Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
Dispersion and deposition of coarser sediment was not modelled as sand
will only be carried a few metres from the point of disturbance. The sediment
distribution of the unconsolidated surface layer in large parts of the area affected
by the cabling is predominantly gravely sand with a small amount of silt.
The high sand/gravel content of the in situ sediment, together with the relatively
small disturbance arising from cable ploughing or trenching to 1m depth,
suggests that for most of the cable route the majority of any disturbed sediment
will fall immediately to the bed in the immediate location of the cable. Because
of the minimal disturbance, fine sand is almost all likely to remain within the
bottom 1m -2m of the water column (even this is probably conservative) and
typical settling velocities of around 10mm/s will ensure that the sand settles
within half an hour (or less) or becomes part of the ambient near bed transport.
Medium or coarse sand will settle within minutes. The vast majority of the
disturbed sediment will initially resettle within 20m of the cable, with almost no
sand being carried more than 100m from the cable except as part of the natural
background transport.
The presence of surface sand/gravel along much of the cable route restricts the
extent of fine sediment dispersion, and the modelled results are only applicable
to those limited areas where chalk or other competent beds are exposed or have
only a very thin surface layer of mobile sand. The plotted model results are
therefore conservative for a 1m depth of burial, but still show that suspended
sediment will be quickly dispersed. In the areas where the export cables are
proposed to be buried up to 3m (shoals / sand waves) the cable is installed in
(mobile) sands only, with no disturbance of the underlying chalk or other beds.
London Array Offshore Wind Farm
An assessment of the cabling operations for the London Array Offshore Wind
Farm has been undertaken by ABP Mer (RPS, 2005). The work assumed that
jetting techniques would be adopted and burial depths would be between 1 and
3m. An assessment of the surface sediments was based on a comprehensive
grab sampling exercise supported by seismic and side-scan surveys. It was
found that the inter-array and export routes were predominately covered with
fine sands. In parallel with the assessment of seabed conditions, suspended
sediment data was collated. This included data from the Southern North Sea
Sediment Transport Study, archive data held at CEFAS and measurements taken
specifically for the project.
Limited modelling was undertaken to quantify the impacts of the cabling
operations. An assessment was completed based on the likely type and volume
of sediment that would be disturbed, an assessment of the fall velocity of the
disturbed sediment and the ambient tidal flows that would carry suspended
sediment.
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