Monitoring Strong Tidal Currents in Straits and Nearshore Regions
The strongest ocean currents occur in coastal regions and have tidal origin. In such regions, high current speeds are typically the result of topographic flow amplification. Despite their sparsity, these sites are optimal for installation of the tidal energy conversion devices, and, therefore, require special techniques for monitoring local currents that often exceed 3 m/s. In this chapter, two prospective techniques for monitoring extreme currents in the nearshore regions are presented. The first one is remote sensing of surface currents by High Frequency radars (HFRs). This method provides high temporal resolution (10–20 min) over large (102–104 km2) domains and long-time intervals, but lacks adequate horizontal resolution and cannot directly monitor the vertical structure of the flow. A complimentary method, based on underway ADCP observations: the use of towed ADCP system presents an opportunity of more focused velocity monitoring within limited (1–10 km2) domains at much higher (up to 50 m) horizontal resolution, over shorter time intervals. This kind of 4-dimensional mapping was performed in the Strait of Dover and in the West Solent. Transient eddies, large horizontal velocity gradients, and vertical shear in the velocity profiles were detected at each site, enabling a detailed characterization of the tidal stream. Combined with the dynamical constraints of a numerical model, the observed velocities significantly increase the accuracy in reconstruction of the full 4-dimensional tidal flow. The presented HFR-based and towed ADCP monitoring systems could be useful for regional model validation and studies of the local hydrodynamics with specific emphasis on resource assessment at tidal energy sites.
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.
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