Page 52 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
either. As such, the accuracy of the breadth or location of the reported axis cannot be
confirmed in or by any study so far.”
4.2.1.2 Alderney’s territorial waters
Tidal Regime
Alderney is located in a macro-tidal setting, with spring tidal ranges in excess of 6 m. Along the
interconnector cable route specifically, the tidal ranges increase with distance towards the
Cotentin Peninsula, and are of the order of 10 m (spring tide) at landfall (Bois et al., 2012).
Figure 8 shows spring and neap tide flow speeds around Alderney. Tidal flows are
predominately to the north-east within Alderney’s territorial waters until, approximately, half way
along the interconnector route when a series of re-circulation patterns occur (Figure 9). Tidal
flow speeds remain highest within Alderney’s waters, reducing along the cable route towards
landfall (Bois et al., 2012). Tidal excursion ellipses for a mean tide are shown to exceed 30 km
at locations with Alderney’s territorial waters (ABPmer, 2008).
A series of Acoustic Dopler Current Profiler (ADCP) measurements have been taken to
characterise the hydrodynamic regime on a more local scale, particularly for the Project 1 area.
A series of metocean campaigns (Osiris Projects, 2009d) undertaken at three discrete locations
within Project 1 (T61, T74, T75; see Figure 6) in circa 42 m water depths, allow for a refined
description of the tidal conditions. Characteristic tidal properties at these three locations at the
seabed are:
ƒ Peak spring currents range from 1.5 m/s to 4.4 m/s;
ƒ Peak neap currents range from 1.0 m/s to 2.5 m/s;
ƒ The regime is ebb dominated;
ƒ Tidal flow is along a NE (flood) to SW (ebb) axis. This tidal direction also occurs on the
opposite side of the island, with flow along a NW to SE axis at the north and south of
Alderney (ABPmer, 2008);
ƒ Surface current perturbations, for example eddies, are contained within 4 m of the
surface; and
ƒ Bed related turbulence is contained within 1 m of the bed.
The interaction of the fast flowing tides with structures, for example headlands and rock
pinnacles, are likely to result in eddy formations. Indeed historic literature documents the
presence of large scale eddies within the Casquets (White, 1835).
Elsewhere, tidal diamond information provides an indication of the tidal flows within the
territorial waters of Alderney:
ƒ In The Swinge, spring and neap peak flows reach 3.5 m/s and 1.4 m/s, respectively;
ƒ Outwith Alderney Harbour, spring and neap peak flows reach 1.7 m/s and 0.8 m/s,
respectively; and
ƒ In the Ortac Channel, spring and neap peak flows reach 2.8 m/s and 1.2 m/s,
respectively.
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