unprecedented detail due to the application of the DF technique in conjunction with
variational 2dVar interpolation. The approach was essential for accurate assessment
of circulation around the islands and in the straits were high horizontal velocity
gradients occur.
Our analysis revealed velocities exceeding 4 m/s and a pronounced asymmetry
between the flood and ebb flow varying in a wide range, 0.5–2.5, around Ushant
Island. The strongest variation of asymmetry was found in the Fromveur Strait.
Neill et al. [15] investigated spatial variation of asymmetry around the Orkney
Islands, where strong tidal currents occur, and documented the values ranging from
0.7 to 1.3. Four times larger asymmetry variation has been found around Ushant
Island, which, to the best of our knowledge, has never been documented at any
other place in the World Ocean.
HFR velocity monitoring in the Strait of Dover documented the strongest currents (up to 2.5 m/s at spring tide) occurring west of the Cape Gris Nez. This highly
energetic area (power density ∼1 kW/m
2 ), located outside the northward navigation
routes and fishing zone, appears suitable for testing energy conversion devices there
[28].
Although HFR-based observations present an adequate tool for monitoring
extreme currents at larger scale, their spatial resolution is often insufficient for the
task of accurate deployment of tidal turbines. In that respect, the presented high
resolution current mapping system Koursk can be seen as complementary to other
coastal observing systems, such as HFRs and moored ADCP arrays. The system
was successfully tested in several sites (the Solent, ports of Dover and Boulogne)
demonstrating a good combination of accuracy, portability and low cost of surveying. The system is equipped with an interpolation algorithm, which allows
reconstructing space-time evolution of the velocity field for surveys whose duration
is comparable or larger than the time scale of tidal variability (1–2 h).
Application of the technique in Boulogne Harbor demonstrated significant (30–
60%) reduction of the model-data misfit for the velocity field obtained as a result of
space-time optimal interpolation. Although the method was applied to recover
surface circulation, it can be extended for assessment of the full 4D tidal flow
dynamics using the data recorded throughout the entire water column. We believe
that the proposed current mapping system may advance understanding and
assessing coastal circulation in tidal environments, especially when used in combination with moored ADCP and/or HFR observations. The data acquired by HFRs
and towed ADCP, combined with the presented methodologies of their processing
could improve the efficiency of ongoing tidal energy conversion projects in France
and UK. The HFR data can be also used for numerical model validation and for
data assimilation in numerical models, which may improve their forecast capability.
Acknowledgements The authors acknowledge the support of the Interreg IVB (NW Europe)
“Pro-Tide” project and support from the US Office of Naval Research. We also acknowledge the
Oceanographic Division of the French Navy (SHOM) for providing ADCP and HF radar data in
the Iroise Sea. The authors thank skipper Eric Lecuyer (LOG) and also Philippe Forget and Yves
Barbin (MIO, Toulon) for their contribution to radar data processing.
Monitoring Strong Tidal Currents in Straits and Nearshore Regions
533
variational 2dVar interpolation. The approach was essential for accurate assessment
of circulation around the islands and in the straits were high horizontal velocity
gradients occur.
Our analysis revealed velocities exceeding 4 m/s and a pronounced asymmetry
between the flood and ebb flow varying in a wide range, 0.5–2.5, around Ushant
Island. The strongest variation of asymmetry was found in the Fromveur Strait.
Neill et al. [15] investigated spatial variation of asymmetry around the Orkney
Islands, where strong tidal currents occur, and documented the values ranging from
0.7 to 1.3. Four times larger asymmetry variation has been found around Ushant
Island, which, to the best of our knowledge, has never been documented at any
other place in the World Ocean.
HFR velocity monitoring in the Strait of Dover documented the strongest currents (up to 2.5 m/s at spring tide) occurring west of the Cape Gris Nez. This highly
energetic area (power density ∼1 kW/m
2 ), located outside the northward navigation
routes and fishing zone, appears suitable for testing energy conversion devices there
[28].
Although HFR-based observations present an adequate tool for monitoring
extreme currents at larger scale, their spatial resolution is often insufficient for the
task of accurate deployment of tidal turbines. In that respect, the presented high
resolution current mapping system Koursk can be seen as complementary to other
coastal observing systems, such as HFRs and moored ADCP arrays. The system
was successfully tested in several sites (the Solent, ports of Dover and Boulogne)
demonstrating a good combination of accuracy, portability and low cost of surveying. The system is equipped with an interpolation algorithm, which allows
reconstructing space-time evolution of the velocity field for surveys whose duration
is comparable or larger than the time scale of tidal variability (1–2 h).
Application of the technique in Boulogne Harbor demonstrated significant (30–
60%) reduction of the model-data misfit for the velocity field obtained as a result of
space-time optimal interpolation. Although the method was applied to recover
surface circulation, it can be extended for assessment of the full 4D tidal flow
dynamics using the data recorded throughout the entire water column. We believe
that the proposed current mapping system may advance understanding and
assessing coastal circulation in tidal environments, especially when used in combination with moored ADCP and/or HFR observations. The data acquired by HFRs
and towed ADCP, combined with the presented methodologies of their processing
could improve the efficiency of ongoing tidal energy conversion projects in France
and UK. The HFR data can be also used for numerical model validation and for
data assimilation in numerical models, which may improve their forecast capability.
Acknowledgements The authors acknowledge the support of the Interreg IVB (NW Europe)
“Pro-Tide” project and support from the US Office of Naval Research. We also acknowledge the
Oceanographic Division of the French Navy (SHOM) for providing ADCP and HF radar data in
the Iroise Sea. The authors thank skipper Eric Lecuyer (LOG) and also Philippe Forget and Yves
Barbin (MIO, Toulon) for their contribution to radar data processing.
Monitoring Strong Tidal Currents in Straits and Nearshore Regions
533