Review of Cabling Techniques and Environmental Effects Applicable

Physical change
variation are high then the degree of impact is likely to be less. There is
considerable variation in the natural and anthropogenic induced levels of
suspended sediment around the UK coast. Suspended sediment concentration
varies around the UK from 1-327 mg/l around the English coast and 1-227 mg/l
around the Welsh Coast but annual mean values are typically 1-110 mg/l (Parr
et al., 1998; Cole et al., 1999). Other areas, particularly near estuaries will have
higher concentrations. In the study area of the proposed Sheringham Shoal
Offshore Wind Farm off the north Norfolk coast (Scira, 2006), concentrations
varied from 10mg/l to 30mg/l and between 30mg/l (in summer) to 60mg/l (in
winter) for the study area for Thanet offshore wind farm, off the east Kent coast
(Royal Haskoning, 2005). The variability in the ambient levels of suspended
sediment and the seasonal variation that can be experienced mean that it is
not possible to state what the potential effect of cabling could be on certain
receptors.
In addition an area of seabed subject to disturbance already will be less affected
by subsequent changes. In order to put the potential effects into context it should
be considered against a number of other activities that occur within the marine
environment, where studies have been completed to investigate potential and
actual effects, e.g. aggregate extraction and fishing activity. Aggregate activity
is a well controlled and monitored activity where levels of suspended sediment
have been measured. There are a number of potential sources of increased
suspended sediment concentrations including the release of material at the
cutter or drag-head, overspill during hopper loading and sieving (which may
be necessary in order to obtain an optimum sediment load). Of these potential
sources the material suspended at the drag-head is the most likely to be similar
to the material suspended during ploughing for cabling activities. Measurement
of the plume generated by the movement of the drag-head alone have shown
that the volume of sediment introduced into the water column is barely
detectable and is in the order of 1% of the material introduced by screening
and overflowing (Hitchcock et al., 1998, John et al., 2000). The scale of effect
will obviously vary dependent on the sediment size and the relative amounts of
overspilling/sieving undertaken but does provide a reasonable indication of the
relative scale of effect when compared to aggregate extraction. It must also be
borne in mind the temporary nature of the effect which is limited to one event
(per cable) over a short time scale along with the immediate start of recovery of
the seabed following disturbance.
The low levels of sediment that are mobilised during cable laying mean that there
will be only low levels of deposition around the cable route. The finer material will
generally remain in suspension for longer but will settle and remobilise on each
tide with no measurable material left in place. Coarser sediments are expected to
settle within a few metres of the cable route and following disturbance are likely
to recover rapidly, given similar communities in the vicinity.
Case studies are provided below which illustrate some of the predicted physical
changes resulting from cable laying activities. The resulting direct and indirect
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