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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Physical change<br />
A similar approach may be adopted for a plough but it should be appreciated<br />
that these tools displace rather than remove sediment from the seabed.<br />
As an example, a wheel cutter tool 250mm wide forming a 1000mm deep trench<br />
at a rate <strong>of</strong> 250m/hour will cut 62m 3 /hr. <strong>Review</strong>ing evidence from l<strong>and</strong> trials <strong>of</strong><br />
these tools <strong>and</strong> also reviewing subsea video a reasonable estimate <strong>of</strong> only 10 to<br />
15% <strong>of</strong> cut material will backfill into the trench with the remainder deposited at<br />
the sides <strong>of</strong> the cut trench or removed as suspended material.<br />
In the absence <strong>of</strong> more specific information, it is reasonable to assume all fine<br />
sediment (clays, silts <strong>and</strong> s<strong>and</strong>s) disturbed during the cable burial operations<br />
would be brought into suspension. The distribution <strong>of</strong> this sediment throughout<br />
the water column will depend upon the size <strong>of</strong> the particles <strong>and</strong> level <strong>of</strong><br />
disturbance caused by the cable burial system. Although coarser sediments are<br />
also likely to be brought into suspension this material will quickly settle back to<br />
the seabed <strong>and</strong> is unlikely to be dispersed by tidal currents.<br />
4.4 Seabed Disturbance by Other Activities<br />
When considering the potential impact <strong>of</strong> cable installation as a whole, it is<br />
necessary to put these into context with other influencing factors including<br />
natural perturbations such as storm activity <strong>and</strong> other seabed users such as oil<br />
<strong>and</strong> gas installations <strong>and</strong> aggregate extraction, both <strong>of</strong> which cause disturbance<br />
to the seabed. The main impact associated with cable installation relates to<br />
the physical disturbance <strong>of</strong> seabed <strong>and</strong> the subsequent creation <strong>of</strong> a sediment<br />
plume. These impacts are however localised in nature <strong>and</strong> are, in general, one<strong>of</strong>f,<br />
short-term effects, with the seabed usually returning to its original state.<br />
When compared with the area affected by other activities, the spatial extent <strong>of</strong><br />
cable installation is very small. Table 4.4 provides an indication <strong>of</strong> the extent <strong>of</strong><br />
physical disturbance caused by other seabed uses in the UK continental shelf. At<br />
certain sites the cumulative <strong>and</strong> in-combination effects with other activities <strong>and</strong><br />
projects will need to be considered during the assessment process in order to<br />
determine the overall effect on existing environmental parameters.<br />
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