Geoscience Reference
In-Depth Information
has mixed the thin sequence. Although sea-bed
sediments deeper than 40 m are generally too
deep for common wave disturbance, and thus
contain silts and clays, sediments deeper than
100 m are able to be mobilized, either by oscill-
atory currents driven from long-period swell
waves, or by wind-driven currents. Over large
areas of the shelf, storms resuspend fine sedi-
ments and may create transient nepheloid layers
near the bed. On the middle shelf, the natural
very fine sands are thus very well sorted and uni-
modal in nature.
On board the vessel, the mining process
involves sorting of sediments into cobbles,
pebbles and tailings (sand, silt and clay), which
fractions are then discarded over the side via a
conveyor belt or a tailings pipe, so that mining
activity tends to increase the patchiness of the
sea bed. Off southern Namibia, in mined areas,
the well-sorted sands are replaced by poorly
sorted and heterogeneous sediments, because
of the addition of gravels and coarser sands
derived from the mined buried lowstand sedi-
ments (Fig. 10.12). In areas of the inner shelf, in
depths of
Between these units runs a network of cables,
usually buried beneath sediment. The UK is a
leader in the use of this technology, and late in
2005, electricity was already being supplied to
the UK National Grid from three offshore wind-
farms. The first sites each occupy
c
.10km
2
of
sea bed and have up to 30 turbines per site, but
this is due to increase greatly in the near future
to as many as 200 -300 turbines per site.
There are potential sedimentary effects during
the construction, operation and decommission-
ing phases of offshore windfarms (Rees, in Judd
et al. 2003). In terms of physical sedimentary
effects, applications for windfarm developments
are considered on a site-by-site basis, requiring a
review of the local and regional coastal processes
and assessment of whether the structures might
change sediment transport patterns, rates and
pathways. Monitoring programmes are under-
taken to test the predictions made by the Environ-
mental Impact Statements. Such work builds
upon basic knowledge of sedimentary impacts
of flows around structures (e.g. Allen 1984), and
includes accurate swath-bathymetric surveys
(Fig. 10.13), repeated at intervals to determine
sediment transport associated with bedform
migration, and overall changes in bed elevation.
Of major interest are the processes of scour
around the monopiles. Observation and model-
ling appear to indicate that, even in areas of
highly dynamic sediment transport, the volume
of material removed by scour around individual
monopiles is insignificant compared with that
associated with bedform migration across the
whole windfarm site. Further, spacing between
monopiles of 300 - 400 m appears to be sufficient
to produce no combined significant sedimentary
impacts (Fig. 10.13). This accords with the 'rule
of thumb' of little impact upon net flow at 6 -10
obstacle diameters downstream. Hence, although
there is a range of research aimed at improving
understanding of the environmental impacts of
windfarms, including a wide range of ecological
effects (Gill 2005), there are unlikely to be major
sedimentary consequences. The Netherlands'
Government has chosen to wait for results from
test sites before committing to significant develop-
ment of offshore windfarms.
40 m, storms have a relatively great
impact in transporting gravel, compared with
mining. Although immediate impacts tend to
be local, the area mined is expanding rapidly,
and 'conservation corridors' are being proposed
between mining lanes to preserve the Holocene
sequence in some areas, and to provide refuges
for benthic organisms to improve recolonization
of the substrate after the cessation of mining.
Such zones are also proposed between aggregate
dredging lanes (Boyd et al. 2004).
<
10.3.2.5 Offshore windfarms
For many countries, a major driver for the
development of Offshore Renewable Energy
Developments (OREDs) is the attempt to reduce
CO
2
emissions in response to the Kyoto Pro-
tocol and to diversify energy supply. The main
active OREDs are offshore windfarms, which
comprise an array of large individual wind tur-
bines, each comprising an impellor (typically of
three or four blades) located on top of a tower
connected to a monopile attached to the sea bed.
