[Coral-List] Peace Corps- Micronesia: Impacts of coral dredging operations in Yap State

Riku_Koskela at ghd.com.au Riku_Koskela at ghd.com.au
Tue Jan 18 04:18:13 EST 2005

Hi Lisa (and Iain),

My apologies for not responding sooner, I have been in the field (on a
dredging project no less) and on Xmas leave along with most others.  Iain
has referred you to a paper our ours that is on our website. We badly need
to put some more work out in the public domain this year as we are doing
really interesting things in tropical systems.  At the moment we a looking
at sediment, metal toxicants and PAH contaminants mobilised from dredging
operations as well as the impacts of Phosphates on corals. All of these are
related to port dredging operations.

There are some really effective things that can be done with sediment
mobilisation around corals. One of these is the use of silt curtains. We
have a report on the application of these (problems/benefits etc). I will
talk to the client and see if I can send you a copy.

In terms of sediment threshholds, we have been undertaking 3-D modelling
and sediment deposition studies on a variety of projects. The upshot of all
this is that the likely trigger value for sediment concentrations will vary
markedly according to seastate and tidal action. During calm conditions and
neap tides we used 4ntu as the trigger value for suspended sediment on a
GBR reef slope while Fringing reef in Darwin Harbour (a far more turbid
environment) was something like 12ntu. During rough weather, natural
mobilisation of sediments may exceed 100ntu in shallow systems and thus
dredging activities have a greater wiindow of opportunity (although they
may not like to work in rough weather).  The distance from the dredge
activity to the key habitat is also of interest. Where dredging is very
close to corals the trigger values for the dredge plume (adjacent to the
dredger) should be very restrictive. In such instances, measures such as
silt curtains should be used (with care). Sediment deposition resulting
from dredging certainly produces coral mortality.  This seems to be avoided
during rough sea state as sediments do not settle out and the plume
disperses over a wide area.

Long-term impacts of dredging are likely to be more difficult to measure.
Dredged areas contain more fine silts and if located next to coral systems,
may act as a new and continuous source of deposited sediment.  However, our
recent investigations of a spoil ground in the GBR suggests that the
formation of new structure has promoted rapid hard and soft coral
colonisation on an otherwise relatively bare sandy seabed. Our paper could
probably be used in a rough fashion to develop your own trigger values.
The performance of discharges and plumes is very site specific and we
develop new models to calculate these for each new job. There is no easy
way around this unfortunately. I have a large modelling team for this
purpose. If there is any further assistance I can give please do not
hesitate to email me.


Dr Riku Koskela
Manager – Aquatic Sciences Group

GHD Pty Ltd

201 Charlotte Street, Brisbane Q 4000
GPO Box 668, Brisbane Q 4001
T (07) 3316 3501 | F (07) 3316 3333 | M 0419 731 694
E Riku_Koskela at ghd.com.au

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             Iain Macdonald                                                
             <dr_iamacdonald at y                                             
             ahoo.co.uk>                                                To 
                                       Lisa Kristine Johnson               
             12/19/2004 04:26          <johnsolk at gmail.com>,               
             PM                        coral-list at coral.aoml.noaa.gov      
                                       Re: [Coral-List] Peace Corps-       
                                       Micronesia: Impacts of coral        
                                       dredging operations in Yap State    

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Dear Lisa and List,

The figure given in Rogers (1990) can, sometimes, be viewed as a limit for
coral reef growth. For your area, however, the limits for coral survival
can be hundreds of mg and not tens. Hodgson has done a lot of work on
sedimentation in SE Asia. A lot of work by Stafford-Smith and Ormond
document sedimentation tolerance of GBR coral species. But more pertinent
to your work from what i have read may be this paper. It is simple but very
good. It is a similar situation but in the GBR. It apparently worked very
well to curtail turbidity.

and Technology

The best thing is that GHD allow you to download for free from their web
site see below. I have no connection to this company. I wonder if Dr. Riku
might like to comment and inform us of any developments / progress in new
papers regarding this topic.


The operation should have at least siltation curtains to minimise impact.
Some dredging-coral impacts have shown no long term impact. see siltation
refs on the NOAA web page. I am afraid i can not send you any copies as i
am 3000 miles from my reference collection.

All the best and keep me informed of anything interesting / progress

Thanks and best regards,
Iain Macd.
PO Box 17458

Lisa Kristine Johnson <johnsolk at gmail.com> wrote:
 Dear Coral List serv recipients,

 I am a new Peace Corps Volunteer in Yap State, FSM working with the
 EPA. My new colleagues have asked for assistance in assessing the
 long term effects of dredging operations around the island. I am just
 getting started with this project, and am faced with obstacles such as
 limited academic resources on island and, unfortunately, my background
 not being in this field (during my masters I was focused on crustacean
 muscle biochemistry/physiology).

 There have been about 10 dredging operations since the 1960s in Yap,
 with one currently underway- about 3 months left, according to the
 operations manager. The dredging operations are for the purpose of
 attaining material to pave roads around the island. Permits, granted
 by the EPA Board, are contingent upon daily monitoring of turbidity by
 the Public Works engineering office and weekly submissions of these
 readings to the EPA. (However, daily monitoring of turbidity nor
 weekly submissions occur as the permits require...) The EPA Board has
 not yet set turbidity limits, although they are open to

 I have read that turbidity around living corals should be no greater
 than 10 mg/cm2/d. Dr. Chris Perry's Feb. 2004 list serv comment
 regarding siltation and turbidity around corals acknowledged Rogers,
 1990 (Marine Ecology Progress Series 62: 185-202) as as good review on
 the topic but warned that this is a complicated issue, and that
 Indo-Pacific corals are adapted to varying levels of sedimentation
 stress. Would anyone be willing to assert an opinion regarding
 acceptable turbidity limits some distance away from the "mixing zone"?

 The long term effects of dredging on this small, marine
 resource-dependent island is of grave concern, and I am really
 committed to this issue and helping the EPA Board make educated
 decisions about granting/or not granting permits in the future- or
 possibly not allowing dredging at all, although ideas concerning an
 alternative means of acquiring road paving materials would need to be

 This study will be conducted by comparing sites- the dredging site
 currently in progress, to sites that have been completed, to sites
 that have not been dredged, but are potential sites to be approved by
 the Board. Information to include would be water quality
 measurements, coral surveys (with the help of a crew of people from
 the EPA office and the Yap Marine Resources Management Division),
 underwater photo and video footage of sites, and interviews with local

 Does anyone have any advice about this subject/project? sources to
 suggest? I am currently using John Clark's Coastal Zone Management
 Handbook (1996, CRC press) as a guide, but I know that there must be
 information out there on this topic, and I would like some advice.

 Thank you! Kammagar!
 Lisa K. Johnson
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