1. 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

2. Introduction Contents Environmental aspects of dredging
Introduction to case studies
Case study: London Gateway Port
Baseline environment
Site specific impacts
Project assessment
Case study: Ichthys Project, Darwin Harbour
Conclusions
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3. Environmental aspects of dredging
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4. Environmental aspects of dredging
Negative perceptions of dredging
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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5. Environmental aspects of dredging
Positive aspects of dredging
Improvement in environment
Removal of contaminated sediments
Restoration of water depth
Improved flow rates and flushing times
Land reclamation
Enhancement of mudflat and saltmarsh habitats
Beach replenishment
Wider economic and social benefits?
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6. Environmental aspects of dredging
What does this mean?
Dredging has an impact on the physical environment
Increases suspended sediment levels
Impact on photic zone (eutrophic level)
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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7. Case study: London Gateway
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8. London Gateway Baseline environment Project description
Location: UK, River Thames
Industrialisation level: High
Tidal range: ~7 metres
River / current: Minor
Habitats: Mudflats / saltmarsh
Turbidity: medium to high
Water quality: Good
Project description
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
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9. Fauna from 2015 benthic survey
London Gateway
Biological receptors
Species adapted for:
Low light conditions (eutrophic level c. few meters)
High suspended sediment concentrations
Dynamic seabed environment
Variations in salinity
Typical SSCs order
50 – >2000mg/l (peak flood tide)
Dredging operations add mg/l (peak)
Equivalent to <10-20% increase
Fauna from 2015 benthic survey
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10. London Gateway Port Predicted suspended sediment concentrations
Comparison with observations
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11. London Gateway In Summary
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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12. Case study: Darwin Harbour
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13. Darwin Harbour Baseline environment Project description
Location: NW. Australia
Industrialisation: Low
Tidal range: 8m
Freshwater input: Minor
Habitats: Mangroves / corals / Seagrass
Turbidity: Very low to low
Water quality: Excellent
Project description
New LNG site
Capital dredge: 16.7Mm3
Dredger: BHD, CSD & TSHDs
Area: Berth and shipping channel
Project duration: 5yrs
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14. Darwin Harbour Biological receptors High profile species:
Mangroves / seagrasses / corals
Marine mammals
Turtles
Species typically adapted for:
Low turbidity
High light levels
Consistent salinity
Dredging operations add mg/l
Equivalent to % increase
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15. Darwin Harbour
Observed and predicted suspended sediment concentrations
Comparison with observations
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16. Darwin Harbour In Summary
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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17. 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:
Within the Thames the effect is relatively minor;
Within Darwin Harbour the effect is relatively major;
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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18. Thank you
Questions?
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