Monitoring tidal currents off the Yarmouth harbour with a towed ADCP system
Report on survey in the Solent by A. Sentchev and M. Thiébaut
Laboratoire d'Océanologie et de Géosciences (LOG), Wimereux, France
Summary
High resolution current mapping surveys, repeatedly performed off the Yarmouth harbour on July 1, 2015,
during spring tide conditions, revealed:
- Very strong ebb tide current with velocity up to 3 m/s
- The highest current speed is observed in the south-western sector of the study area, 300 m offshore the Fort Ru.
- The Solent passage narrowing and the depth increase to 50m, tend to reinforce the current in this area.
- Closer to the shore, in shallow water area (depth < 25m), the peak current velocity is about 2 m/s, and high
current speeds (>1.5 m/s) are observed at ebb and flood flow. This indicates an area and favorable conditions for
scaled turbine test there.
- Another area of high velocity is the north-eastern sector, located in the middle of the Solent – very close to the
Ferry line Yarmouth-Limington.
- In shallow water area, a high velocity shear is observed in the tidal flow. Particular attention should be paid on
current-bottom interaction and turbulence generated by bottom friction. Some major characteristics of the bottom
friction layer will be provided later.
- Flow conditions at site suitable for turbine test will be characterized in more detail later. One or two fixed
mooring surveys will be conducted in September, they will use surface mounted ADCPs, and aim at assessing
the major metrics of the tidal flow and resource characterization.
1. Experimental settings
A high resolution current mapping system (Fig. 1) was used for underwater current velocity measurements
off the Yarmouth harbour, on July 1, 2015. Three 1-2 hour surveys were performed during different stages of
tidal flow under spring tide conditions. A system was also moored during one hour for continuous velocity
measurements in a point between surveys one and two. The system uses a broadband 1200kHz Teledyne RDI
Acoustic Doppler Current Profiler (ADCP) coupled with a high precision GPS, mounted on a floating platform,
towed by a light boat (Yarmouth taxi). The distance from the boat (boat main axis) is controlled by an adjustable
side fin allowing it to avoid contamination by the wake of the boat. Two additional rear fins installed on the hulls
assure stability of the forward propulsion.
Fig. 1 Towed ADCP system
Fig. 2 Area (red rectangle) surveyed with towed ADCP system.
An area of about 3.5 km² (red rectangle in Fig. 2) was repeatedly surveyed by the system towed along
individual tracks oriented in the north-south direction and spaced by roughly 0.5 km. The boat speed varied from
2 to 4 m/s for the majority of the tracks.
The ADCP was set to operate at the pinging rate of 1 Hz, providing velocity profiles within one second
interval (i.d., at 2-4 m spacing). Each ping for velocity was composed of 3 sub-pings averaged within one second
interval, providing velocity error of 4 cm/s. Single-ping bottom tracking was enabled to correct for the boat's
movement and the recorded velocities formed a current vector in the fixed frame relative to the bottom.. The
vertical resolution (bin size) was set to 0.5 m and the centre of the first bin was roughly at 1 m below the sea
surface. The velocity profiles were obtained from 1 m to 22.5 m depth. All the data were stored in an internal
memory unit. After the data retrieval, current velocities were averaged within 30-seconds, geolocalised, and
interpolated on a regular space grid by using a variational interpolation technique 2dVar. The survey time
(UTC+1 hour), duration and the survey mid-time relative to LW in Yarmouth on July 1, 2015 are given in the
Table below.
Survey
Survey time (UTC+1h)
Duration (h)
1
Jul 1, 10:30-11:41
LW-5h – LW-4h
Jul 1, 13:00-14:30
LW-2.5h – LW-1h
Jul 1, 17:00-18:38
LW+1.5h – LW+3h
2
3
1.2
Mid-time of the survey
relative to LW
LW-4.5h
Nb of vel data after 30 s
averaging
142
1.5
LW-1.75
178
1.6
LW+2.25h
196
2. Current vector maps
Fig. 3 shows tidal current vectors in the surface layer (the first bin, 1 m below the surface) recorded by the
system during each survey. The strongest currents are observed during the second survey (LW-1.5h) and in the
area with high depth (SW and NE sectors of the study zone). We assume that the current magnitude curve is
asymmetric with rather fast current reversal and high velocity magnitude observed 1-2 h before and 2 h after the
current reversal. To estimate the tidal current velocity asymmetry, further measurements by moored ADCP are
strongly recommended. Tidal currents are guided by bathymetry in the majority of the domain, and the current
vectors are aligned with bathymetry contours. Some deviation of current from the dominant direction is observed
in the vicinity of Yarmouth harbour. The averaged velocities (after 30 s averaging) are not noisy and show that
the measurements by towed ADCP are of high quality. The highest surface velocities are observed in the SW
sector of the study area during all surveys. The velocity magnitude is close to 3 m/s there at LW-1.5h (survey 2).
Some corrections should be done to account for tidal current evolution during each individual survey. For
example, low velocity values along the eastern track of survey 3 (Fig. 3 right panel) are due to the early tidal
stage: the survey started at LW+1.5h (weak current) and finished at LW+3h when currents are stronger. Such
correction is possible and involves the use of a numerical model for statistically and dynamically consistent
space-time interpolation of velocity measurements. We expect to perform such interpolation and to present the
results later.
Fig. 3 Current vectors measured by towed ADCP in the surface layer (1 m below the sea surface). Each
vector represents velocity averaged within 30 s interval.
We will complete the description of current maps by representing depth averaged current velocities and
velocities measured in the bottom layer.
3. Interpolated current velocity fields
Current velocity profiles measured at 1 s resolution along individual tracks were averaged within 30 s and
interpolated on a regular grid of 100 m spacing. The results for the surface layer (1 m depth level) are shown in
Fig. 4. Current maps allow the assessment of space variations of velocity field in more detail and offer an
opportunity for research of the area for turbine test site locations. The results reveal the existence of stronger
velocities (appeared as current jet) in the area with depth more than 30 m evidencing the effect of bathymetry
and bottom friction on tidal currents. The current jet follows bathymetry contours. The maximum velocity values
are observed in the south-western and northeastern sectors of the domain. Rapid change of depth affects the ebb
flow (surveys 1 and 2), produces flow acceleration and induces the local rise of current velocity up to 2.4 m/s at
LW-4.5 hours, and up to 3 m/s at LW-1.5 hours. The flow convergence along the 20-m depth isobath is another
particular feature of tidal flow. Only accurate space-time interpolation allows the analysis of temporal evolution
of the current velocity field. Some basic features of current evolution can be retrieved from velocity time series
recorded during a mooring at a fix point. The down looking ADCP was moored at an anchored buoy west of the
harbour for a 1 h period between surveys 1 and 2. The results will be presented in the second report.
Peak flow speeds in the south-western sector are excellent for the long-term testing of small-scale (1:2) or
full size devices. Several site locations can be identified for further study: shallow water (close to the shore) and
deep water sites (?). We recommend bottom mounted ADCP measurements and high resolution current mapping
at these sites coupled with numerical modelling for complete site screening. Particular attention should be paid to
direction asymmetry of the flow, tidal current vector rotation, the presence of flow convergence and resulting
vertical velocities (which will be estimated later).
Fig. 4. Tidal currents (blue arrows) in the surface layer (1 m depth level) derived from towed ADCP surveys for
three stages of the spring tide flow. Flow speed is shown by grey shading, bathymetry contours (10, 20, 30 m) by
dashed lines, and ADCP measurement points by grey dots. Black dot shows the location of the black rock (BkR).
Blue dots match the location of the maximum velocity values (m/s) given above.
4. Velocity on cross section
Valuable information provided by current mapping concerns velocity variation with depth. The
knowledge of velocity profile is required in tidal energy projects for accurate evaluation of the available power
within the bottom column. Velocity shear is an important quantity affecting turbulence and loads on tidal
devices.
Fig. 5 shows the velocity magnitude on cross sections during the ebb flow surveys (survey 1 and 2).
Only the western sector with higher velocity values was chosen for analysis. Velocity distribution is not
homogeneous and shows strong variations in horizontal and also vertical directions. Space variations along a
cross section are related to local topography and the effect of flow interaction with the bottom. Globally, slightly
higher velocities are observed in the surface layer. Sometimes, the velocity shear is important. It is also possible
to provide a mean velocity profile at different locations on the cross section and estimate the level of turbulent
kinetic energy (will be provided later). The use of 600 kHz ADCP for velocity profiling will allow the current
mapping to the sea floor, instead of the 22.5 m depth reached in this study.
Fig. 5. Tidal flow speed on a cross section in the western part of the domain for survey 1 and survey 2. Cross
section location is shown in red in the lower-left corner.