Surface Characterisation of Kolk-Boils within Tidal Stream Environments Using UAV Imagery

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.

Comparison of dense optical flow and PIV techniques for mapping surface current flow in tidal stream energy sites

“…Due to the flow speed, high levels of turbulence are present in the Inner Sound. One common turbulent feature is the presence of kolk boils [10]. Kolk boils are rotating vertical plumes of water associated with obstacle interaction and high shear near the seabed where vortices are formed which emigrate towards the surface.…”

“…The surface expressions of kolk boils were present in the video on the left of the frame. Kolk boils are a turbulent feature associated with bottom obstacles and high shear which are common within the Inner Sound [10]. The surface expression of kolk boils is often a very smooth surface associated with the vertical movement of water.…”

“…These systems have been increasingly used in marine science [9]. UAV surveys incur less financial risk, and less physical risk, as the UAVs are typically lightweight and highly manoeuvrable [10].…”

Ecologically-sustainable futures for large-scale renewables and how to get there

“…The design, size and location of arrays can have very significant ecological effects of displacement as discussed above; large offshore windfarm arrays may essentially block the movement of highly mobile animals such as seabirds, and large tidal arrays could block marine mammals and large fishes (basking sharks) from using daily routes to foraging areas and/or annual migration routes. For tidal arrays in particular the 'downstream' changes from both the introduction of structures and extraction of energy have significant impacts on the locations of shear, turbulence structures such as kolk boils [33] upwelling [16], [17]. Very large extractions of tidal energy (> 6 GW) have been shown to have further 'downstream' effects on stratification and mixing [27] with effects of up to 10% changes in physical and biological variables over hundreds of km of distance from the site of extraction [34].…”

Investigation of Turbulent Tidal Flow in a Coral Reef Channel Using Multi-Look WorldView-2 Satellite Imagery

“…This scenario can be taken as representative of high-energy conditions such as: fast-flowing tidal currents of ~1 m/s on the continental shelf [3,4]; tidal flow through straits and inlets [5], and around headlands and islands [2]; and tidal flow over a sill [6,7]. Strong currents and associated turbulence can impact physical structures [8] and be of ecological significance [9], and in some cases can pose a challenge to making in-situ measurements.…”