Page 44 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
of fissures and weathered joints (Osiris Projects, 2009a). The seabed within Project 1 is
characterised by exposed bedrock and rock pinnacles interspersed with sand and gravel
populations of differing sizes. For the remaining seabed within Alderney’s territorial waters,
information is available from Admiralty Charts, which, combined with an understanding of the
hydrodynamic and sedimentological regimes, allows a more generic assumption of the seabed
form to be determined, as presented below.
Small and medium-scale morphodynamic features
Within Project 1 (specifically block T74), along the southeastern flank of Alderney, sand and
gravel waves up to 4.5 m high with wavelengths of 160 m to 190 m have been observed. In
addition smaller megaripples up to 1 m high with wavelengths between 3 m and 7 m have also
been observed (Figure 5) (Osiris Projects, 2009b). These features have also been recorded
both within and adjacent to the proposed interconnector route between Alderney and France
(Osiris Projects, 2009c). It is also expected, given the similar hydrodynamic conditions and
seabed sediments, that these features are present elsewhere within the area of the Draft Plan.
Large-scale morphodynamic features
There are a number of sandbank features within the territorial waters of Alderney. The most
understood feature is South Banks, which is located within Project 1. This headland associated
sandbank is located to the south of Alderney and extends for up to 4 km in length. Such banks
are classified as ‘banner banks’ and are typically separated from the headland by a channel
swept clear by tidal flows (Kenyon and Cooper, 2004). The location of such features is
maintained by the tidal flows. In the instance of South Banks, it is the recirculation of tidal flows
in the lee of Alderney, during the tide’s ebb phase, that maintains this feature (Neill et al .,
2012). Migrating sand waves with rates of 1.4 m per day (Haynes et al., 2013) are present on
the flanks of South Banks, in addition to superimposed megaripples. The head and tail of
South Banks is characterised by trains of sand waves (Figure 6). In contrast to the exposed
bedrock on the surrounding seabed, South Banks represents a substantial reserve of sand and
gravel material, which is interspersed with shell fragments (Figure 7).
A comprehensive suite of surveys undertaken by Seastar Survey Ltd have enabled the
characteristics of and any changes in the size and shape of the South Banks and the long term
pattern of movement to be further understood (Axelsson et al., 2011). It has been shown that,
in response to the tidal flows, sand is continually circulated in a clockwise direction around
South Banks with a bedload transport convergence zone present along the bank crest (Haynes
et al., 2013). However, it is currently not possible to ascertain the sediment source for South
Banks, with potential sources being the intertidal, an active scour area at the north of the island
or further offshore towards the north-east (Haynes et al., 2013).
An example of a similar feature is the Casquets Banks (SSW and SSE) located within the
licence blocks associated with the Project 2 area. It is reported that the SSE bank is composed
of coarse sands and gravels, interspersed with shells. Historic documentation indicates that it
is a large scale feature, having previously been surveyed at approximately 8 km long and 1 km
wide (White, 1835).
R/4001/7 20 R.2129