Review of Cabling Techniques and Environmental Effects Applicable

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