Page 322 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

304 Fundamentals of Ocean Renewable Energy

winter months, when there was significant natural variability due to an energetic,
yet highly variable, wave climate. In summer months, there was less natural
variability, and so a smaller array (52 MW) was sufficient to exceed natural
variability. It is therefore important to consider the timescale that is used to
measure the impact of a TEC array on sedimentary processes, when assessing
how the impact compares to natural variability. This has implications for time
periods when instruments suitable for monitoring are deployed, especially since
winter deployments tend to be more challenging than summer deployments.
All of the studies before considered single TEC arrays. Many regions, such
as the Pentland Firth in Scotland, are expected to accommodate several discrete
arrays. Therefore, the cumulative impact of arrays on sediment dynamics should
be considered in such regions. Fairley et al. [52] demonstrated that the impacts
of four leased sites in the Pentland Firth, somewhat unexpectedly, combined
linearly (i.e. the sedimentary impacts of each array could be considered in
isolation), and then combined. However, the authors comment that this may not
necessarily be the case for very large scales of energy conversion. In addition,
it is noted that the commercial model used (MIKE3) was not explicit about the
energy extraction term, and so it is difficult to compare the methodology against
other studies that used open access code (e.g. [37,38,63]).

10.4.2 Tidal Lagoons

There is very little published research that examines the impact of tidal lagoon
power plants on sediment dynamics. However, there is research into indirect
impacts (e.g. changes to hydrodynamic flow fields) that can be used to infer (but
perhaps not quantify) the possible impacts of lagoons on sediments [51].
There are two physical environments where the sediment dynamics will be
affected by tidal lagoon power plants—the region that is impounded by the
lagoon, and the region that remains outside of the impoundment. In both of
these environments, the operating mode (e.g. ebb-only, flood-only, or flood and
ebb generations) will have a significant influence on the sediment dynamics. For
example, counter-rotating eddy systems may form in the turbine jets [64], lead-
ing to localized changes in sediment transport: an impact that can be minimized
by optimizing the operating mode, and evenly distributing the turbines along
the lagoon structure [65]. Inside the lagoon, and away from the power house,
there will be a significant reduction in energy, both in terms of the wave climate
and tidal currents. In particular, in the absence of any real wave climate (other
than for very short-period waves generated within the severely fetch-limited
local confines of the impoundment), there will be negligible interannual and
intraannual variabilities in sediment dynamics and associated morphodynamics.
Since sediment exchange between the lagoon and the regional ocean would be
so drastically altered, so too would be the sediment dynamics in the nearshore
environment (e.g. beach processes). In addition, processes driven by waves
(e.g. wave radiation stresses leading to longshore transport) will be altered by
the presence of a tidal lagoon power plant, with significant consequences for