Download Same dredger, different environment

Same dredger, different location: Environmental
impacts of dredging
Resistance and tolerance of different environments to dredging
TEP DLG - 22 September 2016
David Sutherland & David Middlemiss
Introduction
Contents
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Environmental aspects of dredging
 Introduction to case studies
 Case study: London Gateway Port
 Baseline environment
 Site specific impacts
 Project assessment
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Case study: Ichthys Project, Darwin Harbour
 Baseline environment
 Site specific impacts
 Project assessment
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Conclusions
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Environmental aspects of dredging
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Environmental aspects of dredging
Negative perceptions of dredging
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Impact of dredging and disposal operations
 Excavation of sediments
 Increase in suspended sediment concentrations
 Possible release of organic matter, nutrients
and / or contaminants
 Disturbance of benthic habitats and communities
 Disturbance of spawning / nursery areas
 Disturbance of sensitive receptors (e.g. seagrass)
 Smothering of benthic / intertidal communities
 Effect on overall water quality
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Environmental aspects of dredging
Positive aspects of dredging
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Improvement in environment
Wider economic and
social benefits?
 Removal of contaminated sediments
 Restoration of water depth
 Improved flow rates and flushing times
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Land reclamation
 Enhancement of mudflat and saltmarsh habitats
 Beach replenishment
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Environmental aspects of dredging
What does this mean?
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Dredging has an impact on the physical environment
 Increases suspended sediment levels
 Impact on photic zone (eutrophic level)
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Provides additional stressor for benthic / intertidal communities
 But species are adapted to the environment that they inhabit….
 How significant is that additional stress?
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Case study: London Gateway
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London Gateway
Baseline environment
Project description
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Location: UK, River Thames
Industrialisation level: High
Tidal range: ~7 metres
River / current: Minor
Habitats: Mudflats / saltmarsh
Turbidity: medium to high
Water quality: Good
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New cargo terminal
Capital dredge: 31Mm3
Dredger: CSD and TSHDs
Area: Berth, turning circle, channel
Project duration: ~ 9 years (ongoing)
Maintenance dredge: Predicted upto
2.25Mm3/yr
© HR Wallingford 2016
London Gateway
Biological receptors
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Species adapted for:
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Low light conditions (eutrophic level c. few meters)
High suspended sediment concentrations
Dynamic seabed environment
Variations in salinity
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Typical SSCs order
50 – >2000mg/l (peak flood tide)
 Dredging operations add 10-200mg/l (peak)
 Equivalent to <10-20% increase
Fauna from 2015 benthic survey
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London Gateway Port
Predicted suspended sediment concentrations
Comparison with observations
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London Gateway
In Summary
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Background Thames SSC varies typically from 20 to >2000mg/L around
London Gateway Port
Dredging was predicted to lead to small and variable increase in suspended
sediment concentrations (order <10 to 20%)
Observed SSCs consistent with predicted SSCs
Organisms in this environment already adapted to high turbidity environments
and therefore expected to have minimal impact on ecology (eutrophic level).
Minimal impact has been demonstrated by monitoring of ecology (diversity,
abundance and biomass)
Mitigation measures included monitoring and compensation of impacted
habitat
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Case study: Darwin Harbour
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Darwin Harbour
Baseline environment
Project description
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Location: NW. Australia
Industrialisation: Low
Tidal range: 8m
Freshwater input: Minor
Habitats: Mangroves / corals /
Seagrass
Turbidity: Very low to low
Water quality: Excellent
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New LNG site
Capital dredge: 16.7Mm3
Dredger: BHD, CSD & TSHDs
Area: Berth and shipping channel
Project duration: 5yrs
© HR Wallingford 2016
Darwin Harbour
Biological receptors
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High profile species:
 Mangroves / seagrasses / corals
 Marine mammals
 Turtles
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Species typically adapted for:
 Low turbidity
 High light levels
 Consistent salinity
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Dredging operations add 100-200mg/l
 Equivalent to 400-2000% increase
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Darwin Harbour
Observed and predicted suspended sediment concentrations
50
40
Turbidity (NTU)
35
5
30
25
0
20
15
-5
10
Water depth at Channel Island (m)
10
45
5
0
01-Jul
-10
06-Jul
11-Jul
CI
16-Jul
WR
21-Jul
NEW
26-Jul
SSI
31-Jul
05-Aug
10-Aug
15-Aug
Measured depth at CI
Comparison with observations
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Darwin Harbour
In Summary
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Background SSC in Darwin Harbour varies typically from 2 to ~50mg/L
Dredging was predicted to lead to significant and sustained increase in
suspended sediment concentrations (order 400 to 2000%)
Model predictions successfully validated.
Organisms in this environment are not adapted to high turbidity levels and
therefore expected to have an impact on ecology.
Predicted to affect mangroves (smothering of pneumatophore, reduction in
eutrophic depth affecting seagrass and corals)
Some impact has been demonstrated - but less than expected.
Mitigation measures included monitoring, restrictions, and development of
marine and land reserves
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Conclusions
Some conclusions……
Dredging can induce environmental change…
 Similar dredging operations can have different effects – this is dependent
upon the environment that the dredging occurs within.
 Focussed on a brief comparison of suspended sediment concentrations – for
similar scale and methodology of dredging:
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 Within the Thames the effect is relatively minor;
 Within Darwin Harbour the effect is relatively major;
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The ecology has adapted to the respective environment, therefore the effect
will be felt more where the increase is significant.
 This is borne out in the modelling and monitoring data for both projects.
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Thank you
Questions?
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