Page 46 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
ƒ Time series of swathe bathymetry which, placed into context using historical chart
analysis, could determine the mobility of any seabed features;
ƒ Seabed sediment grab samples to ‘ground-truth’ the surveys of sediment composition;
and
ƒ Geophysical surveys of the development area.
The collection of this information should provide further understanding of sediment transport
pathways and suspended sediment concentrations (SSC). In addition given the high energy
nature of the environment ongoing monitoring of sites such as the Casquets Banks may be
needed to better understand the baseline geomorphology and natural background fluctuations.
4.1.1.5 Study area
The study area will need to encompass any pathways which connect the Draft Plan with
receptors (e.g. seabed features whose form and function might be altered by the placement of
a tidal turbine device). It is therefore recommended that, at a minimum, the near-field study
area include all of Alderney’s territorial waters and the interconnector cable route, with the far-
field incorporating at least one tidal ellipse excursion from these boundaries. Regional scale
modelling indicates that mean tidal excursions of greater than 30 km can be expected
(ABPmer, 2008). The wider study area shown on Figure 2 encompasses these wider-scale
boundaries.
4.1.2 Impact Assessment
The Draft Plan has the potential to affect the marine geomorphology of the study area through
the following impact pathway:
ƒ Alteration of Seabed Form and Features (Section 4.1.2.1).
The sensitivity of seabed form and features to change is considered to depend on the
recoverability of the feature. Sandbanks are considered to have a greater sensitivity compared
to harder substrate, and therefore the sensitivity of features is considered to range between low
to moderate depending on substrate type and nature of exposure.
Alderney South Banks Subtidal Sandbank is of nature conservation importance and would
meet the criteria for designation as a subtidal sandbank under the EC Habitats Directive (see
Section 5.6.1) and therefore the marine geomorphological importance of this feature, in terms
of its level of protection, is considered to be moderate. The rest of the marine and coastal
environment in the study area is not designated specifically for physical features and is
therefore considered to be of negligible to low importance.
4.1.2.1 Alteration of seabed form and features
Alteration of seabed form and features could occur during the construction, operational and
decommissioning phases of the Draft Plan. Sandwave and megaripple clearing during
construction and/or decommissioning, if required, will alter the seabed characteristics and has
the potential to deposit material on the seabed. Once installed, hydrodynamic changes, in
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