In situ measurements of the Saint-Georges
Bank sedimentary dynamics (Gironde estuary)
Cyril Mallet
(1) , ( )sanne Paireau
2) , David Huntley
Michael Collins (2)
(1) University of Plymouth, Institute of Marîne Studies, Drake Circus, Plymouth PL4 8AA,
United Kingdom
(2) University of Southampton, School of Ocean and Earth Science, European Way,
Southampton S014 3ZH, United Kingdom
C.Mallet@plymouth.ac.uk
Abstract
This study presents the first t«-jyï»measurements of the dynamics over the
Saint-Georges Bank located in the mouth of the Gironde estuary (France).
The sandbank, which is elongated and nearh’ attached to a headland, is
not dynamically stable and shows an anticlockwise rotation of its crest.
We present here the results obtained during the Swamgir-2 cruise in September 1999. Throughout this experiment bathymetric survey, side-scan
sonar records, ADCP transects, deplojmnent of the Tosca benthic Lander
were undertaken. These measurements allow us to estimate residual sand
transport and hydrodynamic pattems over the sandbank.
Introduction
Iinear sandbanks are extensively present on continental shelves and
coastal regions; their location depends on the availability of sand and
the presence of tidal or other current capable of moving the material.
Sandbanks are of considerable economic importance, they can contain
exploitable reserves of sand and gravel but may also form hazards for
shipping. Therefore, numerous studies have focused on the understanding of the processes of their formation, maintenance and stability (Pingree, 1978; Zimmerman, 1981; Pattiaratchi & Collins, 1987; Hulscher et
al., 1993). This study concems the dynamics of an elongated sandbank
(the Saint-Georges Bank) located near the Pointe de Vallière and Pointe
de Suzac headlands, in the mouth of the macrotidal Gironde estuaryr
(SW France) (fig. 1). The bank appeared in 1824 and has built up with
an accretion of 26 10
6 m
3 of sand over the last century. Todayy the bar is
5 km long, 1.2 km wide and up to +1 m above the mean low water
springs. The location of the sandbank corresponds to the accumulation
of marine sands; the sea origin is indicated by the presence of shells in
the medium sand (mean-grain size D = 0.285 mm). The bank is not dynamically stable and shows an anticlockwise rotation of its crest. The
bank is migrating towards the main navigation channel, its dynamics
and mobility are of concern to shipping traffic. Our study focuses on
determining where 2D and 3D hydrodynamic processes are dominant,
223
Bank sedimentary dynamics (Gironde estuary)
Cyril Mallet
(1) , ( )sanne Paireau
2) , David Huntley
Michael Collins (2)
(1) University of Plymouth, Institute of Marîne Studies, Drake Circus, Plymouth PL4 8AA,
United Kingdom
(2) University of Southampton, School of Ocean and Earth Science, European Way,
Southampton S014 3ZH, United Kingdom
C.Mallet@plymouth.ac.uk
Abstract
This study presents the first t«-jyï»measurements of the dynamics over the
Saint-Georges Bank located in the mouth of the Gironde estuary (France).
The sandbank, which is elongated and nearh’ attached to a headland, is
not dynamically stable and shows an anticlockwise rotation of its crest.
We present here the results obtained during the Swamgir-2 cruise in September 1999. Throughout this experiment bathymetric survey, side-scan
sonar records, ADCP transects, deplojmnent of the Tosca benthic Lander
were undertaken. These measurements allow us to estimate residual sand
transport and hydrodynamic pattems over the sandbank.
Introduction
Iinear sandbanks are extensively present on continental shelves and
coastal regions; their location depends on the availability of sand and
the presence of tidal or other current capable of moving the material.
Sandbanks are of considerable economic importance, they can contain
exploitable reserves of sand and gravel but may also form hazards for
shipping. Therefore, numerous studies have focused on the understanding of the processes of their formation, maintenance and stability (Pingree, 1978; Zimmerman, 1981; Pattiaratchi & Collins, 1987; Hulscher et
al., 1993). This study concems the dynamics of an elongated sandbank
(the Saint-Georges Bank) located near the Pointe de Vallière and Pointe
de Suzac headlands, in the mouth of the macrotidal Gironde estuaryr
(SW France) (fig. 1). The bank appeared in 1824 and has built up with
an accretion of 26 10
6 m
3 of sand over the last century. Todayy the bar is
5 km long, 1.2 km wide and up to +1 m above the mean low water
springs. The location of the sandbank corresponds to the accumulation
of marine sands; the sea origin is indicated by the presence of shells in
the medium sand (mean-grain size D = 0.285 mm). The bank is not dynamically stable and shows an anticlockwise rotation of its crest. The
bank is migrating towards the main navigation channel, its dynamics
and mobility are of concern to shipping traffic. Our study focuses on
determining where 2D and 3D hydrodynamic processes are dominant,
223