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The SCAWVEX Rhine Outflow Experiment


Animations 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 comparisons.

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 data loss.

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 areas.

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.

 
 
 
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