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Other Aspects of Ocean Renewable Energy Chapter | 10 285

effects such as wind-driven currents), and less than the uncertainties associated
with both numerical models and observations (which tend to be around 5–10%).
However, tidal power is related to velocity cubed, and so these relatively small
changes in current speed would be amplified in the power signal, leading to a
4–7% increase in instantaneous power. Although they incorporate large-scale
tidal energy extraction in their simulations, De Dominicis et al. [23] similarly
show changes of order 1–2 cm/s in the middle of the 21st century across many
regions of the northwest European shelf seas due to RCP8.5 (Fig. 10.7).

10.2 INTERACTIONS OF ENERGY RESOURCES AND ENERGY
DEVICES (RESOURCE UNCERTAINTY)

Marine renewable energy resources interact with one another through wave-
tide interaction—a process in which short waves interact with long waves. In
addition, the extraction of energy from the marine environment will influence
the resource itself, and this can be an important consideration for large levels
of energy extraction. It is possible to account for the processes of wave-current
interaction and feedbacks between resource exploitation and the resource itself
in numerical models of the marine renewable energy resource. Both issues,
combined under the common theme of ‘resource uncertainty’, are discussed in
this section.

10.2.1 Wave-Tide Interaction
Some regions of the world (e.g. the northwest European continental shelf seas)
concurrently experience both energetic wave climates and tidal current regimes.
The interaction of waves and tides can affect resource characterization, device
performance, and maintenance. In general, we can investigate how waves affect
a tidal energy site, or conversely: how tides can affect a wave energy site.
Wave properties such as group velocity, wave height, and wave power are
altered in the presence of tides. For instance, wave frequency/period is affected
by the Doppler shift. When waves oppose tidal currents, wave height, and
wave steepness increase. Strong opposing tidal currents can even lead to wave
breaking, and a complete blockage of wave energy propagation.
Previous studies have shown that the presence of waves in relatively shallow
coastal environments leads to an enhancement of the (apparent) bed roughness.
This enhancement of bottom friction can reduce the speed of tidal currents,
and lead to a reduction of the tidal energy resource. Further, extreme waves
can damage tidal energy devices; therefore, tidal energy devices do not operate
during these extreme events. If these events happen frequently, they will
significantly reduce the annual energy yield from tidal devices.
Therefore, in sites where wave-tide interactions are significant, further
assessment is required to consider these interactions. Here, we provide a brief