Morphology, internal structure, and reversal of asymmetry of large subtidal dunes in the entrance to Gironde Estuary (France)

The Gironde is a macrotidal estuary characterized by important changes in tidal-current velocity and river discharge. Two surveys were made in the lower estuary in June 1987 and October 1989 in an area with large dunes. The symmetry of these dunes (1.5-6.7 m high) changes from flood-dominated (in upstream areas) to ebb-dominated (in downstream areas). A transition zone consists of symmetrical dunes with a very low vertical form index. The internal structure of the dunes, revealed by seismic records, shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry of the dunes. The lowest part of symmetrical dunes consists of ebb-oriented reflectors, while the upper part has flood-oriented reflectors. The same profile lines run in 1989 show that each large dune can be identified, despite an upstream shift of the transition zone of about 1000 m. Comparison of seismic records shows that some lower reflectors are preserved between the two surveys, and that the dunes moved downstream a mean distance of 30 m. In 1989 the internal structure of symmetrical dunes was reversed, with flood-oriented reflectors in the lower part overlain by ebb-oriented reflectors. Both seasonal changes in river discharge and fortnightly oscillations of the tidal range control the magnitude and orientation of the net bed-load transport in the study area. Because the response time of the large dunes is longer than that of the superimposed small dunes, the reversal of asymmetry of the large dunes requires a long-period process, while fortnightly oscillation or even semidiurnal reversal of the tidal current (during spring tides) could cause reversal of small dunes.