Page 310 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Other Aspects of Ocean Renewable Energy Chapter | 10 295

10.4.1 Impact of TEC Arrays on Sediment Dynamics
Neill et al. [35] first introduced the possibility of nonlocalized, long timescale,
changes in morphodynamics as a result of tidal array operation. Prior to
this research, it had been assumed that either (a) highly energetic tidal sites
were devoid of mobile sediment or (b) any impacts would be localized, i.e.
scouring of sea bed sediments around the base of tidal energy devices would
be the main issue. However, even in highly energetic environments such as the
Pentland Firth (Scotland), the predominant bedrock will be interspersed with
regions of sand (e.g. [52]). Such pockets of mobile sediment are important
habitats for fisheries, and important repositories of sediment that exchange
material with neighbouring beaches over a range of timescales [53]. In addition,
larger offshore sand banks, such as headland sand banks generated by eddies
associated with strong tidal flow past headlands [54], have a significant and
important influence on the flow field [55]. Therefore, any changes in the
morphodynamics of offshore sand banks due to tidal energy extraction will
likely have a significant impact on the tidal energy resource.

Tidal Asymmetry
Since astronomical tides are generated by the combined gravitational forces of
the Sun and the Moon, the frequencies of tidal constituents in the deep oceans
directly relate to lunar or solar days, and can be expressed in terms of diurnal
and semidiurnal components. The propagation of (barotropic) tides in the
deep ocean is primarily governed by linear processes, where their interactions
generate subharmonic tides [56]. For instance, the combination of the principal
semidiurnal lunar (M 2 ) and solar (S 2 ) tidal constituents describes the spring
neap cycle (Section 3.9).
Over continental shelves, and particularly in shallow coastal waters, other
nonlinear forces and processes such as friction, advection (due to advective in-
ertia forces), and diffusion (due to turbulence) become increasingly responsible
for the dynamics of the tides. As a result, the tidal signal is more complex in
such regions, and can no longer be represented by simple linear superposition
of semidiurnal and diurnal components. Using the concept of Fourier series,
by combining higher-frequency tidal components or superharmonic tides, any
nonlinear tidal signal can be reconstructed. Unlike astronomical tides, super-
harmonic tidal components are generated by localized shallow water forces.
Accordingly, the nonlinear interaction of an astronomical tidal component with
itself and other tidal components generates overtides and compound tides,
respectively, with higher frequencies, for example,
), ...
M 4 (2ω M 2 ),M 6 (3ω M 2 ),S 6 (3ω S 2 ),MS 4 (ω M 2 + ω S 2 ),MN 4 (ω M 2 + ω N 2
(10.9)
Overtides and compound tides are the main causes of tidal asymmetry, and
their role in understanding and accurately simulating tides is very important in