River flow control on intertidal mudflat sedimentation in the mouth of a macrotidal estuary

River flow control on intertidal mudflat sedimentation in the mouth of a macrotidal estuary

Geomorphology 239 (2015) 174–181 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph River fl...

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Geomorphology 239 (2015) 174–181

Contents lists available at ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

River flow control on intertidal mudflat sedimentation in the mouth of a macrotidal estuary Antoine Cuvilliez a,⁎, Robert Lafite b, Julien Deloffre b, Maxence Lemoine b, Estelle Langlois c, Issa Sakho b a b c

Université du Havre, CNRS, UMR 6294 LOMC, Laboratoire d'ondes et milieux complexes, F-76058 Le, Havre cedex, France Université de Rouen, CNRS, UMR 6143 M2C, laboratoire de morphodynamique continentale et côtière, F-76821 Mont-Saint-Aignan, France Université de Rouen, ECODIV, Etude et Compréhension de la Biodiversité, F-76821 Mont-Saint-Aignan, France

a r t i c l e

i n f o

Article history: Received 1 July 2014 Received in revised form 18 March 2015 Accepted 19 March 2015 Available online 3 April 2015 Keywords: Mudflats Hydrological variability Development Sediment dynamics High resolution remote sensing Seine estuary

a b s t r a c t The objective of this study is to analyze the impact of hydrological variability influenced by climatic phenomena upon the sedimentary exchange between the turbidity maximum (TM) and a river mouth intertidal mudflat. This study, carried out over a period of 10 years (1997–2006) in the Seine Estuary (France), is specifically focused on two extreme periods: a wet one from 2001 to 2002 and a drier one from 2005 to 2006. This study is based on an original approach combining data gathered via low-altitude remote sensing with altimeter readings and groundlevel measurements. During this 10 year period, we observed a link between climate change and the sedimentary processes on the mudflat surface. The modifications of sedimentary processes are mainly connected to the multiannual variability of hydrological flow rates that control the positioning of the turbidity maximum, the source of the sedimentary material deposited in this intertidal zone. The TM at the mouth of the Seine estuary is well developed; its maximum mass is estimated to be between 300,000 tons and 500,000 tons (Avoine et al., 1981) with maximum concentrations in the surface waters ranging from 1 to 2 g∙l−1 (Le Hir et al., 2001). Most of the fine particles stored within the TM have been found to originate from within the catchment area (Dupont et al., 1994). In the Seine estuary, the dynamics of the estuarine TM, in response to hydrodynamic forcings, have been previously described (Avoine et al., 1981) and modeled (e.g. Brenon and Le Hir, 1999; Le Hir et al., 2001). The TM is upstream of the northern mudflat when the river flow is low (b 450 m∙s−1) and nearby the study area when the river flow is higher. Thus during wet periods, the sedimentation rates increase by +17 cm∙y−1, while during the drier one (when the turbidity maximum is located upstream of the estuary) we observed an erosion rate of 7.6 cm∙y−1. Sedimentation events in the mudflat resulting from spring tides are less frequent during dry periods, and they deposit a smaller quantity of sediment (−23% of total deposition mass per event). Because of the lower flow rates coupled with the impacts of local development, the flood tides have become dominant. This contributes to the addition of sandy or silty sediments on the mudflat, of which the slope has increased 450% over 8 years caused by erosion. © 2015 Elsevier B.V. All rights reserved.

1. Introduction As a result of their strategic positioning, estuarine areas are privileged ecosystems and the site of considerable socioeconomic activity. One of the challenges is the capacity of the estuarine ecosystem to adapt to varying levels of metals and organic and inorganic contamination. Given the high affinity of these contaminants for the particulate phase, especially fine cohesive particles (b 63 μm), we have a specific need to take the sedimentary dynamic of these particles into account. In estuarine systems, suspended solids and cohesive sediments are subject to complex hydrodynamics, controlled by natural processes such as ocean swell, which is expressed on a large scale (Green et al., ⁎ Corresponding author. Tel.: +33 2 35 21 71 02; fax: +33 2 32 74 43 14. E-mail address: [email protected] (A. Cuvilliez).

http://dx.doi.org/10.1016/j.geomorph.2015.03.020 0169-555X/© 2015 Elsevier B.V. All rights reserved.

1997; Ryan and Cooper, 1998; Bassoullet et al., 2000; Da Silva, 2002; Verney et al., 2011), tidal cycles (Eisma, 1998), and the hydrological flow rate of the river (Whitehouse and Mitchener, 1998; Christie et al., 1999; Dyer et al., 2000; Deloffre et al., 2005; Prandle et al., 2006; Talke and Stacey, 2008). Furthermore the river flow alters the position of the TM (turbidity maximum) (Avoine et al., 1981; Lesourd et al., 2003; Uncles et al., 2006), contributing to sedimentary mud deposition on the intertidal mudflats at the mouths of macrotidal estuaries (Avoine et al., 1981; Lesourd et al., 2003; Deloffre et al., 2006). Indeed, when the Seine River flow is high (wet period), the position of the TM is closer to the mudflat surface (Lesourd et al., 2003). Massei et al. (2010) demonstrated that the variability of hydrological cycles in the Seine estuary is controlled by meteorological phenomena such as the North Atlantic Oscillation (NAO). The consequences of such mechanisms for sedimentary transfers within the intertidal mudflats in estuarine areas, however,

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have received very little study. The few studies conducted clearly show the definite role of the NAO in estuarine sediment dynamics, which consists in a strong erosional trend when the NAO index is positive (Kirby and Kirby, 2008; Phillips and Crisp, 2010). One of the reasons for this relative lack of studies is the difficulty of conducting sustained, longterm (i.e., decade-long) studies that allow observers to follow the sedimentary evolution of the intertidal estuarine zones. Another obstacle lies in the superimposition of the consequences of human activity on natural forcing. This situation requires taking into account a mainly multiannual time scale, which is necessary for the readjustment of the hydrodynamic conditions in the modified environment (Cuvilliez, 2008). Multiannual and century-long studies led in various estuaries and deltas (Bourman et al., 2000; Chen et al., 2001; Wolanski et al., 2001; Van der Wal et al., 2002; Van der Wal and Pye, 2004; Lesourd et al., 2003; Blott et al., 2006; Kim et al., 2006; Cuvilliez et al., 2009) have also produced morphosedimentary results that vary greatly as a function of the hydrosedimentary context and the nature of human development. They have highlighted the difficulty of defining the impacts because of human development as opposed to those linked to climatic variation (Cuvilliez, 2008). This study was carried out over a period of 10 years (1997–2006) on an intertidal mudflat located at the mouth of the Seine River estuary (upper Normandy, France, Fig. 1). During these 10 years, two periods of slightly more than one year each were chosen because they illustrate the impacts of climate change on the sedimentary dynamics of the mudflat at the mouth of the Seine estuary. One was a wet period; the other was a dry period. In these cases climate appeared as a dominant factor in the sedimentary dynamics because the mudflat connection with the river remained the same before and after 2005 (Fig. 1). The objectives of this study are (i) to understand and quantify the role of multiannual climate cycles in the sedimentary dynamics of this area, notably during extremes of hydrological flow rate; and (ii) to decipher the impact of natural processes on different time scales. To reach these objectives a

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low altitude remote sensing technique was coupled with selective (ALTUS) and global (LIDAR) altimetric measurements at the level of the mudflat. Recordings of the hydrodynamic conditions and sedimentary sampling followed by analysis were also made. 2. Study area The Seine estuary is a macrotidal estuary with a tidal range maximum reaching 8.5 m during the highest tides. Located in the watershed of Paris, whose surface of 78,650 km2 represents 14% of the French continental territory, the estuary of the Seine, 160 km long, drains the river waters of which the average flow of 450 m3∙s−1 varies between 60 and 2600 m3∙s−1. According to Meybeck et al. (1998) the flood threshold is reached at a rate of 800 m3∙s−1. The northern mudflat is located on the right bank of the Seine River mouth estuary (Fig. 1), accounting for an area of 3.1 km2 (Cuvilliez, 2008). Its easternmost limits are set by the Normandy bridge, built between 1988 and 1995; to the west it abuts against a hook dyke and the Port 2000 dyke, finished in 2004. To the north it is bordered by a salt marsh that has ceased its extension into the mudflat as of 1998, which notably correlates to the impact of the construction of the Normandy bridge (Cuvilliez et al., 2009). Its southern limits historically correspond to the Northern Trench. The hydrological cycle plays a very important role in the morphosedimentary dynamic of the estuarine northern mudflat as it influences the positioning of the TM (Brenon and Le Hir, 1999; Le Hir et al., 2001), the principal source of cohesive sedimentary material in this section of the estuary (Avoine et al., 1981; Dupont et al., 1994). On the annual scale, Lesourd et al. (2003) have shown that subtidal deposits in the Bay of the Seine occur when flooding (flow N 1200 m3∙ s−1) induces the expulsion of the TM from the estuary. During low river flow, sediment deposits are redistributed according to tidal range and ocean swell periods. Furthermore, Deloffre et al. (2006) demonstrated that the sedimentary deposits on the northern mudflat originated in the TM.

Fig. 1. (A and B) Location of the studied area. (C) Right bank of the Seine estuary mouth with dates of the environmental planning (black) and morphologic units of the studied area (white). Black frames show the location of the areas of Figs. 2 and 5.

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The analysis of Seine River flow indicates the multiannual fluctuations of river flow following two variable time scales: one of 17 years and one of 5–9 years, which are linked to the NAO (Massei et al., 2010). This oscillation (i) effectively defines the low-pressure trough situated between northern and southern Europe (Massei and Fournier, 2012), (ii) controls 23% of the flow rate variability of the Seine, and (iii) could explain up to 35% of this variability (Massei et al., 2010). 3. Materials and methods The techniques used have allowed for a survey at two different spatial scales. A local study was conducted during the two chosen periods: from 2002 to 2003 (wet period) and from 2005 to 2006 (dry period). The annual mean flow during the first period was 902 m3∙ s− 1 (Lesourd et al., 2003; Deloffre et al., 2006) and during the second period was 331 m3∙s−1 (data from port of Rouen). The study site was chosen for its dynamic that is representative of the surface of the mudflat during the six years prior to the study period (Cuvilliez, 2008) (Fig. 2). Two ALTUS high spatial and temporal resolution altimeters were implanted in the upper and lower mudflats (Fig. 2, points 1 and 2). They each consist of a 2 MHz acoustic echo sounder, fixed at 27 cm from the surface of the sediment upon implantation. They record local variations of altitude every 10 min with a resolution of 0.06 cm (Jestin et al., 1998). In parallel, the use of a 6 MHz Nortek ADV (acoustic Doppler velocimeter) (Kim et al., 2000) made it possible to record the turbidity at 5 cm from the sediment and water interface during the study period in this reference area (Deloffre et al., 2006; Cuvilliez, 2008). In fact, it is possible to convert the data recorded by the ADV into turbidity units as was shown for example by Ha et al. (2009). Additionally, sediment traps were placed during the two chosen periods during six high tides at the level of the study surface (Figs. 1, 2) because it was the best opportunity to sample in the entire mudflat surface. This approach is intended to complement the quantitative data obtained with the altimeter and the ADV with qualitative information. Granulometric data were

obtained with eight sampling points located at the level of the gray dots in Fig. 2. Each sampling point was sampled three times to compensate for local variation of the spatial distribution of the sediments. Results were obtained with a Beckman Coulter. The organic matter content of the sediment was quantified by the technique of ignition loss at 525 °C. In order to spatially quantify sedimentary dynamics, we needed to generalize local information obtained from a surface chosen for its representativeness. To achieve this second level of study, two methods were linked: a high-resolution remote sensing technique and the LIDAR method (Light Detection and Ranging). The first method was developed (i) to allow an ease of use that is not inherent in satellite imagery and (ii) to obtain a resolution superior to that of platforms such as SPOT 5 (which provides 2.5 m resolution images) or other airborne platforms that only have a resolution of 1.75–0.40 m (Raineya et al., 2003; Brandtberg and Warner, 2006). Thus the method developed was used in a flexible time scale according to the study's scientific objectives. As part of the study, a yearly time scale was chosen during a constant time of year between late April and early June. The overview took place during periods of neap tides. This technique consists of loading digital sensors (Cuvilliez, 2008) on board an ultralight aircraft and flying over the study area at 600 m. The data recorded make it possible to perform georeferenced mapping via control points identified on the ground. Ground resolution obtained for the pixel side is 8 cm. This mapping thus enables us to precisely distinguish (i) the sedimentary facies of the surface of the mudflat, (ii) the limits of different sedimentary units, and (iii) the orientation of the dominant currents that are expressed in the form of specific sedimentary facies (Reineck and Singh, 1980; Cuvilliez et al., 2009) and the deepening of abrasion surfaces located at the front line of clumps of cord grass (Spartina sp.) (Bouma et al., 2007; Temmerman et al., 2007; Van Der Wal et al., 2008). The second method, LIDAR, allows us to distinguish the primary morphological units of the mudflat (Vilas et al., 1999) and also those of the salt marsh (Collin et al., 2010), depending on their altitude, with a precision reaching 10 cm (vertical) (Tignon, 1998) and a ground resolution of 20 cm for a flight altitude of 2000 m. In the context of this study, the boundary between the upper and lower mudflats was set at +5.6 m CMH (local sea level reference). Finally the comparison of recordings made—first at the level of the mudflat with high-resolution remote sensing and LIDAR, and secondly at the local level with high resolution satellite altimetry (ALTUS)—has made it possible to assess the volume of sediment eroded and/or laid down during events observed at the scale of the tidal cycle. 4. Results 4.1. Dynamics of sedimentary volumes during the dry and wet periods

Fig. 2. ALTUS and ADV are located at the level of points 1 and 2 on the mudflat surface. The four gray points near each ALTUS and ADV indicate the locations of sampling points during the study period.

During the drier period chosen (Fig. 3C and E), which follows an annual rhythm (Fig. 3A and B), the linking of the information provided by the two altimeters used in the study reveals two patterns: (i) an overall trend toward the erosion of the surface of the mudflat during a period of extreme low Seine River flow, and (ii) similar erosive behavior in the upper and lower mudflats estimated at − 7.6 cm ∙ y−1. Moreover, regardless of the climatic period under review, the effective wind events (Fig. 3C, with stars indicating wind events) remain a major force in the processes of erosion of the lower and upper mudflats by creating a swell. For example, we recorded 2.1 cm of erosion during a period of only 24 h on 31 July 2006 with winds reaching 18 m∙s−1, oriented between N250°E and N260°E. This result is in accordance with previous work by Da Silva (2002) in the Seine estuary that showed that the wind speed above 10 m ∙ s− 1 oriented between N220°E and N290°E increases the erosion on the mudflat by the swell generated. The same

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Fig. 3. (A) There are similar modes of variability around 17 and 5–9 years of the Seine River flow linked to the North Atlantic Oscillation (NAO) (gray line). (B) The wet and the dry representative periods chosen were linked to the Seine River flow during the 10 years of the study. (C) Evolution of the altitude of the mudflat during the dry period. The evolution of the altitude of the mudflat is shown in relation to episodes of wind characterized as effective or not (see text). Recording of variations in altitude of the mudflat (gray dots) during the drier season. The river flow is correlated to the turbidity. (D) Evolution of the altitude of the mudflat during the wet period. The evolution of the altitude of the mudflat is shown in relation to episodes of wind characterized as effective or not (see text). The river flow is correlated to the turbidity. (E) Amplitude of two representative episodes, one of erosion, the other of deposit, compared with those of a wet period at the same time of year (F). The volumes of sediments implicated in the regions outlined in dotted lines are presented in Table 1. Dark points: upper mudflat. Gray points: lower mudflat. (F) Amplitude of two episodes, one of erosion, the other of deposit, compared with those of a drier period at the same time of year (E). The volumes of sediments implicated in the regions outlined in dotted lines are presented in Table 1. Dark points: upper mudflat. Gray points: lower mudflat. SSC (suspended sediment concentration) in the turbidity maximum is mainly linked to the river flow (gray line on C and D). Compare these SSC during the wet and the drier periods.

approach was used for the wet period (Fig. 3D and F) under similar conditions of tide and wind. Even in the case of erosive swell during the wet period, the bed elevation of the mudflat surface ranged from +5 cm∙y−1 (Fig. 3D) to +17 cm∙y−1 between 2001 and 2002, which was a fairly wet period with a very high river flow (Fig. 3B).

The altimetric data recorded continuously on the mudflat, as shown in the examples in Fig. 3E and F, served to underscore the events of sedimentation and erosion. These recordings were coupled to data obtained from high-resolution remote sensing and LIDAR. Their comparison was used to estimate the volumes of sediment

Table 1 Influence of wind parameters and topography of the northern mudflat during representative episodes of sedimentary deposition and erosion during the wet period (2003) and the drier period (2006). Wind

Sediments (tons)

Speed (m∙s−1)

Direction

Between 0° and 220° Between 220° and 290° Between 290° and 360° Less than 10 More than 10 Upper mudflat Lower mudflat Episode of deposit

A B Episode of erosion C D

March 2003 March 2006 March 2003 Feb. 2006

X X

X X X X

X X X X

36,200 39,200 30,100 1600

12,900 15,000 0 28,400

The critical wind directions and speeds are taken from studies by Da Silva (2002). The indicated volumes correspond to an estimation for the entire mudflat during a tidal episode, and the limit between the upper and lower mudflats was obtained via correlation between high resolution mapping from 2003 and 2006, and data from LIDAR.

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deposited or removed during high spring tides along with river flow and different wind conditions (Table 1). These volumes were calculated based on the average density of fresh mud (1200 kg ∙ m3) and that of consolidated eroded mud (1300 kg ∙ m3) (Da Silva, 2002). 4.2. Qualitative study of the sedimentary dynamics during the dry and wet periods Granulometric analyses of samples collected on the mudflat surface during the dry period (Fig. 4) reveal that, overall, grain size in samples tends to be mainly uni- or bimodal with a recurrence of fractions of sand (peak at 200 μm) with silt (peak at 60 μm) on the upper mudflat and silty sand (peak at 80 μm) on the lower mudflat. The fractions of clay and of fine silts (b25 μm) are only very rarely observed. Comparison of the principal grain size modes recorded from the lower to the upper mudflat shows that during the study years average grain size increases from the lowest to the highest level. These results, consistent with previous work conducted in the Seine estuary (Deloffre et al., 2005), indicate the increasing agitation observed when gradually moving though progressively shallower zones (Alexander et al., 1991; Vilas et al., 1999). The average size of grains deposited on the mudflat surface during the wet period (Fig. 4) shows a large granulometric range of sediments. This confirms that the fine silts and clay particles (b 25 μm) coming from the SSC (suspended sediment concentration) of the river (Deloffre et al., 2006) contribute more to deposits than during the drier period. This is important because fine particles contribute to resistance to shear stress (Soulsby, 1983) and therefore to the forces generated by the swell. The analysis of the organic matter content of the sediments collected during the two study periods reveals a tendency to decrease when phasing from the salt marsh mudflat boundary to the lower mudflat. Levels of organic matter in mudflat surface sediments during the dry period were on average 7% for the upper mudflat and 5% for the lower mudflat, which is very different from the results obtained during the wet period with values falling between 26.6 and 15.2% of OM (Bally et al., 2004). 5. Discussion Quantitative and qualitative comparison of the sedimentary dynamics of the northern mudflat of the Seine estuary before and after 2005 reveals significant differences that cannot merely be correlated to the new construction after 2005. Indeed, during the high tides, the link between the mudflat and the river remained the same before and after 2005. It is this relationship that allows fine sediment deposits to arrive from the

turbidity maximum closer to the mudflat when the river flow is high. The multiannual-scale climatic forcing, which notably controls the flow of the river, is also responsible for the displacement of the TM near the mudflat. Studies of climate variability from the beginning of the 1950s to the present day reveal that observed dry and wet periods correspond to two variable time scales, one of 17 years and one of 5–9 years, of the Seine River flow linked to the North Atlantic Oscillation (NAO) described by Massei et al. (2010). The potentially strong effect of the NAO controls ~23% of the total variance of the Seine River flow and could even reach 35% of this variability. The multiannual study was led during a wet period (maximum Seine flow of N1800 m3∙s−1; annual average: 902 m3∙s−1) (Deloffre et al., 2006) and a drier period (maximum Seine flow of b1300 m3∙s−1; annual average: 331 m3∙s−1) (Cuvilliez, 2008). During the wet period, the general trend of the bed level surface of the mudflat was accretion with + 5 cm ∙ y− 1 (Fig. 3D) up to + 17 cm ∙ y− 1 between 2001 and 2002, while during the dry period the general trend was erosion (− 7.6 cm ∙y−1, Fig. 3C). Notably these different trends are connected to the TM of the Seine River, currently (dry period) upstream (in the middle part of the estuary) (Avoine et al., 1981; Lesourd et al., 2003). While port development activity disrupted the hydrodynamic conditions of the northern trench (Fig. 1) its completion in 2005 has resulted in changes in these hydrodynamic conditions, and the resulting morphological and sedimentary readjustments have led to (i) a reduction in the total surface area of the mudflat by 71.4% (Cuvilliez et al., 2009), (ii) an acceleration of the change of the nature of the sedimentary deposits of the northern mudflat (Cuvilliez et al., 2009), and (iii) an increase in the average slope of the mudflat rising from 0.2% in 1997 (Lesourd et al., 2003) to 0.9% in 2005 (Cuvilliez, 2008). All of these readjustments were amplified during the dry period, which reduced the flow of the Seine thereby maintaining the TM farther upstream in the river (Lesourd et al., 2003). This had the effect of limiting significant mud deposition onto this mudflat, which became unstable during spring tides. As erosion has become dominant, the altitude of the northern mudflat has shown a tendency to decrease on an annual scale. Moreover, even if there are still occasionally significant depositional episodes, this type of event merely very temporarily offsets the recurrent phenomena of observed erosion. Regarding the volumes of sediment deposited on the mudflat, the Seine River flow, impacted by climate change, no longer favors significant mud deposition. Thus the comparison of studies guided by the linkage of ALTUS recordings with remote sensing at low altitude during comparable spring tide events in 2003 and 2006 indicates that an episode of spring tide could elicit a sedimentary deposit estimated to exceed 69,600 tons (representing 33–66% of the SSC: suspended sediment

Fig. 4. Granulometric data. Average of the differential volumes of the sediments sampled in the upper and the lower mudflats during the two study periods.

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concentration) in 2003 (Cuvilliez, 2008) in comparison to 54,200 tons in 2006 (Cuvilliez, 2008). Overall the calculations show at least a 23% reduction in volume deposited. The comparison of qualitative studies of sediments deposited on the northern mudflat before and after 2005 reveals profound differences. On the mudflat, changing hydrodynamic conditions observed before and after 2005 induced a sharp drop in organic matter content, which fell from 26.6% (Bally et al., 2004) to 7% on average on the upper mudflat during the dry period (Cuvilliez, 2008). This can be explained (i) by a phenomenon of continuous erosion of the surface of the mudflat, which mobilizes this portion that is essentially present in the first few centimeters of the mudflat (Bally et al., 2004); and (ii) by the fact that the abundance of fine sands (200 μm) promotes the degradation of organic matter (Bally et al., 2004). The major differences are found in the granulometric data. Thus the fractions of sand (peak at 200 μm) and silt (peak at 60 or 80 μm) have dominated since 2005. They attest to the presence of stronger flood tides. Indeed, the limiting factors of pressure and shear stress are more significant and induce the resuspension of finer particles (Ryan

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and Cooper, 1998; Shi et al., 2012), such as those that were most common in 1999 when two peaks of 15 and 90 μm dominated in this area of the mudflat (Deloffre et al., 2005). This earlier study conducted in a wet season confirmed at that time the important role performed by the TM in the vicinity of the mudflat that was mobilized during spring tides. Are there arguments that allow us to pinpoint the origin of silty and sandy sediments? The answer to this question was obtained through studies of highresolution maps created during the period from 1997 to 2006. It focuses specifically on the evolution of the morphology of clumps of cord grass (Spartina sp.) located at the mudflat–salt marsh interface and their immediate sedimentary environment. Follow-up studies of the morphological evolution of cord grass clumps at the mudflat–salt marsh boundary were conducted in other estuaries (Temmerman et al., 2007; Van der Wal et al., 2008) and have been supplemented by dynamic measurements followed by modeling (Bouma et al., 2007; Temmerman et al., 2007). These studies confirm that the dominant current induces a deepening of abrasion surfaces in the sediment at the front line of these clumps. In the context of this study, the deep abrasion

Fig. 5. Evolution of the orientation of dominant currents (black arrows) based on a study of biogeomorphology of clumps of cord grass (Spartina sp.) (Temmerman et al., 2007), here observed on a scale of two-year intervals to 2005. We can see that in 2003 (wet period), sedimentary deposits were so significant that they succeeded in masking the abrasion surfaces located at the front line of the clumps of cord grass.

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surfaces observed on the maps at the front line of cord grass clumps indicate a change in their position before and after 2005 (Fig. 5). This biogeomorphological study (Temmerman et al., 2007) of tufts of cord grass emphasizes a shift in the orientation of dominant currents ranging from N310°E–N345°E (ebb dominant) to N10°E–N50°E (flood dominant). The construction of the dyke (breakwater, Fig. 1) redirected the flood and increased volumes of sand coming from the Bay of the Seine (Fig. 1) that were deposited on the mudflat surface (Cuvilliez et al., 2009). Moreover, if during wet periods sedimentation was taking place in a relatively homogeneous fashion at the surface of the mudflat regardless of altitude (Cuvilliez, 2008), the situation has changed following the completion of the port development in 2005 and the concurrent dry period. Indeed, the coupling of these phenomena induced instability in sedimentary deposits and started a trend toward erosion on the mudflat. 6. Conclusion As a result of studies conducted over a period of 10 years at the mouth of the Seine estuary and their approach, which coupled techniques performed at the local ground level (for example, altimetry and soil texture) with aerial techniques at the scale of the entire mudflat, it was possible to follow the morphological and sedimentary evolution of the northern mudflat of the Seine estuary. Local human developments have increased the impact of climate change by enhancing the role of the flood. River flow plays a key role in terms of sedimentation/erosion processes on the mudflat situated at the mouth of the estuary. Climatic variability controls the river flow and the position of the turbidity maximum in the estuary, i.e. the role of the TM as a source of material for the intertidal mudflats. It will however be necessary to attentively follow studies conducted at the level of the larger northern mudflat until phenomena associated with local human development are resolved (Cuvilliez et al., 2009) and while the impact of climate change continues (Massei and Fournier, 2012). Acknowledgments This study could not have been successfully completed without the collaboration of Le Grand Port Maritime du Havre and Le Grand Port Maritime de Rouen. This study was financially supported by the scientific program Seine-Aval 4, (R2010-0048) which is coordinated by the Region of Haute-Normandie. We would like to thank Suzanne Thepaut-Hasselback, a master's student at the University of Le Havre, for her contributions to the translation of this article. The authors would like to sincerely thank the reviewers of this article for their thoughtful advice. We also wholeheartedly thank Mr. Richard A. Marston, editor of Geomorphology, for his invaluable guidance which allowed this article to be finalized in the most optimal conditions. References Alexander, C.R., Nittrouer, C.A., Demaster, D.J., Park, Y.A., Park, S.C., 1991. Macrotidal mudflats of the southwestern Korean coast: a model for interpretation of intertidal deposits. J. Sediment. Petrol. 61 (5), 805–824. Avoine, J., Allen, G.P., Nichols, M., Salomon, J.C., Larsonneur, C., 1981. Suspended sediment transport in the Seine estuary, France. Effect of man-made modifications on estuaryshelf sedimentology. Mar. Geol. 40, 119–137. Bally, G., Mesnage, V., Deloffre, J., Clarisse, O., Lafite, R., Dupont, J.-P., 2004. Chemical characterization of porewaters in an intertidal mudflat of the Seine estuary: relationship to erosion–deposition cycles. Mar. Pollut. Bull. 49 (3), 163–173. Bassoullet, P., Le Hir, P., Gouleau, D., Robert, S., 2000. Sediment transport over an intertidal mudflat: field investigations and estimation of fluxes within the “Baie de MarennesOleron” (France). Cont. Shelf Res. 20, 1635–1653. Blott, S.J., Pye, K., Van der Wal, D., Neal, A., 2006. Long-term morphological change and its causes in the Mersey Estuary, NW England. Geomorphology 81, 185–206. Bouma, T.J., van Duren, L.A., Temmerman, S., Claverie, T., Blanco-Garcia, A., Ysebaert, T., Herman, P.M.J., 2007. Spatial flow and sedimentation patterns within patches of

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