ABSTRACT

The bottom sediments in the marine portion of the Gironde estuary have been extensively sampled and studied in relationship to the local wave and tidal current patterns. A semi-empirical analysis was conducted to delimit the areas of wave refraction and the bottom orbital velocity variations.

Compilation of the current and wave energy data from on-site measurements show that distinct energy zones exists, each characterized by different current strength and type. These zones are controlled by the morphological features of the estuary, the tidal and wave energy zones being mutually exclusive. Most of the tidal current discharge occurs in the deeper channels, whereas the shoal areas are the site of important wave energy.

Most of the sediments are sands (phi mean between 2.0 and 0.5). A large gravel deposit however was observed at the entrance of the estuary. The textural parameter distribution patterns of the sands coincide with and reflect the different energy zones. The distinct combination of energy type in a particular zone will cause a distinct textural pattern in the sediments found within it. Moreover, comparison of tim time-averaged tidal velocities with standard sediment erosion curves show that the grain size distribution of the sands seems to be in equilibrium with the hydraulic environment. The gravel deposit is inferred to be a fossil alluvial terrace because it does not seem to be in accord with the present-day current patterns.

A marked functionality exists between the more significant parameters of size, sorting and skewness and the ratio of wave to tidal energy. This is particularly evident in parameter distribution maps. The mean grain size, sorting, and skewness of the sands are inversely proportional to the ratio of wave to tidal activity.

The inferred transportational processes seem also to reflect the established energy zones. A CM curve analysis indicates a graded suspension transport mode for zones where wave energy predominates, and bedload transport for areas influenced by tidal currents. This effect seems to be corroborated by consideration of the orbital bottom wave velocity as a function of depth. The depth limit for the transportational processes due to wave activity seems to be about 15 meters. Beyond this depth, the sediment is transported mainly by the tidal currents.

It is concluded that textural patterns are good indicators of energy variations, even in complex, multi-environmental areas such as estuaries. Future research will probably establish quantitative relationships between energy type and distribution and the resutling textural parameters.