ORC Heavy Rainfall Update #6
ORC Heavy Rainfall Update #6
Attributable to Pete
Stevenson, ORC Duty Flood Officer:
The heavy rain
predicted yesterday by MetService fell overnight and has
dropped off today, with lower intensity rain expected to
continue for the rest of the weekend.
There are still
significant flows feeding into Lakes Wanaka and Wakatipu
from rivers such as the Dart and Matukituki rivers, but
these are starting to recede.
ORC expects water levels
for Lakes Wanaka and Wakatipu to flatten out soon but they
will take time to recede.
Lake
Wanaka
At 8.30am, Lake Wanaka was at
280.29m.
Previous expectations were for Lake Wanaka to
reach 280.5 – 280.8m by the end of the weekend. The latest
modelling predicts Lake Wanaka may reach a lower level of
280.4m. This prediction does not consider the impact of wave
levels due to wind.
This level is above the January 2013
peak (279.4m), but well short of the November 1999 peak
(281.3m). There is a continued possibility that the lakes
could rise again, meaning there is a risk of further
flooding there should be more prolonged heavy rainfall.
Lake Wakatipu
At 8:30am, Lake
Wakatipu was at 311.285m and slowly rising.
Our
prediction for Lake Wakatipu is largely unchanged from
Friday. The lake is expected to reach 311.4m by Monday
morning, not accounting for wind. This lake level for
Wakatipu is still well short of historic flood levels such
as the November 1999 event (312.8m) and May 2010 event
(311.48m).
Downstream River
Flows
Outflows from Roxburgh Dam are being
maintained at 1800cumecs (cubic metres per second).
The
Clutha / Mata-Au River flow at Balclutha is currently
holding steady at 1700cumecs. As the water flows travel down
from the headwaters, this may reach 1800 - 1900cumecs later
on Sunday.
The river is expected to be well contained
within the Alexandra and Lower Clutha Flood Protection
Scheme flood banks.
People operating in the vicinity of
the Clutha / Mata-Au river in the coming days need to be
aware of the possibility that the river level may rise
rapidly because of high rainfall in the headwaters,
regardless of the local weather.
When will
the water levels in Lakes Wanaka and Wakatipu return to
normal?
Should no more rain fall after Sunday,
the level of Lake Wakatipu is predicted to drop 0.6m in the
next week, taking it to 0.6m above the mean December level.
Lake Wanaka is predicted to drop 1.6m, taking it to 1.18m
above the mean December level.
ORC modelling based on
past events predicts that both Lake Wakatipu and Lake Wanaka
will take around 19 days without further rain to return to
average December levels (310.2m and 277.6m respectively).
This does not mean the lakes will be in flood for 19 days,
but is an indicator that lake levels will be high, and
consequently river flows will be elevated, for several
weeks.
ORC has set up a dedicated
webpage for this event (www.orc.govt.nz/cluthaflowsdec19) with
links to up-to-date lake levels and flows, elevation maps
for affected areas, and further
resources.
Local authorities and other
agencies will be sharing information on their social media
and website pages:
QLDC: https://www.facebook.com/QLDCinfo/
CODC:
https://www.facebook.com/centralotagodistrictcouncil
CDC:
https://www.facebook.com/CluthaDistrictCouncil/
MetService:
www.metservice.com
Otago
Civil Defence and Emergency Management: https://www.otagocdem.govt.nz/
Ignite
Wanaka Business Chamber: https://www.facebook.com/IgniteWanakaChamber/
Queenstown
Chamber of Commerce: https://www.facebook.com/QueenstownChamberOfCommerce/
ENDS