Page 317 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

300 Fundamentals of Ocean Renewable Energy

impact on morphodynamics. It is therefore recommended, from both impact
and resource perspectives (since regions of symmetry lead to balanced power
generation between flood and ebb phases of the tidal cycle), that where possible,
regions of tidal symmetry should be favoured for siting TEC arrays. In addition,
Neill et al. [35] found that the impacts of tidal energy conversion influenced
far-field sediment dynamics and morphodynamics, of order 50 km from the
array, and the impacts were not just confined to the site of energy extraction,
as assumed previously.
Offshore Sand Banks
Offshore sand banks are common features of many coastlines around the
world. They represent repositories of sediment that exchange material with
neighbouring beaches over various timescales [53]. In addition, they are an
important natural form of coastal defence, since the shallow waters associated
with offshore sand banks reduce the energy of storm waves via depth-induced
wave breaking. Offshore sand banks are also an important source of aggregate
for the construction industry [61], and can act as nursery grounds for fisheries.
For the above reasons, it is important that tidal energy extraction does not
interfere with the natural cycle of sand bank evolution, and it is this influence
that we investigate here. In particular, many regions of strong tidal flow that are
suitable for tidal energy conversion are associated with offshore sand banks, and
one common category is the headland sand bank.
Strong tidal flow past a headland leads to the generation of eddy systems,
with an opposite sense of vorticity between the flood and ebb phases of the tidal
cycle (Fig. 10.15). Within each eddy system, the outward-directed centrifugal
force (CF) is balanced by an inward-directed pressure gradient force (PG). Since
velocities are weaker close to the sea bed (e.g. Fig. 3.19), the centrifugal force
is weaker at the sea bed, and stronger higher in the water column. The inward-
directed pressure gradient force (which is the same at all depths) can therefore
exceed the centrifugal force at the sea bed, and this leads to flow towards the
centre of the eddy system at the sea bed (balanced by an outward directed flow
at the sea surface, where CF > PG). In fact, the result is actually a ‘spiralling’
flow, since the system is 3D (e.g. consider this secondary flow superimposed
on the 2D horizontal eddy systems shown in Fig. 10.15). The inward-directed
flow at the sea bed leads to net movement of sediment (e.g. via bed load
transport) towards the centre of the eddy system. Over time, this net transport of
sediment leads to the formation of ‘headland sand banks’—one on either side
of the headland (Fig. 10.15). Evolution over long timescales results in a system
7
that is in equilibrium, with the sand bank influencing the flow field, and vice
versa. However, headlands are attractive regions for siting tidal energy arrays

7. Although the tides are relatively constant, sand bank variability will still be influenced by wave
action, for example, at storm and seasonal timescales.