The SCAWVEX Rhine Outflow Experiment
of surface current fields measured by HF radar are available now.
The Rhine Outflow experiment has been scheduled for early spring 1996, to start
at around 15th of February. Due to some delay in delivery of components and
problems with the driver software sent with the A/D converters, the WERA systems
were expected to be ready with 4 weeks delay. As the main aim of this
experiment was on surface currents, CODAR systems have been deployed from
the beginning of the experiment and WERA got operational for the last two
weeks. This solution allowed for a long-term dataset of current data, which
is needed for tidal analysis, and additional two weeks for wave data
To cover the measurement area including the deployments of the current meters,
ADCPs and wave bouys, two sites near s'Gravenzande and on the island
Goeree-Overflake, refered as Arendsduin and Ouddorp, have been selected.
Surface currents have been calculated on a
measurement grid with 2 km spacing shown in figure 1.
As CODAR and WERA have been using the same frequency band, it was not possible
to run them simultanously. Figure 2
shows the schedule of measurement times
for the CODARs and WERAs at Ouddorp and Arendsduin for the whole experiment.
Special care had to be taken to interlock the systems to prevent interfering
eachother during the stand-by time of the inactive system. The antenna
configuration used consisted of a 4-element squared array for the CODAR system
and a 4-element squared array plus a 12-element linear array for the WERA
system. The linear arrays have been set up parallel to the coastline to prevent
distortion of the antenna pattern. The
antenna configuration used at the
Ouddorp site is shown in figure 3.
The advantage of this set-up is
- the possibility to compare CODAR and WERA surface currents to ensure
operation of WERA and
- to compare different algorithms for azimutal resolution as
- 4-element square array direction finding,
- 12-element linear array direction finding and
- 12-element linear array beam forming.
The CODARs have been operating from 18th February 1996 to 26th March 1996. The
data sets are 99.9% complete. Only one short data loss occured due to a line
power failure in Arendsduin. This data has been interpolated for the later
analysis. WERA data are available from 18th March to 27th March 1996 without
CODAR and WERA working range was more then 30 km for currents, so the
measurement area was covered during the whole period. This working range
is less than expected due to former experiments. The reasons are the wide
beach and low antenna hights combined with inflow of fresh water, which
tends to stay at the surface and reduces working range by increasing the
attenuation of the electromagnetic waves. To improve the quality of the
CODAR current data set, some spikes have been eliminated and short gaps
have been interpolated by using a tidal analys.
Figure 4 shows an example for a
surface current field calculated from CODAR
data and figure 5 shows the corresponding
surface current field calculated from
WERA data using direction finding on the 12-element linear arrays.
Depth data have been supplied by Rijkswaterstaat.
As the CODAR antenna can look all around, the Arendsduin CODAR was able to
observe the area in front of Ouddorp; however the accuracry is reduced,
because the radar signal has to travel over land. The Arendsduin WERA could not
reach that area, because the coverage is limited to +/- 60 degrees to the
boresight of the linear array. The data sets will be reprocessed including the
effects of shallow water, when a complete depth map is available. This will
improve the measurements in front of Ouddorp, as there exist some very shallow
First results show, that the WERA working range seems to be better than
the CODAR one. The current vectors correspond well. It can be assumed,
that both systems worked correct and no systematic errors occured.
The tidal analysis of the current data will be refined including the wind
information now available.
The CODAR data sets have been distributed to the SCAWVEX partners, the
reprocessed data sets will be available to the research community.
For the WERA system, an algorithm using beamforming is now ready.
A comparison between CODAR data and WERA data and the different algorithms
is still in progress.