This study investigated the spatio-temporal evolution and preservation of a highstand coastal wedge in the Holocene deposits of the macrotidal Bay of Mont-Saint-Michel (NW France). Vibracores were ...collected in two contrasted environments of the Bay: (1) in the estuary, which is characterized by very strong tidal currents, and (2) along the north-eastern littoral, which is made up of an elongated wave-dominated barrier. Facies analysis associated with radiocarbon dating revealed marked differences in the preservation potential of sedimentary sequences in both environments. In the general framework of a very slow rise in sea level, these two environments are subject to different hydrodynamical, sediment supply and coastal morphology conditions. In the north-eastern area, the coastal wedge is made of successive tidal lagoon infill sequences. Due to their back-barrier origin, each individual sequence has been preserved; the supply of sediments combined with wave dynamics are the main factors that control the way of the system functions. In the estuary, the sediment wedge comprises a single tidal channel infill sequence reaching down to the substratum. In this area, the sediment supply is very high and tidal currents are very powerful. As the active tidal channel occupies all the available space, lateral migration of the channel is the main factor that controls the temporal and spatial development and preservation of the sequence. The potential for the sequence preservation in the estuary is therefore minimal and only the last channel infill sequence is in fact preserved. Dating revealed that the tidal lagoon sequences and the tidal channel infill sequence correspond to millennial and centennial time scales respectively. This study showed that local factors such as the sediment supply and the hydrodynamics, combined with pre-existing topography, play a significant role in the preservation of coastal wedge sequences during sea-level highstand conditions. It also revealed the time scale of the preserved sequences.
A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric ...data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift of the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year.
Shellfish farming, particularly oyster farms, suffers from strong siltation phenomena which are harmful to the production of shellfish. The lack of knowledge about the impact of an oyster farm on the ...wave propagation and on the flow remains a significant difficulty for the comprehension of sediment transport processes in coastal zones. These effects are one of the possible sources of sedimentation. The study presented here focuses on describing finely hydrodynamical phenomena (velocity fields, turbulence parameters) around oyster tables. The analysis is based on experimental trials carried out in a flume tank on reduced models of oyster tables. Experiments highlight the flow perturbations in the near field and constitute a database for validation of numerical models developed at the same time. The model based on Navier–Stokes equations offers the possibility to study the impact of more realistic table lengths on the flow and to simulate the perturbation produced by a group of oyster tables. Both experimental and numerical results are presented in this paper.
PICARD SOL is the ground component of the PICARD mission and is operational since March 2011. A set of instruments including the replica of the space instrument and several atmospheric monitors was ...set up at Calern observatory in order to compare solar radius measured in space and on ground and to better understand and calibrate atmospheric effects on ground based measurements. SODISMII provides full disk images of the chromosphere and photosphere of the Sun in five narrow pass bands ranging from the near ultraviolet to the near infrared. Our preliminary results show a very good instrumental stability. After plate scale calibration using star doublet observations and corrections for atmospheric refraction, first estimates of the mean solar radius at the five wavelengths (393.37, 535.7, 607.1, 782.2, and 1025.0nm) are deduced from measurements recorded between May 2011 and December 2012.
Oyster farming structures are artificial obstacles which disturb tidal flow and wave propagation. These effects can induce modifications of erosion and sedimentation patterns, turbidity changes, ...local silting up and can be threatening for the shellfish farming itself. The understanding of the impact of these structures in terms of hydrodynamics and sediment dynamics in the far-field, i.e. at the scale of a bay, is a very challenging task.
In order to investigate the far-field impact, it is very important to understand in the first place all the changes which occur at a smaller scale, i.e. at the scale of a single table for a farm consisting of oyster tables made of metallic wire structures on which porous bags of oysters are laid. This work is carried out through the idealized representation of the in-situ flow in a free surface flume tank. The flow characteristics around the overall structure are determined from velocity measurements obtained by laser velocimetry. The results highlight an asymmetric development of the boundary layers which suggest the existence of preferential areas for silting up and suspended matter fragmentation under the table.
Many estuaries in the world have been subjected to significant human impact since the 19th century. Natural infilling and human activities, including building of embankments, dikes and jetties have ...modified both the morphology of these estuaries and the distribution of sedimentary facies. The Seine, a macrotidal estuary, provides a good opportunity to study such modifications, as the latest morphosedimentary observations could be compared to old sediment maps as well as to geotechnical drilling data. In the mouth of the Seine, originally a sand/gravel system sediment distribution, has now been transformed into a muddy system. The result is a regressive sequence several meters thick. It is typically fining-up and corresponds to a shift from a distal term (pebbles, gravel, and coarse sands) at its base to a proximal term (mud and fine sands) at its top. Civil engineering works have reduced the available amount of space within the estuary, leading to an increase in the natural downstream shift of the depocenter of mud brought by winter river floods. The deposition area of the mud is today in the open marine zone, where waves and tidal currents render the balance of this process precarious.
This paper presents the results of an investigation on different timescales of the main mechanisms governing fine-grained sedimentation on a macrotidal sandy shoreface-to-inner-shelf setting with a ...supply of terrigenous sediment: the subtidal area of the southeastern Bay of the Seine (Calvados coast, France). Interpretation and calibration of side-scan sonar imagery clearly shows that compact clays crop out in water depths of 3–6 m. Radiocarbon dating and palynological studies of the material sampled using long cores from this subtidal area show that these relict sediments constitute the infilling of lateral valleys of the palaeo-Seine during the last 10 000 years. The lower parts of these deposits consist of compact clays that accumulated in a floodplain setting, later a salt marsh environment, and are succeeded by sediment with sand/mud couplets which formed in a tidal/estuarine system. The top of this sequence has been truncated by a wave erosion surface formed during the Holocene transgression. Today, the sediment accumulating is composed of fine sand, mixed with fine-grained sediment and sometimes temporarily covered by fresh mud. The more recent sedimentological data, compared with surveys in the 1960s–1970s, demonstrate both an increase in the erosion of the submerged earlier Holocene clays and an increase in the mud (silt+clay) content of the superficial sediments. On a seasonal timescale, the seafloor is affected by high-frequency variations in the nature of the contemporary sedimentary cover. Spatial and temporal observations of the seafloor composition have been undertaken during different seasons for four years (1998–2001) to study these sedimentation events. The sedimentation on the inner-shelf is at its maximum when veneers of fresh mud occur after some particular hydrological periods, i.e. sustained high-river outputs following several dry years (i.e. prolonged weak river flows), when significant volumes of mud have been stored within the Seine estuary. Such mud veneers result from: (1) the direct supply of river-born material, (2) the seaward shifting of the turbidity maximum, and (3) the resuspension of mud from the lower estuary (i.e. fluid mud and intertidal flats) under wind-waves, and have been termed ‘estuarine flood deposits’. On a longer timescale of at least the last decades, the southeastern Bay of the Seine is an area of erosion, but it is subjected to ephemeral fine-grained sedimentation on a seasonal timescale.
Side-scan sonar investigations in the eastern part of the macrotidal Bay of Seine have revealed the presence of numerous rippled scour depressions (RSDs) at water depths of 5-9 m. The sediments in ...these depressions consist essentially of coarse-grained shell hash derived from underlying Holocene sediments dated at roughly 6,500 years BP, and arranged in large wave-generated ripples. The shallow marine area where these features occur consists of a wave-generated ravinement surface produced during the marine flooding of the late Holocene transgression. It can be shown that, during the last 20 years at least, erosion of the muddy sand and sandy seabed has exposed underlying relict sediments. These consist of stiff clays, silts and a layer of shell debris which, when exposed, cover the bottom of large scour depressions which appear to be in equilibrium with the local hydrodynamic regime. Morphological and hydrodynamic data suggest that the RSDs are generated by strong cross-shore bottom currents flowing parallel to the features in the direction of the prevailing waves, and probably associated with storm-induced downwelling events. PUBLICATION ABSTRACT
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