Phase
Construction and Decommissioning
Potential Impact
Changes in suspended sediment concentrations as a result of GBS sea bed preparation activities and drilling for jacket installation.
WCS Details
GBS: 1 foundation installed/day. Dredging (in areas of sand waves) of up to 22,500m3 of surface sediment (characterised by grab samples, with 75% being medium sand and only 2% being mud) per foundation and disposal by barge (surface release) in close proximity to each foundation.
Construction and Decommissioning
Changes in bed levels and sediment type at the sea bed as a result of GBS sea bed preparation activities and drilling for jacket installation.
Construction and Decommissioning
Construction and Decommissioning
Construction and Decommissioning
Potential release of contaminants from the Warren Springs Environmental Disposal Site.
Changes in suspended sediment concentrations, bed levels and sediment type as a result of inter-array cable installation activities.
Changes in suspended sediment concentrations, bed levels and sediment type as a result of offshore cable installation activities.
Jacket: 1 jacket installed/48hrs. 50% of the 325 WTG jackets would be drilled, releasing 982m3 of sub-surface sediments (characterised by boreholes, clays, silts and sands) per jacket. 100% disaggregation into component particle sizes assumed (not considering cohesion and clastic properties). As above.
Assessment Method
• Numerical modelling using Delft3D- PART (15 plume releases over a 15 day spring-neap cycle run)
• Standard empirical equations (mobilisation and settling of sediment particles)
• Existing evidence base from marine aggregate dredging industry
• Conceptual understanding of potential impact
• Interpretation against baseline SSC values (summer and winter) and storm effects
• As above.
Assessment of Effect
Short term and localised increases in SSC may affect other receptors (e.g. marine water quality, fish, benthic ecology and marine mammals). Given the sediment types and tidal currents considered, the majority of sediment from GBS installation will rapidly (seconds to minutes) descend to the sea bed as a high concentration dynamic phase plume. It will form a mound on the bed, spreading radially under gravity. The remainder of the sediment will form a passive phase plume and become dispersed by tidal action before subsequently falling to the bed. Sands within this plume will settle within around 20 minutes of release, extending over an area of up to 1km. Finer sediments may persist for longer (hours to days) and travel over a wider area, with net movement to the north. For jackets, due to the finer nature of the sub-surface sediments, material may be transported over tends of kilometres from the release points.
Significance of impact on receptors = Not significant
For GBS, up to 2m thickness of deposition due to dynamic phase plume over a likely worst case area of 100m x 100m (10,000m2) near to each foundation. Less than 0.2mm thickness of deposition of finer material over a wider area during the passive phase plume. For jackets, up to a few centimetres of deposition of sand within a few hundred metres of release, with less than 0.025mm thickness of deposition of finer material over a considerably wider area during the passive phase plume. Significance of impact on receptors = Not significant
Potentially affected by GBS sea bed preparation activities, as described above.
Up to 550km of inter-array cable. Dredging in areas of large ripples and sand waves.
Jetting to bury cable to a depth of 5m along the entire offshore cable route.
• As above.
• Conceptual understanding of potential impact
• Numerical modelling using Delft3D- PART
• Existing evidence base from industry best practice guidance (BERR, 2008) and other wind farms (e.g. Nysted, Kentish Flats, Cromer)
• Conceptual understanding of potential impact
• Interpretation against baseline SSC values (summer and winter) and storm effects
Construction and Decommissioning
Interaction between bed preparation and foundation installation within the East Anglia ONE wind farm and sediment plumes created by installation of the East Anglia ONE offshore cable.
Construction and Decommissioning
Indentations on sea bed left by vessels (vessel jack-up and anchoring operations).
Construction programmes overlap such that plumes coalesce.
• Conceptual understanding of potential impact (based on tidal excursion ellipses)
Fate of contaminants dependent on release and deposition of bed sediments, as assessed above. Significance of impact on receptors = Not significant
Subordinate scale of potential impact compared against foundation installation, assessed above. Significance of impact on receptors = Not significant
Short term and localised increases in SSC due to installation, but baseline SSC values in shallower waters nearer to shore are greater than those further offshore across the wind farm site. Localised (<1km of release) concentrations up to 400mg/l in very shallow water, typically <100mg/l in deeper water (>20m water depth). Dispersion of fine-grained material within 180 hours of release.
Bed level changes of up to 2mm observed within a few hundred metres and up to 0.2mm observed 20km from cable.
Significance of impact on receptors = Not significant
There is only limited opportunity for plume combination due to the arrangement of the layout and cable route with respect to the tidal excursion ellipses. The combined plume may cover a slightly larger geographical area and, for a very short period of time, locally exhibit higher concentrations than assessed for foundation and offshore cable plumes individually. However, this higher concentration plume would not be expected to persist for much longer than a few hours.
Significance of impact on receptors = Not significant
Up to 6 legs of a jack-up barge. Each leg will have a maximum diameter of 16m and form footprint between 50 – 200m2. Penetration
Phys_Proc_background Sept 2013
• Conceptual understanding of potential impact
As each leg is inserted it will cause the already partially consolidated sediments to be compressed downwards and displaced laterally. This may cause the sea bed around the inserted leg to be raised in a series of concentric pressure ridges. As the leg is retracted, some material that has
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