Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
Clay – A permanent scar will be left in stiff clay habitats following cabling
activity. In soft clay, infilling is expected to occur rapidly. In harder or stiffer
clays, a cutting wheel disc is often used which allows a wedge of soil to be cut by
the action of the plough. This wedge is lifted by a ramp on the plough share, the
cable is placed at the bottom of the trench and the wedge of soil then allowed
to naturally backfill onto the cable. This process leaves minimum disturbance to
the seabed with no spoil mounds. Spoil mounds are only found with ‘V’ shape
plough shares more commonly associated with pipeline burial. Clay supports
a species poor community due to the cohesive nature of the substrate. Cabling
through soft clay is likely to put more sediment into suspension than in stiff clay
where the habitat is more cohesive.
Sand – Sand will infill rapidly following disturbance by ploughing or trenching.
Burrowing species may be affected but are generally adapted to change through
natural disturbance due to the mobility of the substrate.
Gravel – Certain types of gravel habitat will infill immediately following cable
laying activity, others may leave a shallow trough following initial infill. Generally,
species inhabiting mobile gravel are adapted to harsh living conditions and
would be expected to recover quickly.
Suspended sediment
Impacts resulting from cabling include the release of sediment into suspension
(see Section 4.3). This can have a number of effects on the benthic species
inhabiting areas adjacent to the cabling activity. The significance of the impact
will be dependent on the type of sediment, the hydrodynamic conditions and the
sensitivity of the species affected in addition to the type of installation method.
The potential change in significance as a result of the type of sediment and
hydrodynamic conditions is outlined in Section 4.3.5 and summarised in Tables
4.2 and 4.3.
Increases in suspended sediment can affect filtering mechanisms of certain
species, such as specific types of worm and brittle stars, through the clogging
of gills or damage to feeding structures. Suspended sediment can also attach
to fish eggs causing abnormalities or death. The sensitivity of the receptor is an
important consideration when determining the significance of this effect. This
includes its tolerance to a given effect, but also to its potential for recovery from
such an effect (discussed in sub-section above). Its tolerance will be similar in
all areas of the UK but will also depend to some extent on its adaptability to
its ambient conditions. If a species has adapted to survive in a wide variety
of conditions (such as in estuaries where suspended sediment concentrations
(SSC) may be measured in grammes per litre), it is more likely to survive a
small increase in SSC than a species which is exposed to a lower variation in
SSC throughout the year. Information on the sensitivity (tolerance) of species
should be obtained from published literature and consultation with relevant
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