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https://doi.org/10.5194/os-2020-23
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2020-23
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  07 Apr 2020

07 Apr 2020

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This preprint is currently under review for the journal OS.

Bardsey – an island in a strong tidal stream: Underestimating coastal tides due to unresolved topography

J. A. Mattias Green1 and David T. Pugh2 J. A. Mattias Green and David T. Pugh
  • 1School of Ocean Sciences, Bangor University, Menai Bridge, UK
  • 2National Oceanography Centre, Joseph Proudman Building, Liverpool, UK

Abstract. Bardsey Island is located at the western end of the Llŷn Peninsula in north-west Wales Separated from the mainland by a channel some 3 km wide, it is surrounded by reversing tidal streams of up to 4 ms−1 at spring tides. These local hydrodynamic details and their consequences are unresolved by satellite altimetry, nor are they represented in regional tidal models. Here we look at the effects of the island in the strong tidal stream in terms of the formation and shedding of eddies, and the budgets for tidal energy dissipation. We show, using local observation and a satellite altimetry constrained product, that the island has a large impact on the tidal stream, and that even the latest altimetry database seriously under-represents the tidal stream due to the island not being resolved. The effect of the island leads to an underestimate of the current speed in the altimetry data in the channel of up to a factor of three, depending on tidal state, and the average tidal energy resource is underestimated by a factor 6. The observed tidal amplitudes are higher at the mainland than at the island, and there is a detectable phase lag in the tide across the island – this effect is not seen in the altimetry data. The underestimate of the tide in the altimetry data has consequences for tidal dissipation and wake effect computation and show that local observations are key to correctly estimate tidal energetics around small-scale coastal topography.

J. A. Mattias Green and David T. Pugh

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J. A. Mattias Green and David T. Pugh

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Bardsey – an island in a strong tidal stream J. A. M. Green and D. T. Pugh https://doi.org/10.17605/OSF.IO/KVGUR

J. A. Mattias Green and David T. Pugh

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Short summary
Bardsey Island lies 3 km offshore the western end of the Llŷn Peninsula in north-west Wales. But the island is too small to show up in tidal databases based on satellite data, so they may not provide the correct local tides. Our new sea-level data from the area shows that the tidal currents in the satellite databases are 1/3 of the observed. Any investigation of other coastal activities, e.g., renewable energy installations, must use local observations to get the correct tides.
Bardsey Island lies 3 km offshore the western end of the Llŷn Peninsula in north-west Wales....
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