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White Water
Features
RAPIDS
A rapid is a swift current of water which runs across a steep
section of river with an uneven bottom and submerged rocks.
These rocky obstructions create 'V chutes, varying currents,
different sized waves, holes, eddies, boils, whirlpools, pressure
areas and drops. The steeper the gradient the bigger, swifter
and more difficult it becomes.
A white water specialist will know what is under the surface
of the river by reading the currents and waves. By predicting
the effect this will have on the boat, appropriate action
can be taken to run the rapid safely.
It is generally recommended that canoeists paddle fairly hard
down rapids - to maintain direction, to avoid becoming broadside
to the current and to penetrate stoppers. In many cases though,
slowing the boat to manoeuvre or even Reverse Ferry Glide,
is absolutely necessary to select the correct line (for example,
among 'rock gardens', in technical turns and heavy water conditions).
Rapids in high water, or rapids of which
you are unsure must be checked from the river bank first.
Rescuers (with throw lines and rescue gear) must be positioned
at the best rescue locations along or beside the rapid, especially
if there are inexperienced paddlers among the group. If you,
or any of your party, cannot tackle the rapid safely, portage
around it.
At Bells Rapid before
the new bridge was built
THE CURRENT
The river flows at various speeds in different locations.
The fastest current is in the middle of the river where it
is deepest. The slowest is near the banks where resistance
occurs. However, there are some exceptions. On sharp corners,
the current increases in speed on the outer side of the bend
and slows down on the inside of the bend, creating an eddy.
Obstacles in the middle of the river cause the flow to vary
when water is deflected. Changes in the shoreline also have
an unexpected effect on the current.
HYDRAULIC
A hydraulic is a general term for the confused and disturbed
currents immediately below an obstruction, eg surges, standing
waves, 'stoppers'.
THE ‘V’ GUIDE (CHUTE)
There are downstream and upstream 'V formations in rapid rivers.
Downstream ' V shapes form when a fast flow of water is compressed
between narrowing shores, boulders or other obstructions.
The 'slack'water behind the obstruction is called an eddy,
and the line that forms the side of the ' V is called an eddy
line. When canoeing down rapids the 'V (or 'tongue') acts
as a guide, as it indicates a safer and deeper passage. The
beginning of the 'V is usually smooth, but it is often followed
by waves, standing waves or stoppers.
When a 'V is pointing upstream,
it is pointing to a rock that may be above the water or just
submerged. Although you may not be able to see a rock, watch
out for the 'V indicators to avoid running into them.
In more complex rapids,
there may be many downstream and upstream 'Vs. Choose the
best downstream 'V sequence available to allow for the safest
route. It is wise to check the route from the bank first and
memorise the features.
PRESSURE AREA
A pressure area is formed by water piling up against the upstream
side of boulders, trees or other obstructions. This area is
often referred to as a 'pillow' or 'cushion'. Stay clear of
this area where possible.
EDDIES
Eddies are located downstream of rocks that protrude above
the water line, behind trees, behind man made obstructions
and on the inside of bends. Most eddies are areas of slow
moving water moving in the opposite direction to the main
current. Depending on the speed of the main current, eddy
currents moving upstream can become quite severe. As the strength
of the current increases, the turbulence on the eddy line
builds up. As a result of this, spiralling currents, whirlpools,
boils and eddy currents also increase in magnitude and cause
difficulties for the paddler when entering or exiting an eddy,
as they can easily throw the boat off line. Once in the eddy
and because the current in the eddy is moving in an opposite
direction to the main current, it is often difficult to prevent
the boat from being drawn upstream towards the obstruction
or into the turbulence. Despite this, most eddies are uncomplicated
and quite manageable.
When racing in the Avon Descent it is
best to paddle in the current and avoid the eddies as much
as possible as eddies will slow your forward speed.
When social paddling in rapids Eddies
allow a paddler to stop, rest and take refuge from potentially
dangerous rapids ahead. In the safety of an eddy, a paddler
can check the route, formulate a plan or wait for the remainder
of the party.
Eddies can also be used as a safe position
from which a rescue can be undertaken, and they also provide
a safe haven for the rescue of boats or capsized paddlers.
Paddlers must be aware of the classic 'eddy roll-over', which
occurs at the bottom of a rapid. Often the paddler is lulled
into a false sense of security after successfully negotiating
a rapid. As the craft glides out of the current and hits the
still water of the eddy, the boat flips over.
STANDING WAVES/WAVES
Standing waves form through the dissipation of energy when
a fast water current reaches a slower, wider pool of water,
or when there is a sudden change in the riverbed gradient
(ie from steep to flat). Waves and standing waves are also
formed downstream of any rock or boulder when a swift current
runs over them. Waves with more of a rounded appearance are
often formed by the swift water colliding with underwater
rocks.
When the standing waves are cone shaped, and explode regularly
from the top, they are called 'haystacks'. As these waves
are not generally caused by water flowing over boulders, a
deep and clear water channel usually exists below the surface.
The only real danger arising from standing waves, is being
tossed into the water by an exploding 'haystack'.
When you can locate good standing waves in a clear rapid without
rocks, you may choose to spend some time surfing and attempting
nose stands, as it is one of the safest places to try out
trick manoeuvres. However, it is always wise to check for
obstacles lurking downstream before you start.
Another form of standing
wave, and one which should be avoided at all costs, is called
a 'rooster tail'. Although it may appear to be an innocent
'haystack', it is in fact formed by rocks or boulders hidden
within the wave explosion. It can be identified by a pronounced
water spray with a pocket of air trapped beneath the wave.
STOPPERS
'Stoppers'are caused by water gushing steeply over a ledge,
weir or large rocks. The fast moving water which tumbles over
the obstruction, starts to recirculate once it hits the bottom.
As the foaming water rolls back upstream, it forms a 'stopper'
which can trap a canoeist within a continuous washing machine
action. Big 'stoppers' must be classed as potentially dangerous
and should always be checked and in many cases, portaged.
When racing, experienced paddlers will avoid stoppers where
possible, as they reduce the boat's forward speed. However,
manageable 'stoppers' are a source of great fun for the white
water enthusiast with well developed skills.
HOLES
Holes are sometimes found at the bottom of drops and just
upstream of stopper waves. They can be deep or shallow depending
on the grade of the ramp or chute. Deep holes are often formed
at the base of waterfalls or when water 'pours over' partially
submerged boulders. Holes are often called 'souse' (to plunge
into water) holes. Holes that are found below partially submerged
boulders often look enormous, because of the much higher water
level that is diverted around the boulder.
WEIRS & FORDS
Man made weirs are without doubt one of the most dangerous
hazards to shoot. Most weirs exist in order to hold back or
divert water. To slow the flow of water and prevent erosion
at these sites, authorities design weirs that are efficient
in dissipating the downstream speed of the water. This usually
creates a continuous 'stopper' with a tremendous holding power
and tow-back, and unnatural straight banks that prevent the
canoeist from escaping the hold of the water via the ends
of the line 'stopper'. There is a high chance of injury or
loss of life if you choose to canoe weirs. Portage them, they
are not worth the risk.
BOILS
A 'boil' is created by water surging vertically upwards and
spilling outwards, in the shape of an umbrella. The boundary
line at its circumference can be quite difficult to negotiate,
due to instability caused by sinking surface water. Passing
over a 'boil' in a canoe can be a trying experience, as unexpected
water surges make the canoe very unstable. 'Boils' vary in
size and those located on large rapids and tidal rapids should
generally be avoided.
BENDS
At a bend, the water is prevented from flowing in a straight
line by the outer bank, which forces it to change direction
and form a swift flowing current. If paddlers lose control
and are forced to the outside edge of the bend, they may be
presented with a number of potential hazards.
For example, some bends have rocky walls which act as barriers
and the result is a continually surging, thrusting turbulence.
Under these conditions, paddlers may find themselves forced
against the wall and unable to remain upright.
Other bends have dangerous undercut banks. Paddlers may be
swept under, become trapped and have an extremely difficult
time trying to escape.
Trees are also extremely dangerous on bends. Remember, never
relax until the complete rapid has been run and you are into
clear flowing water.
ARTIFICIAL OBSTACLES
Artificial obstacles such as bridge pylons, fences etc, are
potentially more hazardous than naturally occurring river
formations and should be avoided at all costs.
From my book Canoeing Down
Under
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