The Star: June 03, 2021

Latest Canterbury news at starnews.co.nz
A retrospective look
at floods in Canty
Thursday June 3 2021 The Star
NEWS 17
After earthquakes, flooding happens to be New
Zealand’s second costliest natural hazard – and
the weekend’s deluge in Christchurch reflects
why. Science reporter Jamie Morton looks at
three questions surrounding the disaster
MANY AREAS in Canterbury
have been struck by major
floods during the past 150
years. In Christchurch,
floodwaters were sometimes
knee-deep in the central city in
the late 1800s and early 1900s.
During early European settlement,
regular river overflows
occurred over the flood plain
areas in and around Christchurch
and Kaiapoi.
Since the primary stopbank
was completed in the 1930s,
the Waimakariri River hasn’t
flooded through the city.
The most recent breakout,
which happened in December
1957, flooded parts of Coutts Island,
in Belfast, and Kainga and
was just under 4000 cumecs.
Being built on the flood plain
of the Waimakariri River,
Canterbury is no stranger to
heavy flooding no matter the
season. The first stopbanks were
built in the 1860s but floods still
regularly happened.
This week marks the anniversary
of several of these
events from the past. From 130
to five years ago, all have caused
damage, fear and anger, but
also given reason to help each
other out
On February 4, 1868, a severe
storm hit Canterbury causing
wide spread damage across the
region. The storm, which began
the day before, continued over
night, wrecking many ships at
anchor in Lyttelton.
The first alarm for Christchurch
was raised by a Fendall
Town (Fendalton) resident who
reported that the Waimakariri
River had flooded and broken
through its banks. This, in turn,
sent a surge of water towards
Christchurch via the Avon.
Just an hour later at 1pm,
the Avon had raised so much
that water had begun to trickle
across the Madras St bridge.
Those unfortunate enough
to live near the Avon and its
streams were now under water
HISTORY: City buildings during the 1868 flood.
PHOTO: PEELINGBACKHISTORY.CO.NZ
– in some places four feet deep.
Tearing away at its own banks,
knocking down fences and
drowning hundreds of hoof
stock, quite a grand pile of
debris rolled into the city with
the torrent.
Police and workmen from the
city council were dispatched out
into the affected areas, helping
people to evacuate their homes
and to clear away any dangerous
debris from the waters.
Victoria St was even dug up
to form a stop bank in the hope
this would halt the flood – it
didn’t. By nightfall, Market
Place (Victoria Square) was
knee deep in water, wrecking
the businesses that called the
place home. One man had to
be rescued when he attempted
to make his way through the
flood. He fell into a hole that
had been dug away by the water
and almost drowned.
Throughout the night, cabbies
drove people back and forth in
their hansom cabs for six dimes
per head. For years, many
remembered the cab’s lanterns
flickering over the eerie black
waters.
Christchurch was not the
only town affected. Kaiapoi
was completely under water
and many bridges, roads and
railways, had been swept away
from the Waimakariri and
Selwyn Rivers. Communication
by telegraph had kept Canterbury
in touch with the rest of
the country but by the time the
floodwaters began to drop back,
only the line between Christchurch
and Lyttelton remained.
An engineer who predicted
that the Waimakariri would
flood eventually was William
Bray (his farm is now the
suburb of Avonhead). His
warnings were not taken
seriously though.
The following poem was
written by Crosbie Ward
of The Lyttelton Times:
At Avonhead lives Mr Bray,
Who every morning used
to say,
“I should not be much
surprised today
If Christchurch city were
swept away
By the rushing, crushing,
flushing, gushing
Waimakariri River”
He told his tale and he
showed his plan,
How the levels lay and the
river ran;
The neighbours thought
him a learned man,
But wished him further
than Isfahan,
With his wearing,
tearing, flaring, scaring
Waimakariri River
Weekend storm a
significant event
WAS IT A one-in-100 year event?
There’s no question the weekend
storm was a significant one.
Places such as Akaroa, Methven
and Winchmore received around
twice their normal monthly rainfall
in fewer than three days.
At Lismore, near Ashburton,
238mm dropped
– that’s the same amount
that it had received in the
previous 187 days.
Such was the storm’s intensity
that some labelled
it a “once in a century”
downpour.
But it’s really not that
simple – and indeed such
a calculation could be impossible
to make.
Dr Rob Bell, a scientist specialising
in coastal hazards, said
categorising events in such a
way – as had become ingrained
because of past practice – was a
“problematic issue”.
“We now face the ongoing
influence of climate change on
weather-related events that no
longer fits with the assumption
of no underlying change in our
weather systems, climate or sea
level.”
The term has long been used as
a measure of the likely time frame,
on average, an event exceeds a
certain level – whether that be
a flood height, rainfall total, or
storm-tide sea level – based on a
Emily Lane
record of historic measurements.
That meant calculating a “onein-100-year”
event from 50 years
– or even 100 years of records –
came with obvious uncertainties.
Bell said a slightly better measure
was the “annual exceedance
probability”, or AEP.
By this, a “one-in-100-
year” event had a chance of
one per cent each year of
being reached or exceeded.
“So the clock doesn’t get
reset once such an event
occurs, such that it won’t
occur again for 100 years,”
he said.
“It’s like a computer
picking randomly from numbers
one to 100 and getting a one, then
five rounds later it could be a one
again.”
The usual way the AEP of a
flood is calculated was with river
gauges, which measure the height
of the water in a river at a given
time.
This was converted into a “flow”
– or how many cumecs, or cubic
metres of water, per second, were
flowing down the river using a
rating curve.
“So for rivers where we have
gauges, we have measurements
of the flow in that river over
time,” said Dr Emily Lane, a
hydrodynamics scientist at
NIWA.
• Turn to page 18
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