Search

Tidal sand dune dynamics is observed for two tidal cycles in the Arcachon tidal inlet, southwest France. An array of instruments is deployed to measure bathymetric and current variations along dune profiles. Based on the measurements, dune crest horizontal and vertical displacements are quantified and show important dynamics in phase with tidal currents. We observed superimposed ripples on the dune stoss side and front, migrating and changing polarity as tidal currents reverse. A 2D RANS numerical model is used to simulate the morphodynamic evolution of a flat non-cohesive sand bed submitted to a tidal current. The model reproduces the bed evolution until a field of sand bedforms is obtained that are comparable with observed superimposed ripples in terms of geometrical dimensions and dynamics. The model is then applied to simulate the dynamics of a field of large sand dunes of similar size as the dunes observed in situ. In both cases, simulation results compare well with measurements qualitatively and quantitatively. This research allows for a better understanding of tidal sand dune and superimposed ripple morphodynamics and opens new perspectives for the use of numerical models to predict their evolution. [ABSTRACT FROM AUTHOR]

Defining a model for the representation and the analysis of spatio-temporal dynamics remains an open domain in geographical information sciences. In this article we investigate a spatio-temporal graph-based model dedicated to managing and extracting sets of geographical entities related in space and time. The approach is based on spatial and temporal local relations between neighboring entities during consecutive times. The model allows us to extract sets of connected entities distant in time and space over long periods and large spaces. From GIS concepts and qualitative reasoning on space and time, we combine the graph model with a dedicated spatial database. It includes information on geometry and geomorphometric parameters, and on spatial and temporal relations. This allows us to extend classical measurements of spatial parameters, with comparisons of entities linked by complex relations in space and time. As a case study, we show how the model suggests an efficient representation of dunes dynamics on a nautical chart for safe navigation. [ABSTRACT FROM AUTHOR]

A traditional taxonomic approach coupled to a biological traits analysis was conducted in order to provide a new insight into macrobenthic communities associated with subtidal sandy environments. Results suggest that the macro-scale distribution of benthic communities is mainly driven by the migration rate of bedforms (sandbank, barchan dune and transversal dune) which changes the sediment grain size and reduces macrobenthic diversity. A classic scheme of species/traits succession was also observed from less to more physically disturbed areas. Finally, the high frequency of migration events homogenized macrobenthic communities between the troughs to the crest of bedforms. As bedforms areas are targeted for the commissioning of offshore windfarms the information provided by the present paper will be particularly useful to implement the environmental impact assessment required for such activities at sea. • Macrobenthic communities vary with the kind of bedforms because of the discrepancy in the intensity of sediment movements; • Massive recruitment by juveniles change the structural diversity in spring but not the macrobenthic ecosystem functioning; • Frequent sediment movements homogenize the macrobenthic communities between the troughs to the crest of bedforms. [ABSTRACT FROM AUTHOR]

Abstract: A one month field campaign featuring two spring–neap tide cycles and three strong storms has been performed in a mobile dune area located in the central part of the Dover Strait. These dunes are known to move in a complex manner as their migration direction varies in space and time (). In order to gain some insights into the dune motion processes we present an analysis of the spatio-temporal variability of currents in the area emphasizing the relative influence of tides and storms. A total of eight different hydro-meteorological regimes have been distinguished during the experiment duration. The analysis of the currents measurements at five locations in the area shows that the eight hydro-meteorological regimes induce very different current responses at the bottom. The residual tidal currents exhibit a significant spatial variability both in direction and in intensity. A numerical model of tidal currents over the Dover Strait confirms the strong spatio-temporal variability of the residual tidal currents featuring three singular points. Amongst them, a saddle point is located just south of the I-dune at the convergence of opposite direction residual tidal currents. The wind-induced currents are almost uniform in space, their intensity and direction however strongly depends on the wind regime and thus on time. The mean total current feature a spatial pattern which can be tidal of wind-induced currents dominated, or either in balance, depending on the regime considered. At the PERMOD campaign time scale, the total current is dominated by the residual tidal current. These results proved to give valuable insights to explain the complex dynamics of dune motion observed in this area by at short and long time scales. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to improve the knowledge of spatio-temporal variability of very large to giant dunes in deep tide dominated environment. Their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations. This motivated the realisation of three recent swath bathymetry surveys across the wide dune field of the Banc du Four located offshore the western Brittany. The Banc du Four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2D to 3D shapes and reaching the largest sizes of such sedimentary structure ever described (wavelength over 1000m and height over 30m). The analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters. Our results show a good height-spacing power law correlation but do not strike the usual statistical regression observed in previous studies. The steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime, interacting with bed load transport and sediment supply. No clear relationship has been observed between asymmetry and size parameters. Water depth cannot be considered as a major factor controlling the size of dunes. Otherwise, the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the Banc du Four were found to range from 3 to 20m·yr−1. Such velocities were only mentioned before by Van Landeghem et al. (2012) on deep continental shelves (>70m deep) and attest of the still present morphodynamical equilibrium of the large dunes. Furthermore, as was previously found for similar dunes in the Irish Sea, no power–law relationship between dune migration rates and dune size can be deduced. Similarly, no strong correlation is established between migration rates and the degree of asymmetry. Nevertheless, the dune asymmetry can be used to predict the migration direction. As for dimension parameters, the evolution asymmetry depends on the study area and migration rates. More precisely, when the crest lines of dunes form straight lines and their migration rates remain below 8–10m·yr−1, the degree of dune asymmetry increases with the migration rate. This trend is reversed for the dunes with higher migration velocities and crests which are sinuous. These results relativize the migratory predictions based on punctual geometrical observations. [Copyright &y& Elsevier]