Hélène Howa

Application of grain size trend analysis for the determination of sediment transport pathways in intertidal areas

Abstract In this paper, we apply the Gao and Collins procedure of grain size trend analysis for the first time in an intertidal sedimentary system. Our purpose is to evaluate a reliable method to determine residual transport directions in intertidal environments, where sediment movement is not well understood. Residual transport patterns can be determined by grain size trend analysis if the following criteria are applied: (1) a single sediment transport unit must be precisely defined and investigated; (2) the thickness of sampling has to be chosen in order to investigate the most recent sedimentary event; and (3) a regular mesh grid is sampled to avoid bias in the calculation of grain size trends. The Gao and Collins method was undertaken simultaneously with a topographic survey and a tracer experiment, during consecutive meteorological conditions of fair weather and a storm event. The topographic changes observed between these two situations and the directions of sediment movement deduced from tracer tracking are coherent with the residual transport pattern calculated from the grain size trend analysis. The method was tested on a ridge and runnel system which corresponds to the intertidal part of a macrotidal beach, called La Salie beach, located along the southwest French coast, in a high wave energy environment. This study permits us to recognize two different residual transport patterns for this intertidal ridge and runnel system; one for fair weather and a second for storm conditions. During fair weather, specific sites are subject to sand accretion, while during storm conditions these sand stocks are subsequently spread out over the whole area, and remain partly on the beach.

Morphodynamique d'une plage macrotidale à barres

The topography, currents, waves, and sediment dynamics of a macrotidal ridge and runnel beach were monitored from April 24 to May 4, 1997 in order to characterise the morphodynamics of this beach type under fair-weather and storm conditions. Sand tracer experiments show moderate longshore and cross-shore transport components while profile surveying highlights a remarkably stable system. These trends are not in agreement with the current pattern, which is dominated by an essentially tidally driven longshore component. The stability of the ridges reflects the high rates of migration of the breaker/swash zone in response to the important vertical tidal excursion across a wide, low-gradient beach.

Utilisation of numerical and statistical techniques to describe sedimentary circulation patterns in the mouth of the Gironde estuary

Abstract Sediment distribution is well defined in the mouth of the Gironde estuary but the sediment dynamics is still poorly understood. The aim of this work is to analyse residual circulation of non-cohesive sediment due to the action of the main hydrodynamic factor: tidal currents. A depth-averaged numerical model emphasises the residual circulation of the Gironde estuary, which is characterised by residual tidal gyres. The comparison between sediment transport calculated from the numerical model and from a statistical analysis of the sedimentary distribution leads to the definition of a simple scheme of non-cohesive sediment circulation in the Gironde estuary’s mouth.

Caractérisation d’une barre sableuse de haut de plage : la berme de la plage du Truc Vert, Gironde

The foreshore of the Truc Vert beach (Bay of Biscay, France) exhibits a sand bar called berm. 86 topographic profiles were surveyed between 1997 and 2003 in order to establish a morphological classification based on profile shape analysis. A principal component analysis and a cluster analysis make it possible to select 6 different types among the 61 berms observed during this study. Distance from the crest to the dune, berm width, its roundness and the slope are the main parameters that are considered. 1.Introduction La plage du Truc Vert (Gironde, France, Figure 1) est une zone étudiée depuis 1997 dans le cadre de l’Action de Recherche Thématique « Hydrodynamique sédimentaire » (ART 7) du Programme National d’Environnement Côtier (PNEC). C’est un environnement particulièrement complexe, caractérisé par des processus hydro-sédimentaires qui agissent sur de vastes échelles de temps et d’espace. La dynamique de cette plage est guidée par l’action des houles de fortes énergies -Hsig = 1,4 m ; Tmoy = 6,5 sdans un environnement méso à macrotidal. Ces agents énergétiques forment des barres sableuses dont la morphologie et l’évolution doivent être étudiées pour comprendre le fonctionnement de ce système côtier.