[MARMAM] Underwater blasting & dredging impacts on coastal dolphins, dugong and other marine wildlife in Darwin Harbour, Northern Territory, Australia

Stuart Blanch ecnt at iinet.net.au
Sun Aug 22 23:16:34 PDT 2010


Dear marine mammal researchers

 

I am seeking information and your assistance regarding the potential impacts
from underwater blasting and dredging on coastal dolphins and dugong, as
well as threatened marine turtles and fish, from shipping channels
associated with the proposed INPEX <http://inpex.com.au/>   LNG plant in
Darwin Harbour in tropical Northern Australia.

 

Marine wildlife at risk:

.         The dolphins are Australian snubfin (Orcaella heinsohni),
Indo-Pacific Humpback (Sousa chinensis) and Indo-Pacific bottlenose
(Tursiops aduncus). 

 

.         In addition, pods of false killer whales (Pseudorca crassidens)
occasionally visit the harbour.

 

.         Dugong (Dugong dugon), which inhabit the seaward-end of the
harbour and are principally threatened by dredging, underwater noise and
potentially boat strike

 

.         IUCN Red-listed marine turtles that use the harbour are flatback,
green and hawksbill turtles.

 

.         Tropical marine and estuarine fish which inhabit the harbour.

 

INPEX's draft EIS for an LNG (natural gas) processing and export facility

Oil & gas company INPEX has lodged a draft
<http://inpex.com.au/ichthys-draft-environmental-impact-statement/welcome.as
px>  EIS proposing to build a massive liquified natural gas plant in Darwin
Harbour in tropical Northern territory.

 

The plan involves constructing a jetty, ship turning basin, and a long
shipping channel through the relatively ecologically intact Darwin Harbour.

 

Dredging the route for the shipping channel involves underwater blasting
<http://inpex.com.au/media/20786/ichthys_eis_ch7.pdf>  (cf p357) to remove a
large rocky underwater navigation hazard (Walker Shoal) which sits in the
middle of their preferred channel route as they claim poses unacceptable
risks to LNG tankers once construction is completed in about 2015.

 

The shipping channel is to be dredged
<http://inpex.com.au/media/20786/ichthys_eis_ch7.pdf>  (cf p305) for up to
3.5 years in the first instance to remove 16.9 million cubic meters of soft
sediments using a backhoe dredge (and perhaps a cutter suction dredge). In
addition, future maintenance dredging is required over the 40+ year lifespan
of the project.

 

Underwater blasting impacts on coastal dolphins?

The blasting would occur 3 times a day over more than 1 year to remove
Walker Shoal which INPEX maintains presents an unacceptable safety hazard to
large LNG tankers because it is very shallow (6m depth at low tide) and very
hard. 

 

What would the impact of this blasting be on dolphins, as well as marine
turtles and fish? 

 

INPEX admit blasting could kill and / or exclude dolphins and other marine
wildlife from that part of the harbour. INPEX's proposed mitigation measures
include using trained spotters on nearby ships to call a halt to blasting if
dolphins surface within the kill zone (radius: 500m, apparently based on an
assumed temporary threshold shift volume of 183 decibels), as well as active
accoustic monitoring by triangulating the location of submerged dolphins
using underwater accoustic meters. INPEX say latter approach is a world
first, which we interpret as being untried and hence unproven.

 

INPEX appears to have no reliable data on the likely impacts of the blasting
on coastal dolphins in partly enclosed harbours. They seem to have
undertaken no modelling of underwater pressure wave impacts on marine
wildlife. They appear to rely on two studies, neither of which is apparently
cited in the draft EIS: one from Binningup in southwest Western Australia on
underwater attenuation of low frequency noise, and a report by 'ICI' on
physical impacts on from blasting on dead dogs in the water (I am not
joking). 

 

Instead, the draft EIS seems to rely on a very limited (and un-cited) number
of peer reviewed research papers as well as grey literature, a lot of which
seems to address underwater sonar impacts on whales from oil & gas
exploration in deep water, rather than on coastal dolphins in partly
enclosed turbid tropical harbours such as Darwin Harbour.

 

We would appreciate any information (particularly peer reviewed papers) you
could provide to us regarding underwater noise impacts on coastal dolphins,
noise attenuation in partly enclosed harbours, adequacy of passive and
active accoustic monitoring in turbid partly enclosed harbours, best
practice underwater accoustic modelling in harbours.

 

Also, is anyone aware of low-noise alternatives to removing very dense
underwater rock, such as other ways of dredging or blasting? 

 

Dredging impacts on coral, sea grass and mangrove habitats

INPEX state they expect a decline in the health of soft corals in the
harbour due to plumes of fine sediments resuspended during dredging, but
state they think tropical soft corals are highly resilient to periodically
being coated in sediments. They state only about 1 mm of sediments are
likely to be deposited on the corals, and this is likely to be removed
during large tides (hence at least monthly). 

 

INPEX estimates mangrove and sea grass death due to dredging and subsequent
sediment deposition to be minor and manageable, eg, 'only' 0.2% of the
harbour's mangroves will be lost.

 

Re-suspension of sediments in slack water areas is thought to be an ongoing
but relatively minor issue, though their models indicate many depositional
areas in mangroves and where tributaries enter the harbour.

 

If you have any information on dredging impacts on tropical marine mammals,
including dolphins and dugong, as well as marine turtles, I would appreciate
any information you could send our way.

 

Submissions close 10 September

If you have experience and knowledge about the potential impacts on coastal
dolphins, dugong and otjer marine wildlife from underwater blasting and
dredging, I urge you to provide a submission to the draft EIS. Submissions
close 10 September.

 

 

Dr Stuart Blanch  |  Coordinator


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T (08) 8941 7439  |  M 0448 887 303  |  E  <mailto:communications at ecnt.org>
coordinator at ecnt.org   |  W  <http://www.ecnt.org/> http://www.ecnt.org/

GPO Box 2120, Darwin NT 0801  |  Unit 3/98 Woods Street, Darwin Northern
Territory - Australia

 

 

 

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