coastal areas, located in their majority along the UK and French coast, not so large
but still impressive current speeds (more than 10 knots) are encountered.
Coastal ocean hydrodynamic models, routinely used for SSH and currents
forecasting, are not always capable of resolving local bathymetric features and
complex physics governing a fast moving tide. In such situations, monitoring of
coastal currents by a bottom-based or towed current recording system remains a real
but not unique opportunity. In turn, in situ observations by bottom-mounted
Acoustic Doppler Current Profiler (ADCP) in strong tidal flows are very difficult to
perform because they require highly qualified staff, boats, specific equipment, and if
the survey is successfully conducted, the acquired data do not allow to characterize
the spatial variability of the flow neither its long-term variation.
Underway velocity measurements by towed or vessel-mounted ADCP is another
efficient tool for tidal flow characterization in nearshore areas. In the early studies
[8, 32], velocity profiles were sampled with sufficient frequency while the vessel
steamed around a circuit allowing to resolve the vertical structure of the tidal
current and its spatial irregularities. Using broadband ADCP surveys
Goddijn-Murphy et al. [9], Polagye and Thomson [18] documented fine scale
velocity variations caused by tidal flow interaction with land and islands. However,
the rapid change of the tidal flow during surveying period tends to induce errors in
the velocity map interpretation. Goddijn-Murphy et al. [9] showed that the accuracy
in reconstruction of the full 4-dimensional tidal flow can be significantly increased
by merging observed velocities with the dynamical constraints provided by
numerical models. More recently, Sentchev and Yaremchuk [25] used the optimal
4-dimensional interpolation technique for reconstructing space-time evolution of
the velocity field derived from towed ADCP surveys in the Boulogne harbour
(English Channel). This approach, which combines underway velocity measurements and modelling, offers a real opportunity for short-term monitoring of the
strong nearshore circulation.
Another technique of coastal flow monitoring involves remote sensing of surface
currents by High Frequency Radars (HFR). Although application of the HF radars
in oceanography has more than 30-year history [17, 19, 26], its efficiency for
monitoring powerful coastal flows, jets and tidal generated transient eddies was
demonstrated only recently (c.f. [22]). A new interest for tidal current monitoring in
straits and narrows is motivated by the growing development of the tidal energy
conversion by in-stream devices. The UK was a pioneer in this field by installing
the first commercial 1.2 MW tidal turbine in the Strangford Loch, a narrow strait
connecting the inland tidal basin to the Irish Sea. In France, the Sabella Co.
installed its first of three 0.5 MW tidal turbines in the Fromveur Strait (W. Brittany). Other projects aim to install tidal stream conversion systems at various sites
in France (Alderney Race) or in Scotland (Pentland Firth). In this context, long term
monitoring of strong tidal flows at these sites is of primary importance for engineering applications and assessing the tidal stream potential.
In this chapter we give an overview of the two prospective techniques for
monitoring strong currents in the nearshore regions with high tidal stream potential:
the remote sensing by HFRs, and underway measurements by a towed ADCP. In
520
A. Sentchev et al.
but still impressive current speeds (more than 10 knots) are encountered.
Coastal ocean hydrodynamic models, routinely used for SSH and currents
forecasting, are not always capable of resolving local bathymetric features and
complex physics governing a fast moving tide. In such situations, monitoring of
coastal currents by a bottom-based or towed current recording system remains a real
but not unique opportunity. In turn, in situ observations by bottom-mounted
Acoustic Doppler Current Profiler (ADCP) in strong tidal flows are very difficult to
perform because they require highly qualified staff, boats, specific equipment, and if
the survey is successfully conducted, the acquired data do not allow to characterize
the spatial variability of the flow neither its long-term variation.
Underway velocity measurements by towed or vessel-mounted ADCP is another
efficient tool for tidal flow characterization in nearshore areas. In the early studies
[8, 32], velocity profiles were sampled with sufficient frequency while the vessel
steamed around a circuit allowing to resolve the vertical structure of the tidal
current and its spatial irregularities. Using broadband ADCP surveys
Goddijn-Murphy et al. [9], Polagye and Thomson [18] documented fine scale
velocity variations caused by tidal flow interaction with land and islands. However,
the rapid change of the tidal flow during surveying period tends to induce errors in
the velocity map interpretation. Goddijn-Murphy et al. [9] showed that the accuracy
in reconstruction of the full 4-dimensional tidal flow can be significantly increased
by merging observed velocities with the dynamical constraints provided by
numerical models. More recently, Sentchev and Yaremchuk [25] used the optimal
4-dimensional interpolation technique for reconstructing space-time evolution of
the velocity field derived from towed ADCP surveys in the Boulogne harbour
(English Channel). This approach, which combines underway velocity measurements and modelling, offers a real opportunity for short-term monitoring of the
strong nearshore circulation.
Another technique of coastal flow monitoring involves remote sensing of surface
currents by High Frequency Radars (HFR). Although application of the HF radars
in oceanography has more than 30-year history [17, 19, 26], its efficiency for
monitoring powerful coastal flows, jets and tidal generated transient eddies was
demonstrated only recently (c.f. [22]). A new interest for tidal current monitoring in
straits and narrows is motivated by the growing development of the tidal energy
conversion by in-stream devices. The UK was a pioneer in this field by installing
the first commercial 1.2 MW tidal turbine in the Strangford Loch, a narrow strait
connecting the inland tidal basin to the Irish Sea. In France, the Sabella Co.
installed its first of three 0.5 MW tidal turbines in the Fromveur Strait (W. Brittany). Other projects aim to install tidal stream conversion systems at various sites
in France (Alderney Race) or in Scotland (Pentland Firth). In this context, long term
monitoring of strong tidal flows at these sites is of primary importance for engineering applications and assessing the tidal stream potential.
In this chapter we give an overview of the two prospective techniques for
monitoring strong currents in the nearshore regions with high tidal stream potential:
the remote sensing by HFRs, and underway measurements by a towed ADCP. In
520
A. Sentchev et al.