Abstract
High-flow tidal stream environments, targeted for tidal turbine installations, exhibit turbulent features, at fine spatio-temporal scales (metres and seconds), created by site-specific topography and bathymetry. Bed-derived turbulent features (kolk-boils) are thought to have detrimental effects on tidal turbines. Characterisation of kolk-boils is therefore essential to inform turbine reliability, control, and maintenance strategies. It will also improve the understanding of potential ecological interactions with turbines, as marine animals use these sites for foraging. Unmanned aerial vehicle (UAV), or drone, imagery offers a novel approach to take precise measurements of kolk-boil characteristics (distribution, presence, and area) at the surface. This study carried out sixty-three UAV surveys within the Inner Sound of the Pentland Firth, Scotland, UK, over four-day periods in 2016 and 2018. Kolk-boil characteristics were examined against relevant environmental covariates to investigate potential drivers of presence and area. The results show that distribution at the surface could be predicted based on tidal phase, with current velocity significantly influencing presence above 3.0 m/s. The technique can be used to inform turbine development, micro-siting and provide better understanding of environmental implications of turbine operation. Finally, it highlights the suitability of UAVs for capturing rapid fine-scale hydrodynamic data in the absence of in situ measurements.
Highlights
With global concern over the impacts of climate change, the 2015 Paris Agreement necessitating net-zero carbon emission energy systems by 2100, and many countries committing to achieving this by 2050, there is an increasing demand to use renewable energy sources [1,2,3]
Spatial clustering of kolk-boils was apparent within the centre of the channel, as well as a clear asymmetry being observed when the distribution of these features was examined based on tidal phase (Figure 6)
Formed kolk-boils detected on the ebb phase (27.1% of all kolk-boils) were predominantly observed at the western side of the Inner Sound, while those detected on the flood phase (72.9% of all kolk-boils) were found in the central and eastern areas of the channel
Summary
With global concern over the impacts of climate change, the 2015 Paris Agreement necessitating net-zero carbon emission energy systems by 2100, and many countries committing to achieving this by 2050, there is an increasing demand to use renewable energy sources [1,2,3]. A key sector of the industry is tidal stream energy extraction (hereafter tidal energy), which offers a predictable, renewable, energy source with a worldwide projected extractable capacity in excess of 120 GW [5,6]. Tidal forcing causes the formation of bedderived turbulent flow structures, such as kolks, bursts and kolk-boils, created by the water interacting with the bathymetry [8]. It generates coastally-derived features (eddies and wakes), as a result of the differences in velocities at boundaries, as well as open-water features (tidal jets) triggered by pressure changes from water height differences [8]
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