Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Physical change<br />
variation are high then the degree <strong>of</strong> impact is likely to be less. There is<br />
considerable variation in the natural <strong>and</strong> anthropogenic induced levels <strong>of</strong><br />
suspended sediment around the UK coast. Suspended sediment concentration<br />
varies around the UK from 1-327 mg/l around the English coast <strong>and</strong> 1-227 mg/l<br />
around the Welsh Coast but annual mean values are typically 1-110 mg/l (Parr<br />
et al., 1998; Cole et al., 1999). Other areas, particularly near estuaries will have<br />
higher concentrations. In the study area <strong>of</strong> the proposed Sheringham Shoal<br />
Offshore Wind Farm <strong>of</strong>f the north Norfolk coast (Scira, 2006), concentrations<br />
varied from 10mg/l to 30mg/l <strong>and</strong> between 30mg/l (in summer) to 60mg/l (in<br />
winter) for the study area for Thanet <strong>of</strong>fshore wind farm, <strong>of</strong>f the east Kent coast<br />
(Royal Haskoning, 2005). The variability in the ambient levels <strong>of</strong> suspended<br />
sediment <strong>and</strong> the seasonal variation that can be experienced mean that it is<br />
not possible to state what the potential effect <strong>of</strong> cabling could be on certain<br />
receptors.<br />
In addition an area <strong>of</strong> seabed subject to disturbance already will be less affected<br />
by subsequent changes. In order to put the potential effects into context it should<br />
be considered against a number <strong>of</strong> other activities that occur within the marine<br />
environment, where studies have been completed to investigate potential <strong>and</strong><br />
actual effects, e.g. aggregate extraction <strong>and</strong> fishing activity. Aggregate activity<br />
is a well controlled <strong>and</strong> monitored activity where levels <strong>of</strong> suspended sediment<br />
have been measured. There are a number <strong>of</strong> potential sources <strong>of</strong> increased<br />
suspended sediment concentrations including the release <strong>of</strong> material at the<br />
cutter or drag-head, overspill during hopper loading <strong>and</strong> sieving (which may<br />
be necessary in order to obtain an optimum sediment load). Of these potential<br />
sources the material suspended at the drag-head is the most likely to be similar<br />
to the material suspended during ploughing for cabling activities. Measurement<br />
<strong>of</strong> the plume generated by the movement <strong>of</strong> the drag-head alone have shown<br />
that the volume <strong>of</strong> sediment introduced into the water column is barely<br />
detectable <strong>and</strong> is in the order <strong>of</strong> 1% <strong>of</strong> the material introduced by screening<br />
<strong>and</strong> overflowing (Hitchcock et al., 1998, John et al., 2000). The scale <strong>of</strong> effect<br />
will obviously vary dependent on the sediment size <strong>and</strong> the relative amounts <strong>of</strong><br />
overspilling/sieving undertaken but does provide a reasonable indication <strong>of</strong> the<br />
relative scale <strong>of</strong> effect when compared to aggregate extraction. It must also be<br />
borne in mind the temporary nature <strong>of</strong> the effect which is limited to one event<br />
(per cable) over a short time scale along with the immediate start <strong>of</strong> recovery <strong>of</strong><br />
the seabed following disturbance.<br />
The low levels <strong>of</strong> sediment that are mobilised during cable laying mean that there<br />
will be only low levels <strong>of</strong> deposition around the cable route. The finer material will<br />
generally remain in suspension for longer but will settle <strong>and</strong> remobilise on each<br />
tide with no measurable material left in place. Coarser sediments are expected to<br />
settle within a few metres <strong>of</strong> the cable route <strong>and</strong> following disturbance are likely<br />
to recover rapidly, given similar communities in the vicinity.<br />
Case studies are provided below which illustrate some <strong>of</strong> the predicted physical<br />
changes resulting from cable laying activities. The resulting direct <strong>and</strong> indirect<br />
91