the next section, we first consider the basic principles of HFR-based observations of
surface currents and then describe application of the method for assessment of
spatial and temporal variability of the tidal flows at two sites where powerful tidal
currents dominate local circulation: off the western coast of France and in the Strait
of Dover. In section “Measurements of Tidal Currents with a Towed ADCP System”, we briefly describe a compact ADCP towing system and demonstrate its
application in two regions (the Solent and Dover Strait) in the English Channel. In
section “Summary”, we summarize the results and discuss the prospects of using
the techniques, in particular, for applications related to tidal energy conversion.
Monitoring Surface Currents with HF Radars
Basic principle High Frequency radars operate in a frequency band 5–45 MHz.
A part of the electromagnetic energy transmitted at these frequencies is trapped in
the “surface-wave” mode and remains bound to the sea surface, following the Earth
curvature. Moreover, in this frequency band, the electromagnetic waves have
wavelengths that are commensurate with wind-driven gravity waves on the ocean
surface. The ocean waves whose wavelengths are exactly half as long as those of
the broadcast radio waves are responsible for the resonant backscatter, an example
of a phenomenon known as Bragg scattering. Crombie (1955) first observed that the
peak energy of HF radio waves reflected from the sea surface occupies a narrow
frequency range near the frequencies ± f b , called the Bragg frequency; f b represents
the Doppler shift of the backscattered signal above and below the HFR frequency, it
is defined as f b = g/πλ, where λ is the radar wavelength, g is the acceleration of
gravity. These two Bragg peaks in the power spectrum are produced by the coherent
scattering of the HFR waves by the Bragg sea surface waves travelling radially
toward and away from the radar.
In an important early study, Stewart and Joy [26] observed that there are often
small frequency differences between the observed Doppler shift of the backscattered HFR waves and the Doppler shift predicted from the surface gravity waves
moving over still water. Using radar-tracked surface drogues to directly measure
ocean currents, they showed that the observed differences resulted from surface
ocean currents and that the current speed toward or away from the radar could be
estimated. In particular, radial velocity v r of the surface current (a projection of the
current velocity vector on the radar beam) can be retrieved from the Doppler shift
Δf of the observed Bragg peaks, compared to their theoretical values, by using the
Doppler formula v r = Δf/2λ.
These principles are the basis for the mapping of surface currents by oceanographic HFRs. Since then, many validation studies comparing HF radar-derived
surface currents with in situ observations have contributed to the growing acceptance and use of HFRs for measuring surface currents (e.g., [4, 16, 21, 23]). Stewart
and Joy [26] assumed that a HF radar samples vertically integrated current velocity.
Earlier work by Kirby and Chen [12] and more recent studies [21, 23] allowed us to
Monitoring Strong Tidal Currents in Straits and Nearshore Regions
521
surface currents and then describe application of the method for assessment of
spatial and temporal variability of the tidal flows at two sites where powerful tidal
currents dominate local circulation: off the western coast of France and in the Strait
of Dover. In section “Measurements of Tidal Currents with a Towed ADCP System”, we briefly describe a compact ADCP towing system and demonstrate its
application in two regions (the Solent and Dover Strait) in the English Channel. In
section “Summary”, we summarize the results and discuss the prospects of using
the techniques, in particular, for applications related to tidal energy conversion.
Monitoring Surface Currents with HF Radars
Basic principle High Frequency radars operate in a frequency band 5–45 MHz.
A part of the electromagnetic energy transmitted at these frequencies is trapped in
the “surface-wave” mode and remains bound to the sea surface, following the Earth
curvature. Moreover, in this frequency band, the electromagnetic waves have
wavelengths that are commensurate with wind-driven gravity waves on the ocean
surface. The ocean waves whose wavelengths are exactly half as long as those of
the broadcast radio waves are responsible for the resonant backscatter, an example
of a phenomenon known as Bragg scattering. Crombie (1955) first observed that the
peak energy of HF radio waves reflected from the sea surface occupies a narrow
frequency range near the frequencies ± f b , called the Bragg frequency; f b represents
the Doppler shift of the backscattered signal above and below the HFR frequency, it
is defined as f b = g/πλ, where λ is the radar wavelength, g is the acceleration of
gravity. These two Bragg peaks in the power spectrum are produced by the coherent
scattering of the HFR waves by the Bragg sea surface waves travelling radially
toward and away from the radar.
In an important early study, Stewart and Joy [26] observed that there are often
small frequency differences between the observed Doppler shift of the backscattered HFR waves and the Doppler shift predicted from the surface gravity waves
moving over still water. Using radar-tracked surface drogues to directly measure
ocean currents, they showed that the observed differences resulted from surface
ocean currents and that the current speed toward or away from the radar could be
estimated. In particular, radial velocity v r of the surface current (a projection of the
current velocity vector on the radar beam) can be retrieved from the Doppler shift
Δf of the observed Bragg peaks, compared to their theoretical values, by using the
Doppler formula v r = Δf/2λ.
These principles are the basis for the mapping of surface currents by oceanographic HFRs. Since then, many validation studies comparing HF radar-derived
surface currents with in situ observations have contributed to the growing acceptance and use of HFRs for measuring surface currents (e.g., [4, 16, 21, 23]). Stewart
and Joy [26] assumed that a HF radar samples vertically integrated current velocity.
Earlier work by Kirby and Chen [12] and more recent studies [21, 23] allowed us to
Monitoring Strong Tidal Currents in Straits and Nearshore Regions
521