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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 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 Baseline environment Site specific impacts Project assessment Conclusions 14/9/2016 © HR Wallingford 2016 Environmental aspects of dredging 14/9/2016 © HR Wallingford 2016 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 14/9/2016 © HR Wallingford 2016 Environmental aspects of dredging Positive aspects of dredging Improvement in environment Wider economic and social benefits? Removal of contaminated sediments Restoration of water depth Improved flow rates and flushing times Land reclamation Enhancement of mudflat and saltmarsh habitats Beach replenishment 14/9/2016 © HR Wallingford 2016 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? 14/9/2016 © HR Wallingford 2016 Case study: London Gateway 14/9/2016 © HR Wallingford 2016 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 14/9/2016 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 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 10-200mg/l (peak) Equivalent to <10-20% increase Fauna from 2015 benthic survey 14/9/2016 © HR Wallingford 2016 London Gateway Port Predicted suspended sediment concentrations Comparison with observations 14/9/2016 © HR Wallingford 2016 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 14/9/2016 © HR Wallingford 2016 Case study: Darwin Harbour 14/9/2016 © HR Wallingford 2016 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 14/9/2016 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 High profile species: Mangroves / seagrasses / corals Marine mammals Turtles Species typically adapted for: Low turbidity High light levels Consistent salinity Dredging operations add 100-200mg/l Equivalent to 400-2000% increase 14/9/2016 © HR Wallingford 2016 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 14/9/2016 © HR Wallingford 2016 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 14/9/2016 © HR Wallingford 2016 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. 14/9/2016 © HR Wallingford 2016 Thank you Questions? 14/9/2016 © HR Wallingford 2016