Home' Afloat : AFLOAT October 2016 Contents 54 AFLOAT.com.au October 2016
by Malcolm Riley*
*Mal Riley is the Media and Communications
Manager for the Bureau of Meteorology in
Hobart. He has worked in meteorology for 40
years. He is a skipper of the tall ship The Lady
Nelson and a keen sea kayaker.
Afriend of mine stated, “It is really
strange to have your second-most-
costly asset tethered by a bit of
rope, where you cannot, and often do not,
see it for days on end.”
He was referring to his new yacht that
was on a mooring in a river estuary many
kilometres from where he lived.
In early June a trough and East Coast
Low brought flood-producing rains along
the coast from southeast Queensland
through to Tasmania. A lesser event added
more rain a couple of weeks later. During
the 2010–12 La Niña, major flooding also
occurred in many rivers in NSW and
Queensland, including coastal locations.
Heavy rain and floods such as these
can cause damage to vessels in a number
Some smaller, open boats simply fill
up with rain; this causes the boat to sit
lower in the water. The freeboard can be
very much reduced so even slight wave
activity can slop more water into these
boats, sinking them at their moorings.
The speed and strength of a flood
current, especially with an ebbing tide,
can be strong enough to break mooring
or berthing lines that may already be very
taut due to the rise in water depth.
Vessels tied up or at moorings that
snag passing floating debris such as
large logs or tree branches will lose their
streamlined shape and no longer ‘cut’
through the water. The additional drag
caused by the debris will add more force
to mooring or berthing lines and increase
their chance of failure.
It is not just boats that get washed
Sometimes the force of the current
is so great that floating pontoon-type
marinas may fail. This happened in
the Brisbane floods of 2011. Similarly a
whole section of marina broke free in the
recent Tasmanian floods at Devonport. A
video of the marina breaking free can be
viewed here <https://ww w.youtube.com/
Apart from the slow increase in drag
from debris collecting on the boat, the
impact from large debris can be the
‘straw that breaks the camel’s back’ and
cause mooring-line failure. This can even
puncture the hull, leading to the vessel
Vessels that break free can collide
with other boats, floating debris or solid
structures, causing further damage and
starting a domino effect in crowded
Vessels can become trapped under
solid structures such as jetties or bridges,
or held down by tight lines in rising
floodwaters. If the water continues to rise,
the vessel cannot escape its own buoyancy
... the force between it and the structure
will increase and damage to the structure
and/or the boat may occur.
During the June floods in Tasmania
the Port of Devonport on the Mersey
River was closed to all shipping, due to
There was forest and other debris
(trees, logs, wire fences kept afloat by
wooden fence posts) flowing downstream,
vessels that had broken free had sunk in
the shipping channel and some of the
navigation aids were no longer functioning.
When departing Devonport the Spirit
o f Ta sm ania (29,000 gross tonnage) initially
heads upriver to a larger swinging basin. It
then turns 180° and heads out to sea. The
ship would have needed tugs to complete
the turn and it would have been a very
difficult operation during these floods.
If during this manoeuvre debris fouled
the tugs’ or ship’s propellers or the ship’s
bow thrusters even for a short period, it
could have caused a dangerous grounding
While the ship was tied up alongside
the wharf in the Mersey River, the speed of
water over the hull at times was measured
at eight knots.
High tides and large waves were
associated with this weather system.
By revisiting an earlier article in Afloat
(May 2015) you should get a clearer
Debris washed down the Emu River in Burnie, Tasmania in June 2016. There were five shipping
containers in the pile of debris not visible in this image.
h Next month we will look at how to find
information regarding potential flooding.
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