Journal ArticleDOI
Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France)
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The Gironde is a macrotidal estuary characterized by important changes in tidal-current velocity and river discharge, and the internal structure of the dunes, revealed by seismic records, shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry as mentioned in this paper.Abstract:
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 semi iurnal reversal of the tidal current (during spring tides) could cause reversal of small dunes.read more
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Earth Surface Processes, Landforms and Sediment Deposits
John S. Bridge,Robert V. Demicco +1 more
TL;DR: In this article, the authors present an overview of the Earth's surface and its geology, including the basic fluid flow, sediment transport, erosion and deposition of soft sediment, as well as a discussion of long-term, large-scale processes.
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Tidal depositional systems in the rock record: a review and new insights
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Holocene sedimentary systems on continental shelves
TL;DR: In this paper, the authors examined the present knowledge of Holocene continental shelf deposits in relation to the processes for their formation, from the prospective of marine sediment dynamics, and developed a modeling approach to the formation of sedimentary records.
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Processes, Morphodynamics, and Facies of Tide-Dominated Estuaries
TL;DR: In this article, the authors define an estuary as a body of water that forms during a shoreline transgression and then fills during a progradational phase that is transitional to a delta.
Book ChapterDOI
Chapter 13 Estuarine Dunes and Bars
TL;DR: The authors examines the morphology, process-response relationships, and internal structures of two larger-scale groups of bed features and dunes, and particularly focuses on the distinction of Barforms from Dunes.