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 />
Table 4.4: Examples <strong>of</strong> physical disturbance <strong>of</strong> the seabed by seabed<br />
uses<br />
Deep water<br />
90<br />
Activity Spatial Extent /<br />
Unit Area<br />
UK Offshore pipelines 5084km length <strong>of</strong> pipeline installed; giving total<br />
length <strong>of</strong> 5847km (assuming a 15% contingency for<br />
interlink pipelines)<br />
UK Oil <strong>and</strong> gas<br />
installations<br />
Shallow water<br />
UK Aggregate extraction<br />
(for 2005)<br />
UK Dredging disposal<br />
(for 2004)<br />
240 <strong>of</strong>fshore platforms<br />
180,000 m 2 (assuming average plan area <strong>of</strong><br />
30 m x 25 m x 240 <strong>of</strong>fshore platforms)<br />
134 km 2 (area actually dredged)<br />
1257km 2 (licensed)<br />
21.2 m tonnes<br />
310 km 2 (licensed)<br />
30.1 m tonnes (wet tonnage disposed)<br />
4.5 Dispersal <strong>and</strong> Re-Deposition <strong>of</strong> Sediment<br />
4.5.1 METHODS<br />
Reference/Source<br />
Ongoing research by Bomel Ltd.<br />
on behalf <strong>of</strong> HSE<br />
Ongoing research by Bomel Ltd<br />
on behalf <strong>of</strong> HSE<br />
Crown Estate (2005)<br />
Cefas (pers. comm.)<br />
Once the sediment has been lifted into suspension it will disperse under the<br />
action <strong>of</strong> the tidal flows. The strength <strong>of</strong> these currents <strong>and</strong> the dispersion <strong>and</strong><br />
settling characteristics <strong>of</strong> the suspended sediment will determine the footprint<br />
over which the sediment will be deposited.<br />
S<strong>and</strong>s <strong>and</strong> gravels disturbed during the cable burial operations will settle back to<br />
the seabed very rapidly <strong>and</strong> the footprint is unlikely to extend any great distance<br />
from the cable route. Silts, clay <strong>and</strong> chalk particles will remain in suspension<br />
for a greater period <strong>of</strong> time <strong>and</strong> will be dispersed over a much greater distance,<br />
depending upon the strength <strong>of</strong> the tidal currents. However, the depth <strong>of</strong><br />
deposition over such a large area is likely to be small.<br />
Relatively simple dispersion <strong>and</strong> deposition calculations can be undertaken<br />
to assess the magnitude <strong>of</strong> the problem based on tidal flow data presented in<br />
the form <strong>of</strong> “tidal diamonds” on published Admiralty charts or measurements<br />
from the site. Alternatively more sophisticated methods may be adopted using<br />
sediment dispersion models to track the movement <strong>of</strong> sediment particles. Such<br />
models require tidal flow inputs from hydrodynamic models.<br />
When calculating the predicted physical changes it is important that the changes<br />
are put into context with natural <strong>and</strong> other anthropogenic changes that occur.<br />
The effect <strong>of</strong> the physical change on receptors within the footprint <strong>of</strong> change will<br />
be dependent on a number <strong>of</strong> factors including the ambient levels <strong>of</strong> suspended<br />
sediment <strong>and</strong> the degree <strong>of</strong> variation throughout the year, for example during<br />
storm events. If the natural levels <strong>of</strong> suspended sediment <strong>and</strong> the seasonal