BLOG – SELKIE TOOL SERIES: Drones to measure surface currents at tidal stream sites

BLOG – SELKIE TOOL SERIES: Drones to measure surface currents at tidal stream sites

To properly understand if a tidal stream site is suitable to generate tidal energy two things are needed: how the currents change from day to day and variation across the site area. Existing tools, based on the use of acoustic doppler current profilers (ADCPs), either give good resolution over time, when mounted in a fixed location on the seabed, or reasonable area coverage, when used to sample different locations from a moving vessel. Moreover, these methods are expensive, incurring vessel, manpower and equipment costs, the potential loss of equipment and the requirement to operate with high safety standards. Various remote sensing techniques have been proposed to provide alternative approaches focused on currents at the water surface. One option is to collect video of the surface flows from a hovering drone and to estimate flow speed and direction from this. The method is based on tracking the movement of features on the waters surface, such as foam patches; this technique is termed large scale particle image velocimetry (LS-PIV). LS-PIV has been successfully applied to rivers, however tidal stream sites provide a greater challenge: land is not often in the field of view which makes image stabilization harder; there are typically fewer surface features to track; and waves in the image may obscure the current signature.

The aim of this Selkie work package is to develop and validate a workflow, and guidance for parameter setting, that will enable the extraction of surface currents at tidal stream sites using existing open-source PIV software and video data collected by commercially available quadcopter drones. We plan to share the method with industry, this will allow them to easily make use of the emerging technology.

It is envisaged that drone-derived surface currents will be of great benefit to the marine renewables industry for a range of purposes. Firstly, the tool will enable low-cost scoping of new sites; this will be particularly beneficial to areas without access to suitable vessels or hydrographic equipment such as remote island communities and developing nations. Secondly, the spatial resolution will be better than vessel mounted ADCP transects and so maps showing surface current patterns can be easily generated: these will compliment ADCP techniques and be beneficial both to assist with device micro-siting and to provide spatial validation for numerical models.

A range of experiments are planned to test the accuracy of the technique. Land based tests of drone hovering stability and the georectification procedure are planned imminently. Flights have been conducted at the Marine Energy Wales META tidal test site and at a location representative of a tidal stream site in Swansea Bay. Data pre-processing methodologies are being developed to remove waves from the imagery and optimise the current signatures. GPS surface drifters are in the process of being constructed to provide validation of the technique; initial validation tests are planned for March 2021 with further validation experiments planned for spring and summer 2021 in Wales. It is hoped that in 2022, assuming COVID restrictions have eased, we may be able to visit sites further afield to collect some data.

Please get in touch with Iain Fairley (i.a.fairley@swansea.ac.uk) if you are interested in learning more. It will be important to validate the techniques at a range of sites and with a range of drones; therefore, if any industry partners have current measurement programmes underway, have access to a quadcopter drone and are willing to collect some video that co-insides with current measurement, we would be interested to hear from you.

Image showing one frame from a video that was used to estimate currents with PIVlab; the results of the current estimation are overlain on top. The arrows show current direction and are coloured by speed (blue slower, red faster).
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