Page 51 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
4.2 Physical Processes
This section considers the hydrodynamic (waves and tides) and sediment transport regimes
within Alderney’s territorial waters and is inherently linked to Marine Geomorphology
(Section 4.1).
4.2.1 Baseline Description
This baseline description initially presents the wider regional setting followed by an outline of
Alderney’s territorial waters and finally, any available local level information.
4.2.1.1 Regional setting
The English Channel is a semi-enclosed sea that narrows towards the east. The hydrodynamic
conditions here are predominately controlled by the tidal regime. The maximum tidal range in
the English Channel varies from 6 to 10 m and is greatest in the Channel Islands Gulf
(Grochowski et al., 1993). Influence is also enforced, though to a lesser degree, by wind and
pressure gradients.
The main tidal wave that propagates through the Channel approaches from the west to the
east. Due to the narrowing in the central and eastern parts of the Channel, regional spatial
variations in tidal velocities are observed. To the east of the Cotentin Peninsula, and including
the Channel Islands, the maximum annual mean spring and neap current speeds are of the
order of 4 m/s to 1.5 m/s respectively.
Waves originating from the North Atlantic enter the English Channel from the west and are able
to propagate directly to Alderney. Locally generated waves will also impart some influence. In
deep water, it is wind that dominates the character of the waves. However, as waves travel
into shallower, nearshore waters they are affected by refraction, shoaling and diffraction due to
depth variation with the wave crests tending to realign with the bed contours; refraction by
currents; and energy dissipation through friction and breaking.
At a regional scale, Alderney is located to the west of a north - south bedload parting zone,
extending from the Isle of Wight, England to the Cotentin Peninsula, northern France (Kenyon
and Cooper, 2004). Net sediment transport either side of this zone is directed away from the
central axis and thus, with respect to Alderney, any potentially mobile sediment is transported
away from the area to the west of the island. Although the presence of a bedload parting zone
near this region is not in dispute, Dix et al. (2007) notes that:
“The precise location of the divergence axis in the central English Channel is not the same in
all reports and is variously located: in a broad strip at an oblique angle between St Catherine’s
Point and the west of the Cotentin Peninsula (Kenyon and Stride, 1970); a north to south line
between St Catherine’s Point and the east of the Cotentin Peninsula (Grochowski et al., 1993);
and, in a north to south strip between The Needles and the centre of the Cotentin Peninsula
(this study). Although Grochowski et al. (1993) predict a very precise and narrow axis, the
present study found the axis region to be much broader; it is unlikely that the results of Kenyon
and Stride (based on interpretation of sparse geophysical data) could resolve a clear line
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