Robert Lafite

Fine sediment transport and accumulations at the mouth of the Seine estuary (France)

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 to 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.

Hydrodynamics of Suspended Particulate Matter in the Tidal Freshwater Zone of a Macrotidal Estuary (the Seine Estuary, France)

The effects of fortnightly, semidiurnal, and quaterdiurnal lunar tidal cycles on suspended particle concentrations in the tidal freshwater zone of the Seine macrotidal estuary were studied during periods of medium to low freshwater flow. Long-term records of turbidity show semidiurnal and spring-neap erosion-sedimentation cycles. During spring tide, the rise in low tide levels in the upper estuary leads to storage of water in the upper estuary. This increases residence time of water and suspended particulate matter (SPM). During spring tide periods, significant tidal pumping, measured by flux calculations, prevents SPM transit to the middle estuary which is characterized by the turbidity maximum zone. On a long-term basis, this tidal pumping allows marine particles to move upstream for several tens of kilometers into the upper estuary. At the end of the spring tide period, when the concentrations of suspended particulate matter are at their peak values and the low-tide level drops, the transport of suspended particulate matter to the middle estuary reaches its highest point. This period of maximum turbidity is of short duration because a significant amount of the SPM settles during neap tide. The particles, which settle under these conditions, are trapped in the upper estuary and cannot be moved to the zone of maximum turbidity until the next spring tide. From the upper estuary to the zone of maximum turbidity, particulate transport is generated by pulses at the start of the spring-neap tide transition period.

Resuspension and advection processes affecting suspended particulate matter concentrations in the central English Channel

Suspended particulate matter (SPM) measurements obtained along a cross-section in the central English Channel (Wight-Cotentin transect) indicate that the area may be differentiated into: (1) an English coastal zone, associated with the highest concentrations; (2) a French coastal zone, with intermediate concentrations; and (3) the offshore waters of the Channel, characterised by a very low suspended-sediment load. The SPM particle-size distribution was modal close to the English coast (main mode 10-12 mu m); the remainder of the area was characterised by flat SPM distributions. Examination of the diatom communities in the SPM suggest:; that material resuspended in the intertidal zone and the estuarine environments was advected towards the offshore waters of the English Channel.

Tidally-induced shear stress variability above intertidal mudflats in the macrotidal seine estuary

Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.

Role of suspended sediments on the distribution of PCB in the Seine Estuary (France)

Abstract PCB (polychlorinated biphenyls) contamination and its relationship to SPM (suspended particulate material) have been studied in the Seine Estuary, which is heavily polluted by these persistent and hydrophobic man-made chemicals. Two sampling cruises have been performed during different freshwater discharge conditions. PCB and SPM concentrations, as well as grain-size distributions in the particulate material have been determined. Water samples have been collected at fixed positions during a tidal cycle, and along transects within the estuary. PCB concentrations vary from 2 ng 1−1 in the marine zone to 250 ng 1−1 within the estuary, and reach 1.3 × 103 ng 1−1 in the turbidity maximum zone. During a tidal cycle, low PCB concentrations are observed at high water, and are in the same range in February as in July. High PCB contamination is observed at low water, but PCB concentrations are about five times higher in February. SPM and PCB variations are well correlated in both periods of observations. Higher PCB contamination during February is explained by higher SPM inputs, mainly due to particles of riverine origin. The transport of PCB within the estuary depends on the quantity and the grainsize composition of suspended material, which varies according to freshwater discharge and tidal amplitude.

Holocene to modern fine-grained sedimentation on a macrotidal shoreface-to-inner-shelf setting (eastern Bay of the Seine, France)

Abstract 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.

Suspended particulate matter fluxes through the Straits of Dover, English Channel: observations and modelling

Suspended Particulate Matter (SPM) concentrations at various levels within the water column, together with salinity and temperature, were measured using water samples collected from six stations across the Straits of Dover. The sampling programme covered a 16-month period, undertaken during 23 cruises. On the basis of the spatial variability in the concentrations, the water bodies are divided by several boundaries, controlled by tidal and wind conditions. Within the water column, SPM concentrations were higher near the sea bed than in the surface waters. Throughout the cross-section, maximum concentrations occurred adjacent to the coastlines. Temporal variability in the SPM concentration exists on daily and seasonal scales within the coastal waters (4.2 to 74.5 mg L-1): resuspension processes, in response to semi-diurnal tidal cycles (with a period of around 12.4 h) and spring-neap cycles (with a period of 15 days) make significant contributions. Distinctive seasonal/annual concentration changes have also been observed. In the offshore waters, such variability is much less significant (0.9 to 6.0 mg L-1). In the summer the English Coastal Zone is associated with relatively high SPM concentrations: the Central Zone has a low and stable SPM concentration between these zones, there is a Transitional Zone, where there is a rapid response of SPM concentration to wind forcing. Finally, the French Coastal Zone is characterized by variable (sometimes high) SPM concentrations. Because of the zonation, SPM fluxes within the Dover Strait are controlled by different transport mechanisms. Within the Central Zone, the flux can be represented by the product of mean water discharges and SPM concentrations. However, within the coastal zones fluctuations in SPM concentrations on various time-scales must be considered. In order to calculate the maximum and minimum SPM fluxes, 10 cells were divided in the strait. A simple modelling calculation has been proposed for this complex area. The effect of spring-neap tidal cycles and seasonal changes can contribute significantly to the overall flux, which is of the order of 20 x 10(6) t.yr(-1) (through the Dover Strait, towards the North Sea). Such an estimate is higher than most obtained previously

Continental/marine ratio changes in suspended and settled matter across a macrotidal estuary (the Seine estuary, northwestern France)

Abstract Suspended matter movements in the macrotidal Seine estuary (France) are mainly controlled by tidal currents. During high river spates, suspended matter is carried out of the estuary into the English Channel. A large part of the estuarine suspended matter comes from continental inputs. Throughout the year, suspended matter is trapped in the Turbidity Maximum Zone and on the estuarine tidal flats. Marine particles which are concentrated in the Turbidity Maximum Zone can be transported upstream into fresh water conditions. Physical and chemical measurements, as well as grain size analysis associated with scanning electron microscopy techniques, are used to study the characteristics of silty sediments and suspended matter. In the Turbidity Maximum Zone of the Seine estuary, particulate tracers, such as diatoms, indicate a continental/marine ratio more than 90% in the suspended matter. These results are similar with those from radiotracers. Sedimentary analyses on fine deposits from the estuarine tidal flats show close similarities with suspended matter coming from the Turbidity Maximum Zone. The sedimentological processes of the Seine estuary are compared with those in some other estuaries.

Relationships between hydrosedimentary processes and occurrence of mercury-resistant bacteria (merA) in estuary mudflats (Seine, France)

The Seine estuary (France) is one of the worlds macrotidal systems that is most contaminated with heavy metals. To study the mercury-resistant bacterial community in such an environment, we have developed a molecular tool, based on competitive PCR, enabling the quantification of Gram-negative merA gene abundance. The occurrence of the Gram-negative merA gene in relation with the topology (erosion/deposit periods) and the mercury contamination of three contrasted mudflats was investigated through a multidisciplinary approach and compared with a non-anthropized site (Authie, France). The higher abundance of the Gram-negative merA gene in the Seine estuary mudflats indicates a relationship between the degree of anthropization and the abundance of the merA gene in the mudflat sediments. In the Seine mudflats, the maxima of abundance are always located in fresh sediment deposits. Therefore, the abundance is closely related with the hydrosedimentary processes, which thus seem to be determining factors in the occurrence of the Gram-negative merA gene in the surface sediments of the Seines mudflat.

Diatoms as particulate tracers in the water column in the eastern english channel

Abstract In the English Channel, diatoms constitute an important part of the suspended particulate matter (SPM). In this study, the diatom contents (allochthonous and autochthonous) in surface waters are used as biogenic tracers to define the sources of suspended material and the exchange of particles between the different water masses in the eastern English Channel. Such an approach is especially worthwhile in this area where diatoms are very numerous. A factor analysis applied to the counts of dead and living diatoms showed that the following zones could be distinguished in order of importance: a. a central zone in which primary production of marine planktonic diatoms is preponderant (autochthonous diatoms); b. a zone with numerous tychoplanktonic marine diatoms, easily resuspended in shallow waters (allochthonous diatoms); c. a deeper zone with a high ratio of marine to marine-brackish benthic diatoms, which indicate particulate input from the Bay of the Seine and the Seine Estuary (allochthonous diatoms); and d. a coastal zone with numerous tidal-zone diatoms (allochthonous diatoms) which originate from littoral erosion (especially along the French coast). The combined analysis of dead and living diatoms allows a description of the passive transport of particles. A spatial study during an annual survey within the Strait of Dover showed the reproducibility of the hydrodynamical response of the allochthonous diatom content for tracing SPM. Such a study in the eastern English Channel allowed the detection of the flux of estuarine particulate input to the open sea.