velocity profiles mav occur for a variety of reasons, the most common
are: unsteadiness in the flow velocity, presence of internal waves, flow
stratification, heterogeneous distribution of bottom roughness, presence
of large bedforms associated with flow separation and eddy shedding.
Here the non-logarithmic profiles are most probably the result of flow
separation effects behind the bank.
Figure 4 - Progressive vector diagrams obtained from the current measurements: (a) Tosca Lander at different
depths from the seabed: (b) RCM8 current meter at 250 cm from the seabed.
The 90 hours deployment of the Tosca Lander and the RCM8 current
meter allows the calculation of residual currents at different heights.
Therefore, we determine the progressive vectors at z = 0.6 m, 1 m and
1.3 m as shown in figure 4. We observe clearly that at the Tosca station
the net flow at z = 0.6 m is directed south-westwards whereas at z =1.3 m
and z =1 m it is directed north-westwards. This veering indicates nonuniformities in the flow and is suggestive of secondaiy 7 circulation effects with the possible presence of helicoidal flows. The difference in net
flow direction obtained with the Tosca Lander and the RCM8 current
meter indicates complex sediment transport pathways but suggests two
major net flows in south-westem and westem directions. This latter
would be in good agreement with the accretion of sand observed on the
top of the bank between 1992 and 1999 during bathymetric surveys.
Conclusion
We have presented here the first results of the Swamgir-2 cruise concerning the general hydrodynamics over the Saint-Georges Bank.
Bathymetric and side-scan-sonar investigations show migration and
increase of bedforms between 1994 and 1999 as well as a deplacement
of the crest towards north-west. The vertical velocity profiles and the
progressive vectors suggest flow separation behind the bank and nonuniform flows due to the topography of the area. Moreover, the veering
observed in net flows may indicate sand transport towards the sandbank crest and upstream.
227
are: unsteadiness in the flow velocity, presence of internal waves, flow
stratification, heterogeneous distribution of bottom roughness, presence
of large bedforms associated with flow separation and eddy shedding.
Here the non-logarithmic profiles are most probably the result of flow
separation effects behind the bank.
Figure 4 - Progressive vector diagrams obtained from the current measurements: (a) Tosca Lander at different
depths from the seabed: (b) RCM8 current meter at 250 cm from the seabed.
The 90 hours deployment of the Tosca Lander and the RCM8 current
meter allows the calculation of residual currents at different heights.
Therefore, we determine the progressive vectors at z = 0.6 m, 1 m and
1.3 m as shown in figure 4. We observe clearly that at the Tosca station
the net flow at z = 0.6 m is directed south-westwards whereas at z =1.3 m
and z =1 m it is directed north-westwards. This veering indicates nonuniformities in the flow and is suggestive of secondaiy 7 circulation effects with the possible presence of helicoidal flows. The difference in net
flow direction obtained with the Tosca Lander and the RCM8 current
meter indicates complex sediment transport pathways but suggests two
major net flows in south-westem and westem directions. This latter
would be in good agreement with the accretion of sand observed on the
top of the bank between 1992 and 1999 during bathymetric surveys.
Conclusion
We have presented here the first results of the Swamgir-2 cruise concerning the general hydrodynamics over the Saint-Georges Bank.
Bathymetric and side-scan-sonar investigations show migration and
increase of bedforms between 1994 and 1999 as well as a deplacement
of the crest towards north-west. The vertical velocity profiles and the
progressive vectors suggest flow separation behind the bank and nonuniform flows due to the topography of the area. Moreover, the veering
observed in net flows may indicate sand transport towards the sandbank crest and upstream.
227