Review of Cabling Techniques and Environmental Effects Applicable

Sheringham Shoal Offshore Wind Farm
Physical change
A broadly similar approach was adopted by HR Wallingford, for the assessment
of ploughing operations for the Sheringham Shoal Offshore Wind Farm.
However, within the study a more sophisticated approach was adopted for the
dispersion of fine sediment using the dispersion model SEDPLUME-RW. The
model used the hydrodynamic output from a TELEMAC-2D flow model and the
assumption of a logarithmic velocity profile through the water column to track
the 3-dimensional movement of fine sediment particles (mobile surface layer
comprising sandy gravel with less than 4% fines). The results indicated that
dispersion of sediment was rapid with concentrations dropping to less than
1mg/l above background within a single flood or ebb excursion. It was noted that
dispersion occurred at a slower rate on a neap tide than a spring tide because
of the lower rate of turbulent diffusion with footprints 4km and 9km either side
of the cable route. The scenarios included ploughing through chalk, ploughing
through a silt/clay/sand mix and trenching through chalk.
For ploughing chalk during a neap tide, the dispersion footprint extends for
around 9km in each direction, with concentrations dropping to levels of less than
1mg/l (above background) within a single flood or ebb excursion. For the spring
tide simulation the higher turbulence causes the chalk concentrations to drop
below 1mg/l (above background) within 4km of the cable route.
The results for fines arising from other bed types with high percentages of fines,
the neap tide footprint extends less than 2km, while the spring tide footprint is
very small. As before the neap tide footprint is larger due to the lower rate of
turbulent diffusion. The extent of the footprint on both tides is less than that of
chalk due to the lower amount of material available per metre length of cable
and the settling of silt during periods of slacker flows (chalk is assumed to have
zero settling). This result is applicable to much of the inter-turbine cabling within
the wind farm site where there are no exposures of chalk.
The volume of material released by trenching through chalk is much higher,
and therefore the extent and persistence of concentrations above 1mg/l is much
greater. The predicted plume extends more than 10km in either direction at a
level of up to 20mg/l (above background) on a neap tide. The model predicts
a gradual drift of the plume towards the shore over the six tides, but that the
plume has dispersed to less than 1mg/l concentration before the end of the
model run.
The footprint of silt deposition was found to extend over a wide area, but at
an undetectable rate. Even under slack water conditions, the maximum rate
of deposition over the six tide simulation was less than 0.5mm in the areas of
greatest deposition, and in most of the footprint area the rate was far less. This
result is anticipated as the deposited fines will be re-suspended on each tide,
with no measurable material left in place.
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