observations. In contrast to the bottom-mounted or moored ADCPs, this method of
surveying does not require deployment of numerous instruments to obtain adequate
spatial resolution and it is not vulnerable to harsh environmental conditions and
extreme tidal velocities. Extensive ADCP surveys demonstrated that tidal currents
are characterized by significant variations at the scales of a few hundred meters [9,
14], and thus, require a very high resolution surveying during multiple tidal cycles.
At the same time, finite speed of the surveying vessel imposes certain limitations on
the accuracy of reconstruction of the 4-dimensional velocity field. To resolve the
issue, observed velocities should be constrained by the relationships governing tidal
dynamics.
This approach was pursued by Goddijn-Murphy et al. [9], who synthesized
vessel mounted ADCP transects in the Pentland Firth (Scotland) with the output of
the Orkney 2D tidal Model (ORKM) and obtained snapshots of the currents during
different tidal phases. Recently, Sentchev and Yaremchuk [25] proposed a 4D
optimal interpolation (OI) technique for processing underway velocity measurements. The method employs statistics of a tidal model to infer the velocity error
covariances required by the interpolation algorithm.
In this section, we discuss the results of high resolution current mapping with a
towed ADCP at two sites in the English Channel characterized by strong tidal
currents. They are of certain interest for industrial companies developing energy
conversion systems, and thus require extended assessment of the local
hydrodynamics.
Methodology High resolution current mapping is performed using an experimental
platform carrying a broadband ADCP (1200 kHz or 600 kHz Teledyne RDI
WorkHorse Sentinel) towed by a light boat (Fig. 5). The distance from the boat is
controlled by an adjustable side fin allowing to keep the platform at a side of the
boat and thus to avoid contamination of the observed velocities by the wake. Two
additional rear fins installed on the hulls assure stability of the forward propulsion.
The ADCP is set to operate at a pinging rate of 1 Hz providing one velocity profile
per second with an accuracy of 0.05 m/s. The velocities are sampled with 0.5 m
vertical resolution; the first bin centred at 0.75 m. Bottom tracking enables to
correct for boat’s movement and the recorded velocities form a current vector in the
fixed frame relative to the bottom.
The boat speed typically varies within 2–4 m/s during a survey. The recorded
ADCP data are merged with high resolution GPS data. The geolocalisation system
GENEQ SXblue, mounted on the side of the platform (Fig. 5) and operating at
1 Hz, provides positioning accuracy of 40–80 cm at towing speeds of 2–4 m/s. The
above described system, called Koursk, was used for high resolution current
mapping at two perspective sites of marine renewable energy in the English
Channel: in the Solent and off the Dover harbour.
Assessment of tidal dynamics in the Solent and off the Dover harbor Tidal
motions in the Solent (Fig. 5, right panel) are very complex and well known for the
unusual phenomena such as the “Double High Water”, the “Young Flood Stand”,
and the short duration of the ebb tide. These peculiarities result from a deformation
528
A. Sentchev et al.
surveying does not require deployment of numerous instruments to obtain adequate
spatial resolution and it is not vulnerable to harsh environmental conditions and
extreme tidal velocities. Extensive ADCP surveys demonstrated that tidal currents
are characterized by significant variations at the scales of a few hundred meters [9,
14], and thus, require a very high resolution surveying during multiple tidal cycles.
At the same time, finite speed of the surveying vessel imposes certain limitations on
the accuracy of reconstruction of the 4-dimensional velocity field. To resolve the
issue, observed velocities should be constrained by the relationships governing tidal
dynamics.
This approach was pursued by Goddijn-Murphy et al. [9], who synthesized
vessel mounted ADCP transects in the Pentland Firth (Scotland) with the output of
the Orkney 2D tidal Model (ORKM) and obtained snapshots of the currents during
different tidal phases. Recently, Sentchev and Yaremchuk [25] proposed a 4D
optimal interpolation (OI) technique for processing underway velocity measurements. The method employs statistics of a tidal model to infer the velocity error
covariances required by the interpolation algorithm.
In this section, we discuss the results of high resolution current mapping with a
towed ADCP at two sites in the English Channel characterized by strong tidal
currents. They are of certain interest for industrial companies developing energy
conversion systems, and thus require extended assessment of the local
hydrodynamics.
Methodology High resolution current mapping is performed using an experimental
platform carrying a broadband ADCP (1200 kHz or 600 kHz Teledyne RDI
WorkHorse Sentinel) towed by a light boat (Fig. 5). The distance from the boat is
controlled by an adjustable side fin allowing to keep the platform at a side of the
boat and thus to avoid contamination of the observed velocities by the wake. Two
additional rear fins installed on the hulls assure stability of the forward propulsion.
The ADCP is set to operate at a pinging rate of 1 Hz providing one velocity profile
per second with an accuracy of 0.05 m/s. The velocities are sampled with 0.5 m
vertical resolution; the first bin centred at 0.75 m. Bottom tracking enables to
correct for boat’s movement and the recorded velocities form a current vector in the
fixed frame relative to the bottom.
The boat speed typically varies within 2–4 m/s during a survey. The recorded
ADCP data are merged with high resolution GPS data. The geolocalisation system
GENEQ SXblue, mounted on the side of the platform (Fig. 5) and operating at
1 Hz, provides positioning accuracy of 40–80 cm at towing speeds of 2–4 m/s. The
above described system, called Koursk, was used for high resolution current
mapping at two perspective sites of marine renewable energy in the English
Channel: in the Solent and off the Dover harbour.
Assessment of tidal dynamics in the Solent and off the Dover harbor Tidal
motions in the Solent (Fig. 5, right panel) are very complex and well known for the
unusual phenomena such as the “Double High Water”, the “Young Flood Stand”,
and the short duration of the ebb tide. These peculiarities result from a deformation
528
A. Sentchev et al.