Results
Sediment dynamics
The comparison of the bathymetry of the sandbank from 1994 to 1999
shows a net transport towards the main channel. This movement corresponds to an anticloclcwise rotation of the main sandbank’s axis: while the attachment point of the coast remains stable, the downstream part migrates
southwestwards. This migration gives the sandbank its asymmetric shape,
with a steep southem stoss side and a gende northem lee side.
The comparison of the side-scan sonar surveys between 1994 and 1999
indicates an increase of the number and the sixe of the bedforms. The recent side-scan sonar sonograms show two distinct provinces consisting of
;tn area of sandwaves in the main channel and an area of megaripples over
the bank and in the Saint-Georges Channel. Megaripples are classified as
having a height H<1.5 m and wavelength L<30 metres.
On the northeast flank of the bank the megartpples are 0.4 to 1.1 m high,
with wavelengths of 9 to 20 m and udth the crest oriented 50°N.
The second province corresponds to the main channel where ebboriented sand waves (H—3-5 m, L~150 m) along with superimposed
megaripples have been observed. This confirms the dynamics of the
ebb-dominant tidal currents in the main channel.
General hydrodynamics
Tlte Gironde is a macrotidal estuary with a spring tidal range of about
five metres. The tidal phase oscillations obtained with the Tosca Lander
show mean water depths vatying between 7 and 11.5 metres (fig. 2).
Figure 2 - Time series of mean values collected during the deployment period :
(a) signifcant wave height H s , (b) zero-crossing wave period T z , (c) tidal elevation h.
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Sediment dynamics
The comparison of the bathymetry of the sandbank from 1994 to 1999
shows a net transport towards the main channel. This movement corresponds to an anticloclcwise rotation of the main sandbank’s axis: while the attachment point of the coast remains stable, the downstream part migrates
southwestwards. This migration gives the sandbank its asymmetric shape,
with a steep southem stoss side and a gende northem lee side.
The comparison of the side-scan sonar surveys between 1994 and 1999
indicates an increase of the number and the sixe of the bedforms. The recent side-scan sonar sonograms show two distinct provinces consisting of
;tn area of sandwaves in the main channel and an area of megaripples over
the bank and in the Saint-Georges Channel. Megaripples are classified as
having a height H<1.5 m and wavelength L<30 metres.
On the northeast flank of the bank the megartpples are 0.4 to 1.1 m high,
with wavelengths of 9 to 20 m and udth the crest oriented 50°N.
The second province corresponds to the main channel where ebboriented sand waves (H—3-5 m, L~150 m) along with superimposed
megaripples have been observed. This confirms the dynamics of the
ebb-dominant tidal currents in the main channel.
General hydrodynamics
Tlte Gironde is a macrotidal estuary with a spring tidal range of about
five metres. The tidal phase oscillations obtained with the Tosca Lander
show mean water depths vatying between 7 and 11.5 metres (fig. 2).
Figure 2 - Time series of mean values collected during the deployment period :
(a) signifcant wave height H s , (b) zero-crossing wave period T z , (c) tidal elevation h.
225