Seasonal variations in the characteristics of superficial sediments in a macrotidal estuary (the Seine inlet, France)

Estuarine, Coastal and Shelf Science 58 (2003) 3–16

Seasonal variations in the characteristics of superficial sediments in a macrotidal estuary (the Seine inlet, France) S. Lesourd*, P. Lesueur, J.C. Brun-Cottan, S. Garnaud, N. Poupinet Morphodynamique continentale et coˆtie`re, UMR 6143 Universite´ de Caen, 14000 Caen, France Received 10 September 2001; accepted 6 August 2002

Abstract Seasonal variations in the sedimentary regime in the mouth of the Seine river, a macrotidal estuary, are described for a 3-year period. The aim of this study is to characterize and to understand the main governing mechanisms, using data from more than a thousand of superficial sediment grab samples or box cores gathered throughout the study period. Analyses of lithofacies and rheological properties were carried out. The distribution of sediments is governed by seasonal meteorological variations. The surface covered by mud reaches a maximum (40% of the total mouth area) during winter. After the winter, the soft mud deposits are progressively redistributed throughout the whole estuary area and onto the shelf. During the lowest freshwater flow at the end of summer, the fine-grained sediments cover less than 20% of the river mouth area. These seasonal variations mainly depend on the river discharge intensity, but are also linked to wave activity. In the study area, the amount of fine-grained deposits after high river flow periods depends on (1) volume of mud erodable within the estuary, (2) the duration of the flood tidal influx, and (3) the duration preceding the particular annual high river flow. During the last decades, filling of the estuary upstream from Honfleur has led to a downstream shift of the fine-grained sediment deposition area; following this, the present-day mud deposition area is in the open part of the estuary, in the subtidal shallow area. Subsequently, fresh mud deposits undergo intense hydrodynamical and meteorological effects, and are partly reworked by waves and tidal currents effects. In this study, it is shown that the behaviour of suspended matter and of superficial sediments is strongly influenced by short but intense events including high river flows and gales. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: estuary; mud; lithofacies; rheology; meteorological event; Seine

1. Introduction Significant silting up can be observed in some estuaries in temperate climate zones. As it can trap many major contaminants, mud is frequently implicated in unfavourable environmental situations and water quality problems. The Seine estuary is a tide-dominated system, according to the classification of Dalrymple, Zaitlin, and Boyd (1992) and Wells (1995), whose infilling is almost completely achieved, according to Harris (1988). The Seine river no longer transports sand (Avoine, 1987). The * Corresponding author. Ecosyste`mes littoraux et coˆtiers, UMR 8013 Universite´ du Littoral Coˆte d’Opale, Maison de la Recherche en Environnement Naturel, 32 avenue Foch, 62930 Wimereux, France. E-mail address: [email protected] (S. Lesourd). 0272-7714/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(02)00340-2

main part of the tidal ebb delta is formed of fine sands that were carried out from the shallow waters of the eastern part of the Bay of the Seine, under the combined influence of waves and flood tidal currents (Auffret & d’Ozouville, 1986). Significant silting up of the downstream part of the estuary can be observed using historical, sedimentological and bathymetric data (Lesourd et al., 2001). However, it is almost impossible to determine the sedimentary budget of the mud (Avoine, 1987; Lesueur & Lesourd, 1999). This is due to the very complex mineralogical and geochemical properties of the fine-grained particles and of their mixing in a macrotidal environment. Clays and silts that converge in the Seine estuary are of both marine and river origin (e.g. Dupont, 1997). A general trend to long-term mud deposition within the mouth of the Seine (i.e. 100-year evolution, Lesourd

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et al., 2001) is modulated by annual variations in the nature of the superficial sediments. The two major questions raised in this study are Why does mud deposition occur in an open macrotidal estuary subjected to high energy influences? What are the main mechanisms involved in the mud deposition process? The present study is a part of the Seine Aval and P.N.E.C. ÔBaie de SeineÕ multi-disciplinary programmes; it deals with an estuary that has been subjected to major engineering works.

2. General setting The Seine is the biggest river that flows into the British Channel. It has a length of 780 km and a catchment area of 78,000 km2 in which 40% of the French population and economic activity is concentrated

(Paris Basin). It ends in the Bay of the Seine (see insert Fig. 1). The Seine estuary is the result of the drowning of the Weichselian incised valley during the Holocene transgression (Lefebvre et al., 1974), as is the case of many modern estuaries (Dalrymple et al., 1992; Fairbridge, 1980; Perillo, 1995). The present-day morphology of the downstream part of the Seine is the result of its filling during the Holocene with a very large quantity of sand and carbonate biogenic material (Lesueur, Lesourd, Lefebvre, Garnaud, & Brun-cottan, 2003). The comparison of old nautical charts and sedimentological data from the 19th century shows an increase in fine-grained sedimentation in the outlet of the Seine, as well as in the shallow waters of the Bay of the Seine. Then, over the last century, sediment distribution has changed from a sand/gravel system to the muddy deposit system that prevails now (Lesourd et al., 2001). The downstream part of the Seine estuary is funnel-shaped with a mouth that opens towards the west to the Bay

Fig. 1. Seine estuary map showing the study area. Position of the RSS (water bathymetric lines are provided by the Service Hydrographique et Oce´anographique de la Marine, France). Points A and B refer to Fig. 4a and b. The sampling line in the insert refers to Fig. 5. Shipeck grabs were collected all over the shown area.

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of the Seine (Fig. 1). Elongated sand bars that are typical of tide-dominated estuaries (Dalrymple et al., 1992) have developed in the lower estuary of the Seine river. Mean river discharge is 400 m3 s
1, with extreme values of 200–2000 m3 s
1. The mean annual mass of suspended particulate matter (SPM) in the river discharge is estimated to be about 6  105 t year
1, with marked variations over years, extreme values being 1.3  105– 1.7  106 t year
1 (Service de Navigation de la Seine, unpublished data). The tidal amplitude at Le Havre is 3 m during neap tides and 7.5 m during spring tides. The Seine estuary is a hypersynchronous estuary with a maximum amplitude of 8 m at Honfleur, 10 km upstream from the opening of the main channel (i.e. the navigation channel). Here, a flow velocity up to 2.5 m s
1 can occur during the rising tide. The tidal cycle is semi-diurnal with a high water slack close to 3 h (Le Floch, 1961). In the Seine estuary, as in other macrotidal estuaries, the turbidity maximum (TM) is mainly governed by tidal currents (Allen, Salomon, Bassoulet, Du Penhoat, & De Granpre´, 1980; Avoine, 1981; Brenon & Le Hir, 1998; Le Hir et al., 2001; Li, Nguyen, Brun-Cottan, & Martin, 1995; Salomon & Allen, 1983; Wells, 1995). An overview of the results concerning the TM in the Seine estuary was published by Avoine, Allen, Nichols, Salomon, and Larsonneur, 1981 and Le Hir et al. (2001). Recent studies do not show major modifications in the TM behaviour in this estuary over recent decades (Dupont et al., 1996): the SPM concentration varies from 0.1 to 1 g l
1 (or more in the layer close to the estuary bed) during a semi-lunar tidal cycle. The total mass of the TM varies from 2  105 t during neap tide to 4  106 t during spring tide (Avoine, 1981). The longitudinal oscillation of the mass gravity centre of the TM during a spring tide is about 20 km. During high river output, a varying but large proportion of the SPM contained in the TM is expelled out to the Bay of the Seine (Avoine, 1987). Fluid mud (100–300 g l
1) expands when the SPM concentration is low, associated with a high vertical gradient of concentration, as it is the case in other estuarine settings (e.g. Dyer, 1995; Eisma, 1992; Kirby & Parker, 1983; Li et al., 1995; Ross & Mehta, 1989). Propagation of swells within the Seine estuary is highly complex and poorly documented. This is due to the uneven morphology (sand bars, embankments) and water depths that vary rapidly with the tide conditions. Nevertheless, the Seine estuary is mainly subjected to westerly to northwesterly swells and to waves generated by local winds in the Bay of the Seine, with a westerly dominance.

3. Materials and methods Sampling of superficial sediments was carried out during 15 cruises throughout the 1993–1998 period.

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Station positioning was accomplished with differential Global Positioning System (G.P.S.) and sampling with Schipek grab. Fluid mud was collected with a horizontal sampling bottle. Box corers were used to sample the mud deposits. Sediments sampled with the grab were split into two fractions. The sand/gravel fraction was sieved in a dry environment with 15 sieves using the French size standard (i.e. AFNOR definition), while the particle size distribution of the fine-grained fraction (<63 lm) was obtained with a Laser Coulter LS 230. A rapid assessment of the sedimentary cover with the Schipek grab enabled to identify the location of the mud deposits and to sample them with the box corers (core length between 10 and 35 cm). Visual description and lithofacies studies were always performed; X-radiographs were taken on a thin (1 cm) slab from each halved core. A sub-sampling technique was applied with each sample of mud recovered with the box core to determine the undrained shear strength (or undrained cohesion, Cu) of this sediment. This parameter allows us to obtain a good estimate of the state of strengthening of the fine-grained sediment (i.e. better than that obtained with sediment water content or sediment porosity). Cu was measured on board with a laboratory vane-test Wykeham Farrance WF-235000. The study zone (Fig. 1) was sampled at a density of 20 samples per square kilometre resulting in about 1000 Schipek grabs, 125 box cores and 150 sets of rheological analyses. First, during the 1993–1996 period, sampling results were used to draw maps of the sedimentary facies showing the superficial sediment variability, and secondly, to define about 20 sites representative of the different areas of the estuarine mouth. These sites, named ÔReference Sampling StationsÕ (RSS), were reoccupied and documented by all the scientific vessels cruising in the area. During the June 1996–October 1998 period, the RSS were sampled 10 times with grabs and box cores. In addition, during 1999 a sampling transect (12 km long 16 stations spaced 600 m) was also performed in the eastern part of the Bay of the Seine, near the outlet of the Seine (see location insert Fig. 1). Based on lithological descriptions and radiographs of the cores, some reference levels were defined to assess the degree of variability of the superficial sediment. Radiographic analysis has allowed a very fine description of sediment, including coarsening or fining up sequences, biological and physical structures, and facies boundaries. These descriptions were completed with rheological analyses giving a quantification of the state of strengthening of the mud. Some variations of facies were mainly characterized by the strengthening of the sediment, involving mud that was difficult to erode (i.e. stiff mud). Some other lithological features, such as noticeable shell lags, were described. The variations in the sedimentary facies were also observed in the cores sampled during the RSS reacquisition. Several cores

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which were sampled quasi-simultaneously in a vicinity of 100 m showed analogous succession of layers, only with a variation in thickness of about a centimetre in the topmost deposit. Thus, one sample was considered to be representative of the few hundreds of square metres around where it was taken.

4. Results 4.1. Seasonal spatial distribution of superficial deposits The results are based on 900 samples of superficial sediments collected along the 1993–1998 period at specific seasonal and river discharge characteristics: (1) during February and March (winter regime), sampling was done after the high flow periods (mean river discharge of 1200 m3 s
1); (2) during April and June (springtime regime, mean river discharge between 600 and 900 m3 s
1); and (3) during September (low river flow period, mean river discharge between 200 and 400 m3 s
1). A map showing the silt + clay content (i.e. the <63 lm

fine-grained content) was established for each of these regimes (Fig. 2). During the winter regime (Fig. 2a), a part of the superficial sediments of the Seine mouth was of homogenous mud deposits (fine sediment content from 25 to 75%) within a restricted area. Within the river mouth, in the Northern channel and in the trough of the ÔRade de la CarosseÕ, fine-grained contents exceeded 75% and some samples showed more than 85%. Outside the mouth of the Seine river, the fine-grained contents sharply decreased from 75 to 0% over a distance of 1 km. During the springtime regime (Fig. 2b), the total surface area of mud deposits (25–75% fine-grained material content) decreased with a drift to the North of the fine-grained sediment areas. Then, the decrease of the fine-grained material content was lower than during the winter period. During the low river flow period (Fig. 2c), the muddy areas with a fine-grained content of about 10–25% or more decreased, while muddy areas with a silt + clay content of about 5% or less increased. During this

Fig. 2. Repartition of the silt and clay content into the downstream part of the Seine estuary, during three given periods of the year. The seasonal character of the sedimentation is pointed out: (a) a high flow rate winter sediment deposit followed by (b,c) a dispersion of the fine-grained material over the area. NC, Northern channel; GPS, Grand Placard Sud; RC, Rade de la Carosse; RB, Ratelets Bank.

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period, the fine-grained content decreased progressively, from 25–75 to 0%. Overall, taking into account the silt + clay fraction in the superficial sediments of the Seine mouth, a mean value of 40% can be proposed for the fine-grained material content in winter, following high river flows. This fine-grained content decreases to 20% at the end of the low river flow period. 4.2. Temporal variability in the fine-grained contents at the RSS Each set of sampling was associated with respect to a specific river discharge value (but not with meteorological conditions that could not be determined before the sampling date). The number of samples per sampling period was constant. The proportion of samples with a fine-grained content higher than 75% was always determined (Fig. 3); this proportional value is related to river discharge and to wind intensity, the latter being taken into account when its velocity exceeded 30 m s
1 for at least 1 day. The occurrence of samples with a high fine-grained content (i.e. >75% silt + clay) was lowest during the

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high river runoff period. This occurrence increased significantly (+30%) during the weeks following high river flow events (Fig. 3); subsequently, the number of sampling stations where muddy areas had increased, decreased. Thus, during high river flow periods, the majority of the fine-grained material brought by the Seine river was deposited over a restricted surface area, whereas after these periods, mud deposits were spread over a larger area. During low river flow periods, the fine-grained sediments were scattered over the inner estuary, due to strong tidal asymmetry with flood dominant character of the currents (i.e. tidal pumping, Gue´zennec, Lafite, Dupont, Meyer, & Boust, 1999; Le Hir et al., 2001; Wolanski, Moore, Spagnol, D’Adamo, & Pattiaratchi, 2001) and in the surrounding shallow water of the Bay of the Seine, where they are mixed with sands and gravel of the inner shelf. 4.3. Thickness of the recent mud deposits At the RSS where the extend of muddy sediment cover was variable over time, the processes governing the sedimentation were studied by determination of lithofacies, thickness of the sediment layers and rheological

Fig. 3. Seine river discharges during 1996–1999. The flow rate is measured at the lock of Poses (data provided by Service de Navigation de la Seine, Rouen). The mean high flow rate is 700 m3 s
1 and the mean low river flow it is 300 m3 s
1. Dots at the upper part of the graph mark the gale and storm periods (data provided by Se´maphore de la He`ve, near Le Havre). The vertical line at the 03-99 date points the Bay of the Seine sampling along the transect. The dots figured on the solid line (sampling dates) show the proportion of silt + clay content greater than 75%: after the high flow rate period, the number of RSS having more than 75% silt + clay is small (5% during February 1997 and 20% during March 1998).

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characteristics of the mud. There was no evidence of tidal cyclicity in the RSS cores, unlike those found in some protected areas in the inner part of the Seine estuary (Lesourd, 2000) and in other estuaries in the English Channel (e.g. Tessier, 1993). Two sets of cores were sampled in an area where recent mud deposits had occurred: the first one in March 1997, a week after a high river flow period (1200 m3 s
1) and the second one in February 1998, 2 weeks after a high river flow period (1250 m3 s
1, Fig. 3). The fresh mud deposits were identified with respect to their lithofacies and their physical characteristics. The March 1997 cores showed at their top a layer of olive grey soft silty mud, having a thickness of 1–10 cm, and the February 1998 cores showed at their top a layer of soft mud 1–8 cm thick (mean thickness of 3 cm). An estimation of the total mass of the fine-grained deposit after high river flow periods was carried out by taking into account the surface of the muddy area, its thickness and its water content. This mass is of about 1  106 t (uncertainty of 30%) for the March 1997 period and of 7  105 t (uncertainty of 30%) for the February 1998 period. However, the February 1998 fresh mud deposits stretched out on a wider area of the Seine mouth than the March 1997 ones. The March 1997 and February 1998 high river flows were characterized by similar river discharge values and comparable wave energy levels (Fig. 3). Nevertheless, the 1998 maximum runoff occurred after a long period of high river flow, while in 1997 the maximum runoff was a single, brief event inducing a flush effect. Moreover, the 1998 coring was carried out a week later than the 1997 one, regarding the high river flow event. These deposit conditions and coring period disparities could explain the thickness variability of the two mud deposits. 4.4. Temporal variability of sediment properties Two-year sedimentological variations are described, based on the lithological descriptions. Two examples have been chosen among all the studied stations (Fig. 4). Results from reacquisition of two RSS are described below: the first station is in the inner part of the estuary, in the Northern channel (point A, Fig. 1) and the other example is in the outer part of the estuary (point B, Fig. 1). 4.4.1. Temporal variations in superficial sediments into the inner estuary This part of the estuary mainly consists of the Northern channel, with 4-m water depth. In the A-RSS, the initial water/sediment interface was defined as the surface of a stiff black mud with interlaminae of sand/silts and shell fragments (Fig. 4a). In March 1997, after a brief high river flow period, this reference facies was topped by a soft mud layer reaching a thickness of 10 cm. In

September 1997, the residual part of this mud layer was strengthened, as revealed by a marked increase of the Cu value (0.3–0.6 to 4 kN m
2), linked to reduction in thickness. During the September 1997–February 1998 period, a thin soft mud deposit was sampled; it was probably due to the high river discharge values during the December 1997–January 1998 period. Another thin mud layer, which was difficult to distinguish from the former, was observed in June 1998, after two high river discharge peaks (1200–1300 m3 s
1 in April and May 1998; Fig. 3). To sum up, during the high river flows periods: (1) the mud deposits are mainly located in the shallow subtidal areas of the Seine mouth; and (2) the thickness of both the soft and poorly consolidated mud (Cu=0.1– 0.5 kN m
2) and the associated fluid mud can locally exceed 10 cm in this part of the estuary. Hydrological data obtained in the Northern and the main channel during these periods (Lesourd, 2000) showed a predominance of seaward transport of water and SPM, and the system formed by the turbidity maximum and the associated fluid mud drifts towards the outlet and the Bay of Seine (Avoine, 1981; Lesourd, 2000). Conversely during low river flows, the turbidity maximum and the associated fluid mud drift towards the inlet and landward in the estuary. 4.4.2. Temporal variability in superficial sediments at the outer boundaries of the estuary During the same sampling times, soft mud accumulates in the shallow subtidal areas (5–10 m water depths) of the Seine outlet, with a lag time varying from a week to several weeks with respect to the higher river runoff. For example, in this area (Fig. 4b) a recent homogenous soft mud stretched out on fine sands in June 1997, 2 months after high river flows. Later, a part of this mud layer had consolidated. Depending on the meteorological conditions, the residual deposit was topped with: (1) thin but homogenous (i.e. non-structured) mud layers which enclosed some more consolidated mud fragments (i.e. mud balls or mud clasts) during low river flow; or (2) fine sand laminae during winter; or (3) a thick soft mud deposit with interlaminated silts after a long high river flow period during spring (e.g. April– June 1998; the later persisted until autumn 1998). In the shallow water depth area of the eastern part of the Bay of Seine, offshore of the Seine estuary and the Calvados coast, veneers of fluid-to-soft mud generally occur temporarily. Deposition and erosion periods occur alternately, depending on the season and on meteorological conditions (Garnaud, Lesueur, Lesourd, Poupinet, & Brun-Cottan, 2002). The periods of finegrained sedimentation are mostly in relation with the SPM brought by the rivers, mainly the Seine, during the high river flow periods. Nevertheless, these shallow water mud deposits are also affected by tidal currents

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Fig. 4. Time evolution of lithofacies based on cores sampled at different dates at the RSS. Cores were sampled with a box corer and were interpreted after visual description, photography and X-ray photography, and measurements of the undrained shear rate with a vane-test. (a) Northern channel (see location Fig. 1, RSS A at 4 m below lower sea level). Note the homogeneous soft mud associated with high river flow period. (b) Subtidal area of the outlet (see location Fig. 1, RSS B at 7 m below lower sea level). Note that in this area, the soft mud deposit thickness increases during the low river flow. The soft mud is interlaminated by silt.

and wave effects. A representative situation of these successive mechanisms was observed in 1999. The Seine winter high flow was long (7 months) and intense (mean river discharge of 770 m3 s
1; Fig. 3). In March 1999, when the maximum river flow occurred, the mud deposit was very weighty, with soft mud deposits containing more than 30% of fine-grained particles and reaching 5 cm in thickness at the seabed at some locations (seaward RSS; Fig. 5). After winter, the thickness of the mud deposit was reduced; in May 1999, the thin persistent layer of soft mud had shifted, resulting in a spreading of the winter mud deposits over the eastern part of the Bay of the Seine. This annual tendency leads to a

residual increase of the mud content in this shallow offshore area, by mixing of fine-grained estuarine supply with marine shelly fine sands.

5. Discussion The variations of the characteristics of the sedimentary cover of the Seine estuary, like with other estuaries, are strongly related to the river discharge intensity (Cooper, 2002; Uncles, Stephens, & Harris, 1998; Woodruff, Geyer, Sommerfield, & Driscoll, 2001). As it is the case with other estuaries subjected to engineering

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Seaward

Coastward

Silt + clay content (%)

60

March 99

50

May 99

40 30 20 10 0 22

23

24

25 26

27

28 29 30 31 32 Station number

33

34 35

36

37

Fig. 5. Time evolution of the silt + clay content of sediments sampled in the southeastern Bay of the Seine study area (see localization insert Fig. 1, sampling line). Short-term variations of silt and clay content are shown from winter to spring 1999; see Fig. 3 for Seine river flow rates and wave activities.

works (O’Connor, 1987; To¨nis, Stam, & van de Graaf, 2002; Wolanski et al., 2001), in the Seine estuary the areas of deposition available for moveable sands and mud are now located in the outlet, where the current velocity field intensity decreases. The upstream part of the Seine estuary acts as a Ôbuffer systemÕ with respect to the fine-grained material (Gue´zennec et al., 1999); this is due

to the loss of tidal energy in the upper reaches (Avoine, 1981) linked with dykes and embankments effects (Lesourd et al., 2001). Thus, the characteristics of the mud deposit are linked to seasons; however, the mud concentration is not clearly related to the tidal amplitude (i.e. spring-neap tides), as it is shown in other man-altered estuaries, e.g. the Scheldt (Fettweis, Sas, & Monbaliu, 1998). A schematic scenario is proposed to describe the cycle of the fine-grained material within the Seine mouth area (Fig. 6). This scenario synthesizes the results obtained by data based on more than 100 box cores that were collected in representative areas (the referred RSS) and during variable hydrodynamic conditions (tide, waves, river flows). It is based on short-time processes, including tidal activity and meteorological situations and on medium- and long-time processes including low and high river flow regimes; it also takes into account the fluid mud phase. 5.1. Short-time processes effects The tidal short-time erosion/deposition balance furthers the deposition term, which is due to some specific characteristics of the Seine estuary: (1) the long high tide slack (more than 2 h) that favours the deposition of a

Fig. 6. A descriptive synthesis of the two main periods in a seasonal mud cycle at several stations (the RSS) in the mouth of the Seine estuary (a) after a brief high river flow regime (corresponding to Fig. 8a), (b) long low river flow regime (corresponding to Fig. 8b). This description is set on a synthesis of core observations, grab sampling and SPM concentration measurements.

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large fraction of the SPM contained by the water column; and (2) the fast settling of SPM and the strengthening of the mud are due to a high silt content (quartz, calcite) and a low clay mineral content (Lesourd, 2000; SOGREAH, 1991). Meteorological events also play an important role; low pressures are associated with heavy rains that are often correlated with high river flow regimes, but they also generate waves that can stress areas of shallow subtidal areas and the intertidal mud flats, mainly on the northern bank of the estuary. Wave activity cancels out the potential tidal signature in mud deposits and impedes the mud strengthening process. If high-energy waves occur during high river flow periods, consolidated mud is eroded and transported downstream to the Seine outlet. As it is the case with other settings (Dyer, 1986; Maa & Mehta, 1987; Mehta, 1989, 1991; Ward, 1985), consolidated mud laminae or beds are then reworked, the product of which are mud clasts which are frequently sampled in the superficial sediments over the Seine mouth. Wind-induced waves and storms have a dramatic effect on the fine-grained sediment cycles; they are known to control resuspension process of the fine sediment and the resultant pattern of SPM in waters, and to generate an increase in the turbidity maximum (e.g. Eisma, 1992; Jiufa & Chen, 1998; Weir & McManus, 1987). As an example in the Seine estuary, the total mass of sediments eroded from the intertidal Northern mudflat during a given gale period lasting over few days was estimated to be of the same order as the total SPM mass of the TM (i.e. 2–4  105 t on spring tide following Avoine, 1981). 5.2. Medium-time processes regimes The medium-time (i.e. annual) behaviour of the sediment cover is mainly governed by the seasonal effects of the short high river flow periods and the long low river flow periods, along years. 5.2.1. Short high river flow periods During the brief and sometimes repetitive high river flow periods, mainly during the winter season, the mud deposits are into restricted areas and show homogenous lithofacies (massive beds), suggesting a rapid accumulation process (see Nichols, 1984/1985). Thus, the mud thickness would depend on: (1) the availability of the sedimentary built-up during the low river flow (Van Leussen, 1991); and (2) the duration and intensity of the high flow period; an intense, brief and high river flow runoff, would then supply a large amount of fine material to the lower estuary by comparison with a high river discharge that has been progressively established. Once formed from high-concentration SPM flows, fine-grained deposits could develop sufficient cohesive strength to withstand high-energy conditions (McCave,

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1984). Thus, the thickness of the initially settled finegrained material may control the preservation of recent superficial mud layers (Migniot, 1989). When an initial layer is thick enough, only the topmost part of the fresh mud deposit would be eroded, while its basal part is preserved during a time long enough to allow strengthening and protection against erosion (Fig. 7). The composite fine-grained material, from both continental and marine origins, can be removed by waves and then mixed with direct fluvial supply. As in many other coastal settings (e.g. Goff, Dunbar, & Barrett, 1998) during flood events, wind conditions (i.e. direction, speed and duration) have a significant influence on fine-grained sediment accumulation. This phenomenon occurs during short periods corresponding to very high increases of the river flow. Thus, the resulting solid discharge to the mouth is not only a terrestrial output of fine sediments, but must be considered as an Ôestuarine floodÕ supply. 5.2.2. Low river flow regime Present-day areas of mud deposition in the Seine estuary are down to the mouth after high river flow periods. This is due to the lack of areas available for deposition in the upstream part of the estuary (Avoine et al., 1981; Lesourd et al., 2001), and to the decrease of the ebb current velocity at the exits of the Northern and main channels. The SPM and settled matter of continental origin dominate in the lower estuary (Dupont, Lafite, Huault, Homme´ril, & Meyer, 1994). During the long (i.e. from springtime to autumn) low river discharge periods, due to the tidal pumping, the composite fine-grained material (marine and fluvial material) is stored within the estuary (Gue´zennec et al., 1999). This is frequently seen in macrotidal estuaries, e.g. the Elbe (Schoer, 1990) and Scheldt (Verlaan, 2000). Depending on the thickness of the mud deposits after the high flow regime, a part of the fresh mud is resuspended and then can be swept out of the estuary. Another part of the deposit consolidates (strengthening effect) and favours the long-term residual mud deposit balance (Fig. 8). 5.3. Fluid mud behaviour Fluid mud is considered as an important state in the cycle of fine sediment in estuarine systems, where it allows net accumulation, even in energetic environments (Nichols, 1984/1985). In the Seine estuary, fluid mud is not as developed as in the Gironde (Allen, 1972) or the Loire (Gallenne, 1974) estuaries, but it is still important. In the study area, this kind of very dense suspensions mainly occurs: (1) during high river flow events and after periods of westerly or southwesterly winds (SOGREAH, 1991) in the downstream part of the main

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Fig. 7. Reconstruction of mud erosion/deposition and strengthening processes in the Seine estuary. (a) During the initial period of high river flow events (few days) involving an intense SPM supply, a thick layer of mud deposit having fast strengthening takes place; the upper part of this deposit is eroded. The brief initial period is followed by a monthly period characterized by an increase of the critical erosion shear stress value of the mud. (b) During low or mean river flow periods, the mud supply remains moderate and erosion predominates; then, mud layers are reworked and the resulting material is scattered during the following tidal cycles.

central channel; and (2) at the beginning of the low river flow period in the upstream part the Northern channel (Lesourd, 2000). Fluid mud is also found inside the transverse channels of the intertidal flats, at the beginning of the low river flow period. Some weeks after it was formed, this very fluid mud has partly consolidated and behaves as soft mud that fills the transverse channels during summer time (Fig. 9). To summarize this study at an year-scale, it can be shown that the Northern channel and the Northern mudflat operate as follows: during the low river flow period, these areas act as temporary mud zones of accumulation, during high river flow and winter periods, these areas act as source zones for the fine-grained material, including the erosion of the Northern mudflat; the latter increases the turbidity maximum. By the same time, the TM and the associated mud

deposition areas drift downstream, towards the outlet.

6. Conclusions The Seine mouth shows a long-term (i.e. pluridecade) mud (i.e. silt + clay) accumulation tendency, regarding the fine-grained erosion/deposition balance. Nevertheless, this tendency is modulated by meteorological events linked to seasonal variations. The main results of the present study are 1. The silting up of the Seine mouth is primarily the result of the long-term downstream shift of the TM and the associated fluid mud. This seaward movement of fine-grained material occurs mainly during estuarine floods (i.e. continental supply plus temporary fine-grained deposits removed from the inner

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Fig. 8. Residual SPM discharges directions (arrows) during high and low river flow periods. Calculations are set on turbidity and horizontal current velocity fields. Tidal effects during these periods are smoothed out. Results concerning the main navigation channel are from Avoine (1981, 1987). During a high river flow regime (a), all the residual SPM discharges directions point out to the sea (west direction). During low river flow periods (b), the residual SPM discharge is still west-oriented in the main channel, while in the Northern channel it is east-oriented. RB, Ratelets bank; NC, Northern channel.

estuarine system) brought by intense fluvial runoff. Subsequently, mud accumulates into the shallower subtidal depths of both the Seine mouth and of the Eastern Bay of the Seine. 2. The sediment erosion/deposition cycle of the finegrained material is a seasonal one that can be divided into two main successive periods: during winter, the fine-grained sediment supply is the result of high river discharge periods, amplified by ebb currents during spring tides. The mud deposit areas are then limited to the proximal outlet part of the estuary. The associated mean silt þ clay content generally exceeds 75% in the topmost 10 cm of the superficial sediment. during low river flow, the fine-grained sediments accumulated in the estuary outlet are reworked by waves and tidal currents. A fraction of this material is scattered away to the Bay of the Seine, but another part is transported landward into the estuary where it contributes to the deposition of estuarine mud. SPM has a tendency to settle in the lateral zones and in some sink holes. These deposits (i.e. residual mud) make up a superficial

cover that contributes to the filling of the Seine outlet. In the open areas, at the outer boundaries of the sand banks, the thickness of the annual residual mud deposit does not exceed a few millimetres, being interlaminated with sands; in a few rare shallow areas where strong tidal currents and wind waves occur, there is a lack of recent muds. Finally, the wide-ranging results obtained here allow to conclude that the high spatial and temporal variability of the sedimentary cover in the Seine mouth is governed by the two main seasonal period regimes that correspond to meteorological effects. After the newly projected large-scale engineering works, the inner estuary will be closely infilled, that will dramatically stress the Seine estuary behaviour and sedimentary processes. Acknowledgements The authors thank the Service de Navigation de la Seine (S.N.S.) for supplying hydrologic data, the semaphore de la He`ve, the National Institute of Sciences of the Universe (I.N.S.U./C.N.R.S.) and the crews of the

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Fig. 9. Two photos of the Northern mudflat (a) after a high river flow (08/01/98), corresponding to Fig. 8a, (b) during low river flow (20/07/98), corresponding to Fig. 8b. During high river flow events (a), the mud stored before into the inner estuary is then deposited into the Seine mouth and expelled to the Eastern Bay of the Seine (with the corresponding shift of the turbidity maximum). Thus, the subordinate transverse channels of the mudflat are empty. During low river flow periods (b), a large amount of mud is deposited on the Northern mudflat and the transverse channels are fully loaded of soft mud.

N/O ÔCoˆte de NormandieÕ and ÔCoˆte d’AquitaineÕ for their operational support. This work was carried out within the framework of the Seine-Aval scientific programme, financed by the following partners: the French State, the Haute-Normandie Region and the other Regions of the Paris basin, the Agence de l’Eau Seine-Normandie, and the industrial firms of Haute-Normandie.

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