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