Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
effects that could occur on the environmental characteristics are discussed in
Section 5. Section 5 also puts the potential environmental effects into context
with other activities within the marine environment.
4.5.2 RECENT EXPERIENCE
Introduction
This section covers recent experience and case studies identified during the
literature and data search completed for the study. As part of this search
discussions were held with ABP Marine Environmental Research Ltd who, with
Cefas and HR Wallingford, are undertaking RAG Sedimentation Theme Study
SED01 – Review of Round 1 Sediment Process Monitoring Data – Lessons Learnt.
The discussions were to ensure that any relevant data collected under SED01
was made available to this study.
Norfolk (Cromer) Offshore Wind Farm
As part of the environmental impact assessment for the Norfolk (Cromer)
Offshore Wind Farm (Norfolk Offshore Wind, 2002) Royal Haskoning undertook
an assessment of the fate of sediment released during ploughing operations in
superficial silts, sands and gravels and the underlying boulder clay and chalk.
An assessment of the relative proportions of the fine and coarse sediments
was made based on seabed samples and the depths to consolidated materials.
Advice was provided by Engineering Technology Applications Ltd. on the volume
of material displaced during ploughing and, conservatively, it was assumed all
this material was brought into suspension. An allowance was also made for the
volume of sediment disturbed by the plough skids.
The re-deposition of the coarse sediment was assessed by considering settling
velocities of the particles, tidal current speeds and the height of the release point.
The calculations indicated that the footprint for the re-deposition of the coarse
sediment was sensitive to both the tidal conditions (i.e. spring or neap) and
the time of release within the tidal cycle. The results indicated that the largest
deposition footprint but smallest depositional depth (200m either side of the
cable with a deposition depth of a few millimetres) would occur with release at a
mid spring tide. In contrast, the smallest footprint but greatest depth (20m either
side of the cable with depth of approximately 10mm) occurred with release at
high water neap (see Figures 4.1 and 4.2). For fine sediments it was considered
that particles would disperse throughout the water column and background
suspended sediments concentrations would only be raised by a few percent.
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