll," or simply "Roll." Kayaks are paddled in a sitting
position, with a two-bladed paddle.
Rafts,
are also often used as a whitewater craft; more stable than typical kayaks,
they are less maneuverable. Rafts can carry large loads, so they are often
used for expeditions. Typical whitewater rafts are inflatable craft, made from
plastic - much like an air mattress.
Catarafts, are similar to rafts,
constructed from two inflatable pontoons on either side of the craft which are
bridged by a support carriage where the occupants sit. Catarafts can be
smaller and more maneuverable than many typical rafts.
Canoes,
are often made of fiberglass, kevlar, plastic or a combination of the three
for strength and durability. They may have a spraycover, resembling a kayak,
or "open," resembling the typical canoe. Whitewater canoes are
paddled in a kneeling position, with a one-bladed paddle. Open whitewater
Canoes have large airbags to prevent the boat from being swamped by big waves
and holes. Like kayaks, whitewater canoes can be righted after an upset with
an Eskimo Roll.
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