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Morphological responses of an estuarine intertidal mudflat to constructions since 1978 to 2005: The Seine estuary (France)
Geomorphology 104 (2009) 165–174
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Morphological responses of an estuarine intertidal mudflat to constructions since 1978 to 2005: The Seine estuary (France) Antoine Cuvilliez a,c,⁎, Julien Deloffre a, Robert Lafite a, Christophe Bessineton b a b c
Laboratoire de Morphodynamique Continentale et Côtière, Université de Rouen, UMR CNRS 6143 M2C, 76821 Mont St. Aignan Cedex, France Maison de l'estuaire de la Seine (Center of Scientific culture and Technique of the Estuary), 76600 Le Havre, France Université du Havre, FST, 76058 Le Havre cedex, France
a r t i c l e
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Article history: Received 10 March 2008 Received in revised form 18 August 2008 Accepted 21 August 2008 Available online 3 September 2008 Keywords: Remote sensing Tidal mudflat Human impact Macrotidal estuary
a b s t r a c t Since 1834 the Seine estuary (France) has been the site of numerous construction projects with the aim to accommodate and secure boat traffic. Since 1978, the increasing of the activities of Le Havre port, located at the mouth of the estuary, has accelerated the construction work rate. Several dykes, a bridge, and new port facilities have been constructed in rapid succession, modifying considerably the hydrodynamic conditions which sustain a partially vegetated sandy–muddy tidal flat located in the North bank of the estuary between the new port of Le Havre and the Normandy bridge achieved in 1995. The present study deals with the morphological evolution of this zone from 1978 to 2005. The use of a low altitude remote sensing technique combined with traditional methods of ground survey and probes allow to demonstrate the impact of human activities on sedimentary and vegetation dynamics. The Northern mudflat of the estuary is the most affected by these human activities, which surface have reduced of 62% during the last 27 years with an intensified local erosion during the last 27 months corresponding to a loss 1250 000 m3 of fine-grained sediment. At the same time, the general sanding up in the channel of the zone has caused a loss of more than 31% of the tidal prism, more than three quarters of which occurred during the last three years. Results also establish that the response times of the sedimentary or topographic readjustment to an installation depend on the extent and the nature of the construction. In fact, the sedimentary readjustments to an installation can be delayed by up to 10 years in the case of the Seine estuary. This delay is explained by a rapid succession of construction works which may occult the effect of a single installation. Except in the case of a dyke built perpendicular to ebb and flood currents, the impacts of these installations reach a hydrosedimentary equilibrium on the level between 1 and 7 years after their completion. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Intertidal muddy zones are typical and characteristic environments in macrotidal estuaries which also include usually megaripple sand banks and salt marshes (Hayes, 1975; Dalrymple et al., 1992). Various environmental parameters influence the aspect and the evolution of these structures. Thus, Whitehouse and Mitchener (1998) underline the impact of the seasonal variations which include the evolution of the flows river (Christie et al., 1999; Dyer et al., 2000; Prandle et al., 2006), the swell and storms (Stevenson et al., 1988; Green et al., 1997; Roman et al., 1997; Ryan and Cooper, 1998; Anthony, 2000; Bassoullet et al., 2000; Verney, 2006). In addition, estuarine systems act as a ‘hub’ acting
⁎ Corresponding author. Université du Havre, FST, 76058 Le Havre cedex, France. Tel.: +33 2 32 74 43 23; fax: +33 2 32 74 43 14. E-mail address: [email protected] (A. Cuvilliez). 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.08.010
as a linkage point between the inland river and the ocean and thus are prone to construction, both in historical and modern times (Davis and Barnard, 2000; Mellalieu et al., 2000). Also, for these natural parameters, it is necessary to add the impact of the anthropogenic activities which modify the hydrodynamic conditions on the surface of the mud flat (Lafite and Romaña, 2001; van der Wal and Pye, 2004; Kim et al., 2006; Jaffe et al., 2007). This evolution of the intertidal muddy zones under the effect of constructions was also studied on various scales of time: about 20 years (Kim et al., 2006) or longer periods going from 30 to 160 years (Lesourd, 2000; Bourman et al., 2000; Chen et al., 2001; Wolanski et al., 2001; van der Wal et al., 2002; van der Wal and Pye, 2004; Blott et al., 2006). These studies show that it can be difficult to discern which changes may be due to natural evolution as opposed to anthropogenic evolution of estuarine systems. This difficulty is mainly related to (i) the need for long-term (i.e. decade to century), detailed studies and (ii) the variability of the response time of the system to one installation. The objective of this paper is to assess and quantify the responses of an intertidal mudflat to construction works based on a combination of original techniques that allows to study medium to long-term processes
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(i.e. several decades). The responses to these works are at the same time morphological and sedimentary. They relate to (i) the evolution of surfaces of a slikke, a salt marsh, sandy banks, of a channel but also (ii) the evolution of the nature of the sediments which cover the bare intertidal zones. This study was undertaken on the Northern mudflat located at the mouth of the macrotidal Seine estuary (Upper-Normandy, France, Fig. 1a, b) between The new Le Havre port (port 2000) and the Normandy bridge (Fig. 1c). On its right bank this environment presents a complex and dynamic mud flat/salt marsh units, corresponding to the most extensive tidal flat of the Seine estuary. The rhythm of silt deposit is monthly, related to the cycle of the strongest spring tides in this macrotidal context (Deloffre et al., 2006). These sedimentary dynamics also depend on the swell (Ricardo da Silva, 2002), the Seine river flow (Brenon and Le Hir, 1999) and the many construction (Lesourd et al., 2001). In order to understand and quantify the role of these installations on the evolution of this intertidal mudflat, a specific strategy was developed. The morphological evolution is studied by coupling aerial, topographic, bathymetric and sedimentological data. This strategy allows to exanimate the morphological evolution of three specific areas which are restricted by engineered dykes: the mudflat, salt marshes system (i.e. aerial, topographic and sedimentological data) and the subtidal zone (i.e. bathymetric data). These data have been acquired at different key periods of the morphological evolution of the intertidal mudflat during the last 27 years (from 1978 to 2005) which correspond to the increase of the construction work rate. Consequently, this approach allows us to focus on the spatial and temporal variations in this area with respect to its constructions, in order to (i) understand the mudflat morphological evolution over the past 27 years, and (ii) specifying the impacts of human activities on the mudflat dynamics.
2. Study site The macrotidal Seine estuary (maximum tidal range of 8 m) is one of the main estuary of the European continental shelf in an oceanic climate context. It is fed by a catchment area of 78 650 km2 that makes up 14% of the French territory and is occupied by 26% of the population, that is 16 million inhabitants. The estuary length is approximately 160 km long, the tidal limit in this estuary is a dam, where the tidal range is still ∼0.5 m. The mean annual Seine river discharge measured at this dam is 450 m3 s− 1, ranging from 50 to 2200 m3 s− 1. Two historical French harbours are located along the Seine estuary: Le Havre and Rouen. The boat traffic is still important in these two ports (respectively the 2nd and the 6th French harbour for traffic). 50% of French river traffic travels through this estuary as far as it allow the delivery of goods to Paris. During the last 30 years, three main projects have been achieved in the Seine estuary: the dykes constructions to secure navigation (achieved in 1980), the Normandy Bridge (1988– 1995), and the expansion of the Le Havre port, namely ‘Port 2000’ (2002–2006). These projects are focused on the mouth of the estuary (Fig. 1c), and induce morphological and sedimentary modifications of this part of the system. In this paper, we will focus on the impact of the constructions on the Northern intertidal mudflat at the mouth of the Seine estuary located between the new Port 2000 and the Normandy bridge (Fig. 1c). In 2005, this Mudflat corresponds to a muddy–sandy field of 2.9 km2 located on the right bank at the Seine estuary mouth (Fig. 1c). This area is limited in the North by an estuary road which corresponds to the old Northern high dyke (Fig. 1c). A hook dyke in the West and the Normandy bridge in the East limit this mudflat (Fig. 1c) which is limited in the South in the axial zone by a cliff whose origin is related to an erosion by the currents which animate the Northern trench (Fig. 1c).
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).
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The Northern mudflat (Fig. 1c) is mainly made up of silty-clay particles, sometimes associated with fine or coarse sands (Hoyez and Cuvilliez, 2001; Lesourd et al., 2003; Deloffre et al., 2007). The sedimentary dynamics of this mudflat has been examinated for several decades (Avoine, 1981; Lesourd et al., 2001; Deloffre et al., 2005) as well as its hydrodynamics (Verney, 2006). This work has led to the modelling of the hydro-sedimentary evolution of the zone (Brenon, 1997; Le Hir et al., 2001). Whereas these studies have highlighted the natural mechanisms at work, they have not sufficiently considered anthropic pressure. Indeed, they do not take into account the multiannual dynamics (i.e. several decade) of the mudflat nor do they help to explain or quantify the impact of its installations. Currently, this 2.9 km2 mudflat surface represents what remains of what was once a much larger area estimated at 130 km2 in 1834 (Avoine, 1981). The surface of the mudflat has been reduced over the decades because of harbour, river or industrial installations (Hoyez and Cuvilliez, 1998; Lesourd, 2000; Cuvilliez, 2003). At the mouth, four large dykes were constructed (Fig.1c). From north to south, we can observe (i) the non-submersible Northern high dyke which is currently limited by the estuary road, (ii) the Hook dyke, and (iii) the submersible Northern and Southern low dykes. In the distal part, work on the new Port 2000 (2002–2006), a vast construction project, required the extraction of more than 46 Mm3 of sediments to provide for nautical access and the construction of an external dyke measuring 5790 m. The Normandy bridge completed in 1995 is the longest stayed girder bridge of Europe. Of a length of 2141 m and two pylons separated by 856 m, it required seven years of work. A whole of dykes was also built near the studied area since 1970 in order to facilitate navigation. Thus, the Northern low dyke (Fig. 1c) ensures, with the Southern low dyke, a single navigation channel by increasing the current velocity in the central channel, in particular at ebb tide. The Hook dyke, built between 1978 and 1979 (Fig. 1c), creates areas where dredged sediments from the channel can be deposited and limits the western zone of the Northern mudflat. It greatly reduces the wetted cross section of the mouth. Between this zone and the Northern low dyke there is a shallow channel known as the Northern trench. Following the damming up carried out upstream, this channel was gradually infilled, and in 1980 necessitated the opening of an engineered breach in the Northern low dyke in order to limit the phenomenon of sedimentation and to preserve fishing activities in the Northern trench (Desprez and Dupont, 1985). Since 1988, the Northern mudflat has been limited in its eastern part by new embankments necessary for the construction of the Normandy bridge.
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Thus, this vast muddy zone evolves in a system contained by the built dykes and the embankments of the Normandy bridge. Starting in 2001, when the first installations for Port 2000 began, this partially closed system has undergone new and rapid changes. Following one after another in quick succession, first there was the construction of the Southern low dyke for Port 2000 (from 2002 to 2004), next there was the construction of the breakwater (2003–2004) and finally, within the context of the accompanying work for this installation, the creation of a breach in the Northern low dyke as well as the opening of a meander upstream from the Normandy bridge (2005). In addition to these successive installations, the area of the Northern mudflat has been subjected to dredging which began in January 2002. Located south of Port 2000 and at the level of the Northern trench, the purpose of this dredging is to increase the wetted cross section and to favour sediment transfer, thus limiting excessive silting of the zone. 3. Materials and methods The identification of the sedimentary and biological units which constitute the surface under study is carried out by a location at different periods. Because of the extent and type of the surface, in situ measurements on these mudflats are physically difficult. The cartography is thus used in this study and based on ortho-rectified aerial photographs, acquired by low altitude shots taken from a microlight aircraft. Although it is very sensitive to weather conditions, the microlight makes it possible to obtain much higher precision and better repeatability than with satellites. Indeed, the pixel side currently attains 8 cm at an altitude of 600 m instead of the 2.5 m suggested by SPOT 5. Thanks to ground control geo-referenced points identified on the photographs a geo-referenced cartography of the studied surface can be obtained. This cartography allows to identify the location of the different sedimentary and vegetation units and also the limits of the mudflat and the sandy banks (Figs. 2 and 3). The method of evaluating the retreat of the cliff in finer detail (i.e. southern/lower limit of the mudflat), as for any other morphological element, consists in initially seeking ground control points between two series of shots, such as for example a tuft of spartina. Once located on successive shots, these ground control points are used to measure the distances which separate them from mobile structures. Thus, in Fig. 3c, we can easily identify the relative position of the ground control points compared to the cliff which is the southernmost of the mudflat. Their position compared to that of the cliff over time demonstrates the intensity of the erosion process. In order to support
Fig. 2. Evolution of the salt marsh limit (1 to 5) and the South mudflat limit (1′ to 5′) for the last 27 years. A to E are described in the text.
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Fig. 4. Evolution of the retreat of the cliff from 1997 to 2005 according to the distance between the mudflat cliff and ground control points (1 to 5 of Fig. 3).
these observations, the distances which separated the ground control points from the top of the cliff were measured. That makes it possible to evaluate the rate of erosion over time (Fig. 4). Only 5 years (1978, 1988, 1998, 2003 and 2005) are charted (Fig. 2) because they represent key years. We chose them because they correspond to the beginning or ending of great works. The quantification of the morphological evolution in surface area as well as in volume allowed us to create differential maps by combining the data from the low altitude flights (intertidal area) with bathymetric surveys (subtidal area). With regard to the evolution of the surface of the Northern mudflat (Fig. 5), the results are obtained by superimposing two masks where each one represents the exact form of the mudflat on the same scale. Because the pixel has a known surface, an excess or lack of pixels between two masks shows the amount of increase or reduction in the surface of the Northern mudflat for a known period of time. In order to estimate volumes of moving sediment in the study area, a single beam echo sounder of 33 kHz is used to take into account only the hardened floor and to exclude the unconsolidated mud. To obtain a high resolution map, the boat fact trajectories separated by 100 m. The operation is repeated every one or two months according to the intensity of installations being built and the last recorded variations. Thus, the recorded differences allow to create differential bathymetric maps. In a study limited to zone D of Fig. 2 using these two techniques combined with in situ bottom sediment sampling, we were able to evaluate the volumes (Fig. 6) and the nature of the moved sediments. 4. Results 4.1. Multiannual survey of the limit of the upper limit of the mudflat The limit between the mudflat and the salt marsh is at 7.5 m above 0 (lowest sea-level). Its evolution was recorded over a period of 27 years (1978–2005). The lines (1 to 5) mentioned in Fig. 2 specify the limit of the maximum extension of the spartina belt on the bare mud, respectively 1978, 1988, 1998, 2003, and 2005. The observation and recording of the evolution of the southern limit of the salt marshes for 27 years highlight the variable rates of displacement of this boundary. Two distinct zones can be clearly discerned from this chart: a relatively narrow (1.2 km) western part (A), extending from the dyke to the tip of a dune ridge (Fig. 2) and the eastern part (B) corresponds to a 3-km long zone going from the tip of the dune to the Normandy bridge. Concerning part A, we observe a slow extension of the salt marsh surface from 1978 to 1988 with a mean rate of 8 m a− 1 and since 1988 there is a stabilization of this
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extension. In part B, the rapid initial extension of 44 m a− 1 between 1978 and 1988 slows down significantly as from 1988 with 26 m a− 1 between 1988 and 1998. A relative stabilization of the salt marshes is observed as from the year 2000, in particular close to the Normandy bridge. Since 2002, certain parts of the salt marsh limit show even a local retreat. The relative stability of the southernmost salt marsh since 1988 observed very early with respect to the dune (part A) can be explained by a certain perenniality of the hydrodynamic conditions in this part of the Northern mudflat (Cuvilliez, 2003). The extension recorded over time in part B is explained by the construction of the Northern low dyke, the Hook dyke and the breakwater upstream (Lesourd, 2000) which since 1978 have had an effect on the empoldering of this zone by significantly attenuating the swells. The Hook dyke is used as an anchoring point for salt marsh while stabilizing, along with the Northern low dyke, the wetted cross section in the mouth. During the second period (since 1988), the relative stability of the environmental conditions slows down the expansion of the salt marshes which at the present time have stopped spreading towards the south because of the erosion that the mudflat (Fig. 2) has undergone since the 1980s. This has modified not only the topography of the mudflat (Bessineton, 2006) but also increased the hydrodynamic conditions that became unfavourable for the colonization of spartina (Desprez and Dupont, 1985). 4.2. Multiannual survey of the lower limit of the mudflat The southern limit of the mudflat is defined by the level of the low tides in spring (Fig. 2) in the lower part of which the subtidal zone extends. This limit evolves differently from east to west over the past 27 years and allows to subdivide this surface into three large zones. The first, namely zone C, represents a surface whose northern limit is marked out by two perennial structures, i.e. the Hook dyke and a dune. In the south, this zone is separated from the Northern low dyke by a subtidal area. The 1.8 km long surface of the mudflat is relatively stable since the maximum retreat recorded during the period 1978–2005 was 102 m, giving a mean retreat rate of 3.8 m a− 1. The second zone, called D in Fig. 4, measures 2.5 km long and corresponds to a mudflat crossed by tide channels draining water from the salt marshes and making up its northern limit. In the south, this zone is influenced by the strong ebb and flood currents present in the Northern trench included between 1.5 and 2 m s− 1 (Waeles, 2005). This part of the mudflat, limited by a cliff, regressed quickly since the maximum retreat recorded over 27 years is 450 m, representing a mean rate of 16.7 m a− 1. The third zone, called E, approximately 350 m long, presents as for the zone C a soft inclined limit with the northern channel and an evolution similar, that is to say a maximum retreat of 120 m during the period 1978–2005 with a mean rate of 4.4 m a− 1. The morphology and its evolution of the zones C and E are to be put in relation to an orientation of the currents parallel or subparallel with these limits and speeds lower than 1 m s − 1 (Waeles, 2005). The installations carried out since 1978, i.e. the dykes and the Normandy bridge, have modified slightly the conditions of deposition and erosion in zones C and E. However, this is not the case for the axial part of the Northern mudflat where the opening of a breach in the Northern low dyke in 1980 (Desprez and Dupont, 1985), the installation and the enlargement of the bank of La Passe, then the construction of the breakwater influenced the evolution of this zone by creating an acceleration of the currents, inducing erosion of the mudflat. The retreat of the cliff which borders the mudflat in the south (zone D, Fig. 2) has evolved at three different rates. From 1978 to May 1999 the average retreat is estimated at approximately 25 m per annum. From 1999 to 2003 it increased to a rate of approximately 58 m per annum (Fig. 4). Then, a stabilization of the retreat of the top of the cliff limiting the low
Fig. 3. a. Location of the Northern mudflat area used for the study of the retreat of the cliff. b. Location of the ground control points used for the valuation of the retreat of the cliff. c. Extracts of aerial cartographies from 1997 to 2005 showing positions of ground control points and cliff. All extracts are at the same scale.
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Fig. 5. Environmental planning and evolution of the Northern mudflat these last 27 years.
mudflat has been observed as from 2003. The bathymetric data reveal that the low mudflat was initially truncated at its top (visible on Fig. 3c in April 2003 and May 2004) and thereafter underwent erosion at its base that tended to make it more vertical (Fig. 3c, June 2005). Currently, this erosion is stabilized in the axial part of zone D (Fig. 2) and is migrating more to the west subsequent to the installation of the breakwater (Fig. 1c) and to the change in geometry of the sandy bank following construction of this dyke.
4.3. Evolution of the intertidal muddy surface Basically, the entire surface of the mudflat (Fig. 7) has changed on average according to two broad tendencies: a rapid reduction of surfaces between 1978 and 1985, then a slower reduction between 1985 and 2005. The first reduction phase is explained by a considerable spread of the salt marsh (Hoyez and Cuvilliez, 1998). During this period, the rate of disappearance of the mudflat is 0.28 km2 y− 1 and the Fig. 5, curves C, D,
Fig. 6. Differential of soundings made between March 2005 and December 2002. The cartographied surface is the 2005 one. Red: La Passe bank limit. Pink: isobath 0.
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Fig. 7. (a) Grey: area which was the subject of a follow-up of the surfaces evolution between 1980 and 2005. The results are on (b). (b) Evolution during 25 years of the surfaces of salt marsh, mudflat, intertidal sand banks and subtidal of the limited area on (a).
and E) shows that it's linked to the construction of the Hook dyke and the Northern low dyke. In addition, the installation of the engineered breach resulted in an increase in the current velocities at flood and ebb tides (Desprez and Dupont, 1985; Hoyez and Cuvilliez, 1998; Lesourd, 2000). Consequently, these new hydrodynamic conditions induced a new system made up of unstable silt–sand units and a trench. The slow reduction phase begins in 1985: the surface reduction of the mudflat continues much more slowly, at about 0.13 km2 y− 1 over 20 years. This corresponds to the new dynamics of this estuarine system which is contained by dykes and a restricted extension of the surface of the salt marsh which gradually tends to be stabilized. Occasionally, this regression still takes place in the central zone D but is compensated for from 1998 to 1999 by the growth of the western and eastern zones of the mudflat in zones C and E (Fig. 5) subsequent to the construction of the Normandy bridge and the installation of the embankments. Indeed, during these last 27 years human activities have induced a reduction of more than 62% of the surface of the Northern mudflat of the Seine estuary. By coupling the bathymetric data and aerial shots of the Northern mudflat, it is possible to estimate sediment volumes. For the period going from December 2002 to March 2005, in zone D (Fig. 2) the volume of sediments moved in this zone over a period of 27 months represents approximately 1.25 Mm3. This corresponds to an estimated reduction of 0.18 km2 in the surface of the mudflat corresponding to 5.5% of its surface. These sediments, derived from the mudflat, are
primarily consolidated silts associated with sand lenses (Lesourd et al., 2001) transported by the flood current towards the east of the trench. In addition, the observations carried out by this study show that the silt is either stored on the level of the raised breach, or exported towards the Seine via the recently dug meander. As the orientation of the megaripples observed on its surface underlines, the bank of La Passe is currently enriched by a large contribution of 200 µm fine sand (Fig. 6) typical of the Seine estuary, thanks to the action of a dominating flood current. In addition, the Hook dyke, located in the west, acts on this bank not only by giving it the bifurcated form visible in Fig. 6, but also by favouring the continuous increase of its northernmost surface. This involves a displacement of the trench towards the north. As a result, it pushes back the southernmost limit of the low mudflat and digs the bottom of the channel in the same direction, making the cliff more abrupt (Fig. 3c, (I) June 2005). The studies of sedimentary and granulometric dynamics carried out in this zone reveal variations that are a consequence of the constructions. Thus, the principal granulometric analysis done on the mudflat surface showed a change from 15 and 90 µm in 1998 (Lesourd, 2000) to 200 µm in 2005. If we refer to the data collected over the two last years, we can note that the construction of the foundations of the breakwater from July to September 2003 was accompanied by an inversion of the sedimentary dynamics, going from an episode of sedimentation to one of erosion and propagated from the low mudflat towards the high mudflat in zone C (Fig. 2). On the contrary, the raising
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of this same breakwater simultaneously changed the phenomenon of generalized surface erosion of the mudflat in zone D into a process of sedimentation that propagates progressively from the south towards the north. Lastly, the data collected from the mudflat as well as from the bank of La Passe show an evolution of the grain size resulting in an overall increase in the mean size of the deposited particles, originally at 20 µm and reaching 200 µm between 2002 and 2004 for zones D and E (Fig. 2). The absence of finer particles underlines the increase in the hydrodynamic conditions at the Northern trench. 4.4. Overall assessment of the Northern trench The evolution of the intertidal muddy surfaces in the Seine estuary depends on both the evolution of the mudflat–salt marsh boundary and the spatial extent of the low mudflat. Consequently, it is necessary to take into account all of the estuarine zone which is submerged at high water, i.e. both subtidal and intertidal zones. We thus extended our study to the entire northern zone of the estuary which has been evolving since 1978, the date of the beginning of this survey. This 25 km2 zone is limited by the Northern main dyke, the Northern low dyke, the mouth of the Seine and the breakwater located upstream of the estuary (Fig. 7a.). In this area, we calculated not only the surfaces of the subtidal zone but also those of the intertidal sands made up of the sandy megaripple banks found in the low mudflat, the bank of La Passe (ranging between 0 and 5 m), the mudflat (ranging between 5 and 7.5 m) and the salt marsh (greater than 7.5 m). The installation of the longitudinal dykes (Hook dyke and Northern low dyke) and of an upstream breakwater until 1980 changes the direction of the currents and decreases their force. So (1) the intertidal area (sandy mudflat and banks) significantly decreased until 1985, (2) the subtidal zone developed at a rate of 0.28 km2 per year and (3) the salt marsh developed quickly (Figs. 2 and 7b). To avoid a stronger mud deposit in the study zone, a breach was set up in 1978 inducing strong currents in the Northern trench (Desprez and Dupont, 1985). The result of this was the build up of the sandy intertidal bank of La Passe, observed for the first time in 1985, which gradually reduced the surface of the subtidal zone at a rate of 0.08 km2 per year and increased the depth of the navigation channel (Avoine, 1985). This phenomenon still persists today, 20 years later. The installation of the embankment for the Normandy bridge in 1988 has maintained the slow and continuous encroachment of the salt marsh. The sandy intertidal zone continues to increase due to a strong dynamism in the Northern channel, accentuated by the recalibration of the breach, then the installation of a breakwater in 2003. Thus, 78% of the variation in the bathymetric profile occurred between 2002 and 2005. The reduction observed in the wetted cross section has caused an increase in the erosion of the southernmost limit of the low mudflat which has become more abrupt since 2004 (Fig. 3c. (H)). The general sanding up of the zone, observed since 1985 by Desprez and Dupont (1985) and after by Hoyez and Cuvilliez (2001), favours a raising of the bottom of the subtidal part which is transformed into a sandy intertidal zone. The mudflat, located in the concave part of the Northern channel, erodes quickly under the effect of the currents. 5. Discussion The morphological evolution of intertidal mudflats was and makes still the subject of studies based on recordings continuous or discontinuous realized with various steps of time and are completed sometimes by mathematical models necessary to explain and to consider the modifications of the units which compose these fields. Recordings carried out from the daily to the secular scale made it possible to follow the evolution of estuaries like those of Severn (Harris and Collins, 1985; Kirby, 2000), of Ribble (van der Wal et al., 2002, 2004),of Keum (Kim et al., 2006), of Mersey (Blott et al., 2006), of Ord
(Wolanski et al., 2001), and of Gironde (Fenies and Tastet, 1998). The mathematical models like those presented by Roberts et al. in 2000, Pritchard in 2002 and 2007, D' Alpaos and Defina in 2007 supplement these morphological studies. In this study, the pluriannual scale seems to be adapted to show the sedimentary and morphological impacts consecutive with constructions, contrary to finer scales (diurnal and semi-diurnal), better to understand the impact of punctual phenomena like storms or flooding (Deloffre, 2005). The estuary of the Seine was the subject of studies concerning its sedimentary cover since 1834 per Beautemps-Beaupré (Lennier, 1885) showing until in the 70s that this zone was mainly made up of sandy sediments (Avoine, 1981; Laignel, 1991). In the 90s, new studies showed a clear silting of the mouth of the Seine (Avoine, 1994; Lesourd, 1995). However, the historical data concerning the estimate of surface of the studied zone before 1978 remain approximate and in 1974 it seems that the slikke occupied a surface from approximately 10 km2 and the salt marsh less than 2 km2 (Lesourd, 2000). With the increase in the rhythm of constructions in the estuary since 1978, the regular estimates of the surface of the northern mudflat were realized and their results refined by the remote sensing at low altitude which makes it possible to obtain an instantaneous calculation of surfaces of the various units which make all this mudflat. The results obtained within the framework of this study during these 27 last years make it possible to evaluate the morphological and sedimentary impacts of two types of installation on an intertidal mudflat in a macrotidal estuary: (1) the construction of dykes and (2) that of a bridge. (1) 3 types of dykes are to be distinguished: (i) those which are parallel or subparallel with the current of flood and ebb (the hook dyke and the Northern low dyke), (ii) that which blocks these currents (the breakwater dyke) and (iii) those which limit the extension of Port 2000 more downstream from the zone of study. (i) This type of dyke led to two responses spread out in time. (1) First of all they increase sedimentation in the northern part of the estuary, which leads in 7 years (1978–1985) to an increase in the surface of salt marshes (Hoyez and Cuvilliez, 1998; Lesourd, 2000; Cuvilliez, 2003). The same type of construction led to the same result in various estuaries like in Ribble (van der Wal and Pye, 2004) or in Keum (Kim et al., 2006). (2) In addition, these dykes have a longer-term effect by decreasing oscillating volume of the zone, which is isolated from the channel of navigation, of 31% from 1981 to 2005. This continuous reduction in the oscillating volume, which accelerates with the construction of the hook dyke in 2003, reinforces the phenomenon of sedimentation which tends to decrease the capacities of the biological compartment. (ii) the breakwater dyke, perpendicular to the current of flood and ebb, has an impact different from the dykes parallel with the major currents. Indeed, as of the end of its construction in 2003, it causes a fast erosion of the slikke located at a hundred meters of its end by diverting the currents of flood and ebb. It also supports the fast growth of a silty and sandy bank, the bank of La Passe (Fig. 6), in the most sheltered zone of flood current dominating. It is however difficult to estimate with precision the longer-term impact of this dyke but it is possible that these phenomena of erosion of the slikke and sandy deposit still continue over a period ranging between 4 and 7 years. (iii) the construction of Port 2000 dykes causes an erosion of the adjacent slikke and the growth of sandy banks with megaripples. This is due to an increase current velocity in this zone and these effects are similar to those recorded with the construction of the breakwater dyke. This similarity is explained by the fact why the principal dyke of Port 2000 is also parallel or subparallel with the main currents.
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(2) During the construction of the bridge of Normandy, the installation of embankment perpendicular to the major currents onto the stable salt marsh increased sedimentation. The impact of the installation of the embankments was delayed by the construction of the pillars of the bridge, in the lower part of the mudflat, which disturbed the processes of sedimentation. Thus, the accretion of the Eastern zone of the northern mudflat was entirely completed only 10 years later (zone E, Fig. 5). The assessment of surfaces should lead to a relative stabilization over a 30 year period if no new installation intervened, as in the estuary of the Mersey (Blott et al., 2006). However, in estuary of the Seine, continuous installations do not make it possible to observe this stabilization. In the same time, the sedimentary assessment related to installations must also take into account the significant activities of dredging carried out on the level of the channel of navigation. They are estimated at 4.106 m3 in 2005 according to the Port authority of Le Havre. This has certain effects, in particular by supporting estuarine sedimentation like that was recorded in the estuary of the Mersey (Blott et al., 2006) or of Quequén in Argentina (Perillo et al., 2005). 6. Conclusions Thanks to the recordings carried out during 27 years starting from traditional methods (bathymetric, granulometric for example) but also original and precise like the low altitude remote sensing, we could estimate the morphological and sedimentary impacts of the principal types of installation carried out by the man in the estuary of the Seine these last years. These impacts are variable and depend on the nature and the extent of work. They can be immediate, in the case of constructions of the dykes, with an erosion of the muddy zones, sensitive to the variations of currents and continue over 7 years before a new hydrodynamic balance is reached. These morphological and sedimentary impacts can also be differed in time and be different from that observed with dyke constructions. It is the case of the construction of the bridge of Normandy from which the effects on the growth of the salt marshes in the Northern mudflat were reached ten years after the beginning of work. Finally, at the present time, the whole of constructions undertaken induce a sedimentary assessment which shows that intertidal mudflats are drastically reduced during this period of 27 years. It is appropriate however to indicate that the constant rhythm of the work completed since 2002 in the estuary of the Seine complicates the study of the morphosedimentological impacts related to constructions. Indeed, the time between two works is shorter than the time of the morphological equilibrium. In addition, it is advisable to also take into account the role of the climatic variations that decrease the flow of the river in period of winter rising since 2001, and climatic changes that increase the marine level in the Seine from 1 to 5 mm per annum (Waeles, 2005). Lastly, if the morphosedimentological response can be estimated with more or less difficulties, one can put questions about the consequences even more complex on the habitats and the biological compartment. Acknowledgements This study could not have been completed without the collaboration of the members of La Maison de l'estuaire and the people in the dredging and the hydrography departments of both the Le Havre and Rouen Port Authorities. This work was carried out in the framework of the Scientific Seine-Aval Program coordinated by the Conseil Régional de Haute-Normandie. References Anthony, E.J., 2000. Marine sand supply and Holocene coastal sedimentation in northern France between the Somme estuary and Belgium. In: Pye, K., Allen, J.R.L. (Eds.), Coastal and Estuarine Environments: Sedimentology, Geomorphology and Geoarchaeology. Special Publication N°175. Geological Society, London. 435 pp.
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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g e o m o r p h
Morphological responses of an estuarine intertidal mudflat to constructions since 1978 to 2005: The Seine estuary (France) Antoine Cuvilliez a,c,⁎, Julien Deloffre a, Robert Lafite a, Christophe Bessineton b a b c
Laboratoire de Morphodynamique Continentale et Côtière, Université de Rouen, UMR CNRS 6143 M2C, 76821 Mont St. Aignan Cedex, France Maison de l'estuaire de la Seine (Center of Scientific culture and Technique of the Estuary), 76600 Le Havre, France Université du Havre, FST, 76058 Le Havre cedex, France
a r t i c l e
i n f o
Article history: Received 10 March 2008 Received in revised form 18 August 2008 Accepted 21 August 2008 Available online 3 September 2008 Keywords: Remote sensing Tidal mudflat Human impact Macrotidal estuary
a b s t r a c t Since 1834 the Seine estuary (France) has been the site of numerous construction projects with the aim to accommodate and secure boat traffic. Since 1978, the increasing of the activities of Le Havre port, located at the mouth of the estuary, has accelerated the construction work rate. Several dykes, a bridge, and new port facilities have been constructed in rapid succession, modifying considerably the hydrodynamic conditions which sustain a partially vegetated sandy–muddy tidal flat located in the North bank of the estuary between the new port of Le Havre and the Normandy bridge achieved in 1995. The present study deals with the morphological evolution of this zone from 1978 to 2005. The use of a low altitude remote sensing technique combined with traditional methods of ground survey and probes allow to demonstrate the impact of human activities on sedimentary and vegetation dynamics. The Northern mudflat of the estuary is the most affected by these human activities, which surface have reduced of 62% during the last 27 years with an intensified local erosion during the last 27 months corresponding to a loss 1250 000 m3 of fine-grained sediment. At the same time, the general sanding up in the channel of the zone has caused a loss of more than 31% of the tidal prism, more than three quarters of which occurred during the last three years. Results also establish that the response times of the sedimentary or topographic readjustment to an installation depend on the extent and the nature of the construction. In fact, the sedimentary readjustments to an installation can be delayed by up to 10 years in the case of the Seine estuary. This delay is explained by a rapid succession of construction works which may occult the effect of a single installation. Except in the case of a dyke built perpendicular to ebb and flood currents, the impacts of these installations reach a hydrosedimentary equilibrium on the level between 1 and 7 years after their completion. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Intertidal muddy zones are typical and characteristic environments in macrotidal estuaries which also include usually megaripple sand banks and salt marshes (Hayes, 1975; Dalrymple et al., 1992). Various environmental parameters influence the aspect and the evolution of these structures. Thus, Whitehouse and Mitchener (1998) underline the impact of the seasonal variations which include the evolution of the flows river (Christie et al., 1999; Dyer et al., 2000; Prandle et al., 2006), the swell and storms (Stevenson et al., 1988; Green et al., 1997; Roman et al., 1997; Ryan and Cooper, 1998; Anthony, 2000; Bassoullet et al., 2000; Verney, 2006). In addition, estuarine systems act as a ‘hub’ acting
⁎ Corresponding author. Université du Havre, FST, 76058 Le Havre cedex, France. Tel.: +33 2 32 74 43 23; fax: +33 2 32 74 43 14. E-mail address: [email protected] (A. Cuvilliez). 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.08.010
as a linkage point between the inland river and the ocean and thus are prone to construction, both in historical and modern times (Davis and Barnard, 2000; Mellalieu et al., 2000). Also, for these natural parameters, it is necessary to add the impact of the anthropogenic activities which modify the hydrodynamic conditions on the surface of the mud flat (Lafite and Romaña, 2001; van der Wal and Pye, 2004; Kim et al., 2006; Jaffe et al., 2007). This evolution of the intertidal muddy zones under the effect of constructions was also studied on various scales of time: about 20 years (Kim et al., 2006) or longer periods going from 30 to 160 years (Lesourd, 2000; Bourman et al., 2000; Chen et al., 2001; Wolanski et al., 2001; van der Wal et al., 2002; van der Wal and Pye, 2004; Blott et al., 2006). These studies show that it can be difficult to discern which changes may be due to natural evolution as opposed to anthropogenic evolution of estuarine systems. This difficulty is mainly related to (i) the need for long-term (i.e. decade to century), detailed studies and (ii) the variability of the response time of the system to one installation. The objective of this paper is to assess and quantify the responses of an intertidal mudflat to construction works based on a combination of original techniques that allows to study medium to long-term processes
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(i.e. several decades). The responses to these works are at the same time morphological and sedimentary. They relate to (i) the evolution of surfaces of a slikke, a salt marsh, sandy banks, of a channel but also (ii) the evolution of the nature of the sediments which cover the bare intertidal zones. This study was undertaken on the Northern mudflat located at the mouth of the macrotidal Seine estuary (Upper-Normandy, France, Fig. 1a, b) between The new Le Havre port (port 2000) and the Normandy bridge (Fig. 1c). On its right bank this environment presents a complex and dynamic mud flat/salt marsh units, corresponding to the most extensive tidal flat of the Seine estuary. The rhythm of silt deposit is monthly, related to the cycle of the strongest spring tides in this macrotidal context (Deloffre et al., 2006). These sedimentary dynamics also depend on the swell (Ricardo da Silva, 2002), the Seine river flow (Brenon and Le Hir, 1999) and the many construction (Lesourd et al., 2001). In order to understand and quantify the role of these installations on the evolution of this intertidal mudflat, a specific strategy was developed. The morphological evolution is studied by coupling aerial, topographic, bathymetric and sedimentological data. This strategy allows to exanimate the morphological evolution of three specific areas which are restricted by engineered dykes: the mudflat, salt marshes system (i.e. aerial, topographic and sedimentological data) and the subtidal zone (i.e. bathymetric data). These data have been acquired at different key periods of the morphological evolution of the intertidal mudflat during the last 27 years (from 1978 to 2005) which correspond to the increase of the construction work rate. Consequently, this approach allows us to focus on the spatial and temporal variations in this area with respect to its constructions, in order to (i) understand the mudflat morphological evolution over the past 27 years, and (ii) specifying the impacts of human activities on the mudflat dynamics.
2. Study site The macrotidal Seine estuary (maximum tidal range of 8 m) is one of the main estuary of the European continental shelf in an oceanic climate context. It is fed by a catchment area of 78 650 km2 that makes up 14% of the French territory and is occupied by 26% of the population, that is 16 million inhabitants. The estuary length is approximately 160 km long, the tidal limit in this estuary is a dam, where the tidal range is still ∼0.5 m. The mean annual Seine river discharge measured at this dam is 450 m3 s− 1, ranging from 50 to 2200 m3 s− 1. Two historical French harbours are located along the Seine estuary: Le Havre and Rouen. The boat traffic is still important in these two ports (respectively the 2nd and the 6th French harbour for traffic). 50% of French river traffic travels through this estuary as far as it allow the delivery of goods to Paris. During the last 30 years, three main projects have been achieved in the Seine estuary: the dykes constructions to secure navigation (achieved in 1980), the Normandy Bridge (1988– 1995), and the expansion of the Le Havre port, namely ‘Port 2000’ (2002–2006). These projects are focused on the mouth of the estuary (Fig. 1c), and induce morphological and sedimentary modifications of this part of the system. In this paper, we will focus on the impact of the constructions on the Northern intertidal mudflat at the mouth of the Seine estuary located between the new Port 2000 and the Normandy bridge (Fig. 1c). In 2005, this Mudflat corresponds to a muddy–sandy field of 2.9 km2 located on the right bank at the Seine estuary mouth (Fig. 1c). This area is limited in the North by an estuary road which corresponds to the old Northern high dyke (Fig. 1c). A hook dyke in the West and the Normandy bridge in the East limit this mudflat (Fig. 1c) which is limited in the South in the axial zone by a cliff whose origin is related to an erosion by the currents which animate the Northern trench (Fig. 1c).
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).
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The Northern mudflat (Fig. 1c) is mainly made up of silty-clay particles, sometimes associated with fine or coarse sands (Hoyez and Cuvilliez, 2001; Lesourd et al., 2003; Deloffre et al., 2007). The sedimentary dynamics of this mudflat has been examinated for several decades (Avoine, 1981; Lesourd et al., 2001; Deloffre et al., 2005) as well as its hydrodynamics (Verney, 2006). This work has led to the modelling of the hydro-sedimentary evolution of the zone (Brenon, 1997; Le Hir et al., 2001). Whereas these studies have highlighted the natural mechanisms at work, they have not sufficiently considered anthropic pressure. Indeed, they do not take into account the multiannual dynamics (i.e. several decade) of the mudflat nor do they help to explain or quantify the impact of its installations. Currently, this 2.9 km2 mudflat surface represents what remains of what was once a much larger area estimated at 130 km2 in 1834 (Avoine, 1981). The surface of the mudflat has been reduced over the decades because of harbour, river or industrial installations (Hoyez and Cuvilliez, 1998; Lesourd, 2000; Cuvilliez, 2003). At the mouth, four large dykes were constructed (Fig.1c). From north to south, we can observe (i) the non-submersible Northern high dyke which is currently limited by the estuary road, (ii) the Hook dyke, and (iii) the submersible Northern and Southern low dykes. In the distal part, work on the new Port 2000 (2002–2006), a vast construction project, required the extraction of more than 46 Mm3 of sediments to provide for nautical access and the construction of an external dyke measuring 5790 m. The Normandy bridge completed in 1995 is the longest stayed girder bridge of Europe. Of a length of 2141 m and two pylons separated by 856 m, it required seven years of work. A whole of dykes was also built near the studied area since 1970 in order to facilitate navigation. Thus, the Northern low dyke (Fig. 1c) ensures, with the Southern low dyke, a single navigation channel by increasing the current velocity in the central channel, in particular at ebb tide. The Hook dyke, built between 1978 and 1979 (Fig. 1c), creates areas where dredged sediments from the channel can be deposited and limits the western zone of the Northern mudflat. It greatly reduces the wetted cross section of the mouth. Between this zone and the Northern low dyke there is a shallow channel known as the Northern trench. Following the damming up carried out upstream, this channel was gradually infilled, and in 1980 necessitated the opening of an engineered breach in the Northern low dyke in order to limit the phenomenon of sedimentation and to preserve fishing activities in the Northern trench (Desprez and Dupont, 1985). Since 1988, the Northern mudflat has been limited in its eastern part by new embankments necessary for the construction of the Normandy bridge.
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Thus, this vast muddy zone evolves in a system contained by the built dykes and the embankments of the Normandy bridge. Starting in 2001, when the first installations for Port 2000 began, this partially closed system has undergone new and rapid changes. Following one after another in quick succession, first there was the construction of the Southern low dyke for Port 2000 (from 2002 to 2004), next there was the construction of the breakwater (2003–2004) and finally, within the context of the accompanying work for this installation, the creation of a breach in the Northern low dyke as well as the opening of a meander upstream from the Normandy bridge (2005). In addition to these successive installations, the area of the Northern mudflat has been subjected to dredging which began in January 2002. Located south of Port 2000 and at the level of the Northern trench, the purpose of this dredging is to increase the wetted cross section and to favour sediment transfer, thus limiting excessive silting of the zone. 3. Materials and methods The identification of the sedimentary and biological units which constitute the surface under study is carried out by a location at different periods. Because of the extent and type of the surface, in situ measurements on these mudflats are physically difficult. The cartography is thus used in this study and based on ortho-rectified aerial photographs, acquired by low altitude shots taken from a microlight aircraft. Although it is very sensitive to weather conditions, the microlight makes it possible to obtain much higher precision and better repeatability than with satellites. Indeed, the pixel side currently attains 8 cm at an altitude of 600 m instead of the 2.5 m suggested by SPOT 5. Thanks to ground control geo-referenced points identified on the photographs a geo-referenced cartography of the studied surface can be obtained. This cartography allows to identify the location of the different sedimentary and vegetation units and also the limits of the mudflat and the sandy banks (Figs. 2 and 3). The method of evaluating the retreat of the cliff in finer detail (i.e. southern/lower limit of the mudflat), as for any other morphological element, consists in initially seeking ground control points between two series of shots, such as for example a tuft of spartina. Once located on successive shots, these ground control points are used to measure the distances which separate them from mobile structures. Thus, in Fig. 3c, we can easily identify the relative position of the ground control points compared to the cliff which is the southernmost of the mudflat. Their position compared to that of the cliff over time demonstrates the intensity of the erosion process. In order to support
Fig. 2. Evolution of the salt marsh limit (1 to 5) and the South mudflat limit (1′ to 5′) for the last 27 years. A to E are described in the text.
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Fig. 4. Evolution of the retreat of the cliff from 1997 to 2005 according to the distance between the mudflat cliff and ground control points (1 to 5 of Fig. 3).
these observations, the distances which separated the ground control points from the top of the cliff were measured. That makes it possible to evaluate the rate of erosion over time (Fig. 4). Only 5 years (1978, 1988, 1998, 2003 and 2005) are charted (Fig. 2) because they represent key years. We chose them because they correspond to the beginning or ending of great works. The quantification of the morphological evolution in surface area as well as in volume allowed us to create differential maps by combining the data from the low altitude flights (intertidal area) with bathymetric surveys (subtidal area). With regard to the evolution of the surface of the Northern mudflat (Fig. 5), the results are obtained by superimposing two masks where each one represents the exact form of the mudflat on the same scale. Because the pixel has a known surface, an excess or lack of pixels between two masks shows the amount of increase or reduction in the surface of the Northern mudflat for a known period of time. In order to estimate volumes of moving sediment in the study area, a single beam echo sounder of 33 kHz is used to take into account only the hardened floor and to exclude the unconsolidated mud. To obtain a high resolution map, the boat fact trajectories separated by 100 m. The operation is repeated every one or two months according to the intensity of installations being built and the last recorded variations. Thus, the recorded differences allow to create differential bathymetric maps. In a study limited to zone D of Fig. 2 using these two techniques combined with in situ bottom sediment sampling, we were able to evaluate the volumes (Fig. 6) and the nature of the moved sediments. 4. Results 4.1. Multiannual survey of the limit of the upper limit of the mudflat The limit between the mudflat and the salt marsh is at 7.5 m above 0 (lowest sea-level). Its evolution was recorded over a period of 27 years (1978–2005). The lines (1 to 5) mentioned in Fig. 2 specify the limit of the maximum extension of the spartina belt on the bare mud, respectively 1978, 1988, 1998, 2003, and 2005. The observation and recording of the evolution of the southern limit of the salt marshes for 27 years highlight the variable rates of displacement of this boundary. Two distinct zones can be clearly discerned from this chart: a relatively narrow (1.2 km) western part (A), extending from the dyke to the tip of a dune ridge (Fig. 2) and the eastern part (B) corresponds to a 3-km long zone going from the tip of the dune to the Normandy bridge. Concerning part A, we observe a slow extension of the salt marsh surface from 1978 to 1988 with a mean rate of 8 m a− 1 and since 1988 there is a stabilization of this
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extension. In part B, the rapid initial extension of 44 m a− 1 between 1978 and 1988 slows down significantly as from 1988 with 26 m a− 1 between 1988 and 1998. A relative stabilization of the salt marshes is observed as from the year 2000, in particular close to the Normandy bridge. Since 2002, certain parts of the salt marsh limit show even a local retreat. The relative stability of the southernmost salt marsh since 1988 observed very early with respect to the dune (part A) can be explained by a certain perenniality of the hydrodynamic conditions in this part of the Northern mudflat (Cuvilliez, 2003). The extension recorded over time in part B is explained by the construction of the Northern low dyke, the Hook dyke and the breakwater upstream (Lesourd, 2000) which since 1978 have had an effect on the empoldering of this zone by significantly attenuating the swells. The Hook dyke is used as an anchoring point for salt marsh while stabilizing, along with the Northern low dyke, the wetted cross section in the mouth. During the second period (since 1988), the relative stability of the environmental conditions slows down the expansion of the salt marshes which at the present time have stopped spreading towards the south because of the erosion that the mudflat (Fig. 2) has undergone since the 1980s. This has modified not only the topography of the mudflat (Bessineton, 2006) but also increased the hydrodynamic conditions that became unfavourable for the colonization of spartina (Desprez and Dupont, 1985). 4.2. Multiannual survey of the lower limit of the mudflat The southern limit of the mudflat is defined by the level of the low tides in spring (Fig. 2) in the lower part of which the subtidal zone extends. This limit evolves differently from east to west over the past 27 years and allows to subdivide this surface into three large zones. The first, namely zone C, represents a surface whose northern limit is marked out by two perennial structures, i.e. the Hook dyke and a dune. In the south, this zone is separated from the Northern low dyke by a subtidal area. The 1.8 km long surface of the mudflat is relatively stable since the maximum retreat recorded during the period 1978–2005 was 102 m, giving a mean retreat rate of 3.8 m a− 1. The second zone, called D in Fig. 4, measures 2.5 km long and corresponds to a mudflat crossed by tide channels draining water from the salt marshes and making up its northern limit. In the south, this zone is influenced by the strong ebb and flood currents present in the Northern trench included between 1.5 and 2 m s− 1 (Waeles, 2005). This part of the mudflat, limited by a cliff, regressed quickly since the maximum retreat recorded over 27 years is 450 m, representing a mean rate of 16.7 m a− 1. The third zone, called E, approximately 350 m long, presents as for the zone C a soft inclined limit with the northern channel and an evolution similar, that is to say a maximum retreat of 120 m during the period 1978–2005 with a mean rate of 4.4 m a− 1. The morphology and its evolution of the zones C and E are to be put in relation to an orientation of the currents parallel or subparallel with these limits and speeds lower than 1 m s − 1 (Waeles, 2005). The installations carried out since 1978, i.e. the dykes and the Normandy bridge, have modified slightly the conditions of deposition and erosion in zones C and E. However, this is not the case for the axial part of the Northern mudflat where the opening of a breach in the Northern low dyke in 1980 (Desprez and Dupont, 1985), the installation and the enlargement of the bank of La Passe, then the construction of the breakwater influenced the evolution of this zone by creating an acceleration of the currents, inducing erosion of the mudflat. The retreat of the cliff which borders the mudflat in the south (zone D, Fig. 2) has evolved at three different rates. From 1978 to May 1999 the average retreat is estimated at approximately 25 m per annum. From 1999 to 2003 it increased to a rate of approximately 58 m per annum (Fig. 4). Then, a stabilization of the retreat of the top of the cliff limiting the low
Fig. 3. a. Location of the Northern mudflat area used for the study of the retreat of the cliff. b. Location of the ground control points used for the valuation of the retreat of the cliff. c. Extracts of aerial cartographies from 1997 to 2005 showing positions of ground control points and cliff. All extracts are at the same scale.
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Fig. 5. Environmental planning and evolution of the Northern mudflat these last 27 years.
mudflat has been observed as from 2003. The bathymetric data reveal that the low mudflat was initially truncated at its top (visible on Fig. 3c in April 2003 and May 2004) and thereafter underwent erosion at its base that tended to make it more vertical (Fig. 3c, June 2005). Currently, this erosion is stabilized in the axial part of zone D (Fig. 2) and is migrating more to the west subsequent to the installation of the breakwater (Fig. 1c) and to the change in geometry of the sandy bank following construction of this dyke.
4.3. Evolution of the intertidal muddy surface Basically, the entire surface of the mudflat (Fig. 7) has changed on average according to two broad tendencies: a rapid reduction of surfaces between 1978 and 1985, then a slower reduction between 1985 and 2005. The first reduction phase is explained by a considerable spread of the salt marsh (Hoyez and Cuvilliez, 1998). During this period, the rate of disappearance of the mudflat is 0.28 km2 y− 1 and the Fig. 5, curves C, D,
Fig. 6. Differential of soundings made between March 2005 and December 2002. The cartographied surface is the 2005 one. Red: La Passe bank limit. Pink: isobath 0.
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Fig. 7. (a) Grey: area which was the subject of a follow-up of the surfaces evolution between 1980 and 2005. The results are on (b). (b) Evolution during 25 years of the surfaces of salt marsh, mudflat, intertidal sand banks and subtidal of the limited area on (a).
and E) shows that it's linked to the construction of the Hook dyke and the Northern low dyke. In addition, the installation of the engineered breach resulted in an increase in the current velocities at flood and ebb tides (Desprez and Dupont, 1985; Hoyez and Cuvilliez, 1998; Lesourd, 2000). Consequently, these new hydrodynamic conditions induced a new system made up of unstable silt–sand units and a trench. The slow reduction phase begins in 1985: the surface reduction of the mudflat continues much more slowly, at about 0.13 km2 y− 1 over 20 years. This corresponds to the new dynamics of this estuarine system which is contained by dykes and a restricted extension of the surface of the salt marsh which gradually tends to be stabilized. Occasionally, this regression still takes place in the central zone D but is compensated for from 1998 to 1999 by the growth of the western and eastern zones of the mudflat in zones C and E (Fig. 5) subsequent to the construction of the Normandy bridge and the installation of the embankments. Indeed, during these last 27 years human activities have induced a reduction of more than 62% of the surface of the Northern mudflat of the Seine estuary. By coupling the bathymetric data and aerial shots of the Northern mudflat, it is possible to estimate sediment volumes. For the period going from December 2002 to March 2005, in zone D (Fig. 2) the volume of sediments moved in this zone over a period of 27 months represents approximately 1.25 Mm3. This corresponds to an estimated reduction of 0.18 km2 in the surface of the mudflat corresponding to 5.5% of its surface. These sediments, derived from the mudflat, are
primarily consolidated silts associated with sand lenses (Lesourd et al., 2001) transported by the flood current towards the east of the trench. In addition, the observations carried out by this study show that the silt is either stored on the level of the raised breach, or exported towards the Seine via the recently dug meander. As the orientation of the megaripples observed on its surface underlines, the bank of La Passe is currently enriched by a large contribution of 200 µm fine sand (Fig. 6) typical of the Seine estuary, thanks to the action of a dominating flood current. In addition, the Hook dyke, located in the west, acts on this bank not only by giving it the bifurcated form visible in Fig. 6, but also by favouring the continuous increase of its northernmost surface. This involves a displacement of the trench towards the north. As a result, it pushes back the southernmost limit of the low mudflat and digs the bottom of the channel in the same direction, making the cliff more abrupt (Fig. 3c, (I) June 2005). The studies of sedimentary and granulometric dynamics carried out in this zone reveal variations that are a consequence of the constructions. Thus, the principal granulometric analysis done on the mudflat surface showed a change from 15 and 90 µm in 1998 (Lesourd, 2000) to 200 µm in 2005. If we refer to the data collected over the two last years, we can note that the construction of the foundations of the breakwater from July to September 2003 was accompanied by an inversion of the sedimentary dynamics, going from an episode of sedimentation to one of erosion and propagated from the low mudflat towards the high mudflat in zone C (Fig. 2). On the contrary, the raising
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of this same breakwater simultaneously changed the phenomenon of generalized surface erosion of the mudflat in zone D into a process of sedimentation that propagates progressively from the south towards the north. Lastly, the data collected from the mudflat as well as from the bank of La Passe show an evolution of the grain size resulting in an overall increase in the mean size of the deposited particles, originally at 20 µm and reaching 200 µm between 2002 and 2004 for zones D and E (Fig. 2). The absence of finer particles underlines the increase in the hydrodynamic conditions at the Northern trench. 4.4. Overall assessment of the Northern trench The evolution of the intertidal muddy surfaces in the Seine estuary depends on both the evolution of the mudflat–salt marsh boundary and the spatial extent of the low mudflat. Consequently, it is necessary to take into account all of the estuarine zone which is submerged at high water, i.e. both subtidal and intertidal zones. We thus extended our study to the entire northern zone of the estuary which has been evolving since 1978, the date of the beginning of this survey. This 25 km2 zone is limited by the Northern main dyke, the Northern low dyke, the mouth of the Seine and the breakwater located upstream of the estuary (Fig. 7a.). In this area, we calculated not only the surfaces of the subtidal zone but also those of the intertidal sands made up of the sandy megaripple banks found in the low mudflat, the bank of La Passe (ranging between 0 and 5 m), the mudflat (ranging between 5 and 7.5 m) and the salt marsh (greater than 7.5 m). The installation of the longitudinal dykes (Hook dyke and Northern low dyke) and of an upstream breakwater until 1980 changes the direction of the currents and decreases their force. So (1) the intertidal area (sandy mudflat and banks) significantly decreased until 1985, (2) the subtidal zone developed at a rate of 0.28 km2 per year and (3) the salt marsh developed quickly (Figs. 2 and 7b). To avoid a stronger mud deposit in the study zone, a breach was set up in 1978 inducing strong currents in the Northern trench (Desprez and Dupont, 1985). The result of this was the build up of the sandy intertidal bank of La Passe, observed for the first time in 1985, which gradually reduced the surface of the subtidal zone at a rate of 0.08 km2 per year and increased the depth of the navigation channel (Avoine, 1985). This phenomenon still persists today, 20 years later. The installation of the embankment for the Normandy bridge in 1988 has maintained the slow and continuous encroachment of the salt marsh. The sandy intertidal zone continues to increase due to a strong dynamism in the Northern channel, accentuated by the recalibration of the breach, then the installation of a breakwater in 2003. Thus, 78% of the variation in the bathymetric profile occurred between 2002 and 2005. The reduction observed in the wetted cross section has caused an increase in the erosion of the southernmost limit of the low mudflat which has become more abrupt since 2004 (Fig. 3c. (H)). The general sanding up of the zone, observed since 1985 by Desprez and Dupont (1985) and after by Hoyez and Cuvilliez (2001), favours a raising of the bottom of the subtidal part which is transformed into a sandy intertidal zone. The mudflat, located in the concave part of the Northern channel, erodes quickly under the effect of the currents. 5. Discussion The morphological evolution of intertidal mudflats was and makes still the subject of studies based on recordings continuous or discontinuous realized with various steps of time and are completed sometimes by mathematical models necessary to explain and to consider the modifications of the units which compose these fields. Recordings carried out from the daily to the secular scale made it possible to follow the evolution of estuaries like those of Severn (Harris and Collins, 1985; Kirby, 2000), of Ribble (van der Wal et al., 2002, 2004),of Keum (Kim et al., 2006), of Mersey (Blott et al., 2006), of Ord
(Wolanski et al., 2001), and of Gironde (Fenies and Tastet, 1998). The mathematical models like those presented by Roberts et al. in 2000, Pritchard in 2002 and 2007, D' Alpaos and Defina in 2007 supplement these morphological studies. In this study, the pluriannual scale seems to be adapted to show the sedimentary and morphological impacts consecutive with constructions, contrary to finer scales (diurnal and semi-diurnal), better to understand the impact of punctual phenomena like storms or flooding (Deloffre, 2005). The estuary of the Seine was the subject of studies concerning its sedimentary cover since 1834 per Beautemps-Beaupré (Lennier, 1885) showing until in the 70s that this zone was mainly made up of sandy sediments (Avoine, 1981; Laignel, 1991). In the 90s, new studies showed a clear silting of the mouth of the Seine (Avoine, 1994; Lesourd, 1995). However, the historical data concerning the estimate of surface of the studied zone before 1978 remain approximate and in 1974 it seems that the slikke occupied a surface from approximately 10 km2 and the salt marsh less than 2 km2 (Lesourd, 2000). With the increase in the rhythm of constructions in the estuary since 1978, the regular estimates of the surface of the northern mudflat were realized and their results refined by the remote sensing at low altitude which makes it possible to obtain an instantaneous calculation of surfaces of the various units which make all this mudflat. The results obtained within the framework of this study during these 27 last years make it possible to evaluate the morphological and sedimentary impacts of two types of installation on an intertidal mudflat in a macrotidal estuary: (1) the construction of dykes and (2) that of a bridge. (1) 3 types of dykes are to be distinguished: (i) those which are parallel or subparallel with the current of flood and ebb (the hook dyke and the Northern low dyke), (ii) that which blocks these currents (the breakwater dyke) and (iii) those which limit the extension of Port 2000 more downstream from the zone of study. (i) This type of dyke led to two responses spread out in time. (1) First of all they increase sedimentation in the northern part of the estuary, which leads in 7 years (1978–1985) to an increase in the surface of salt marshes (Hoyez and Cuvilliez, 1998; Lesourd, 2000; Cuvilliez, 2003). The same type of construction led to the same result in various estuaries like in Ribble (van der Wal and Pye, 2004) or in Keum (Kim et al., 2006). (2) In addition, these dykes have a longer-term effect by decreasing oscillating volume of the zone, which is isolated from the channel of navigation, of 31% from 1981 to 2005. This continuous reduction in the oscillating volume, which accelerates with the construction of the hook dyke in 2003, reinforces the phenomenon of sedimentation which tends to decrease the capacities of the biological compartment. (ii) the breakwater dyke, perpendicular to the current of flood and ebb, has an impact different from the dykes parallel with the major currents. Indeed, as of the end of its construction in 2003, it causes a fast erosion of the slikke located at a hundred meters of its end by diverting the currents of flood and ebb. It also supports the fast growth of a silty and sandy bank, the bank of La Passe (Fig. 6), in the most sheltered zone of flood current dominating. It is however difficult to estimate with precision the longer-term impact of this dyke but it is possible that these phenomena of erosion of the slikke and sandy deposit still continue over a period ranging between 4 and 7 years. (iii) the construction of Port 2000 dykes causes an erosion of the adjacent slikke and the growth of sandy banks with megaripples. This is due to an increase current velocity in this zone and these effects are similar to those recorded with the construction of the breakwater dyke. This similarity is explained by the fact why the principal dyke of Port 2000 is also parallel or subparallel with the main currents.
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(2) During the construction of the bridge of Normandy, the installation of embankment perpendicular to the major currents onto the stable salt marsh increased sedimentation. The impact of the installation of the embankments was delayed by the construction of the pillars of the bridge, in the lower part of the mudflat, which disturbed the processes of sedimentation. Thus, the accretion of the Eastern zone of the northern mudflat was entirely completed only 10 years later (zone E, Fig. 5). The assessment of surfaces should lead to a relative stabilization over a 30 year period if no new installation intervened, as in the estuary of the Mersey (Blott et al., 2006). However, in estuary of the Seine, continuous installations do not make it possible to observe this stabilization. In the same time, the sedimentary assessment related to installations must also take into account the significant activities of dredging carried out on the level of the channel of navigation. They are estimated at 4.106 m3 in 2005 according to the Port authority of Le Havre. This has certain effects, in particular by supporting estuarine sedimentation like that was recorded in the estuary of the Mersey (Blott et al., 2006) or of Quequén in Argentina (Perillo et al., 2005). 6. Conclusions Thanks to the recordings carried out during 27 years starting from traditional methods (bathymetric, granulometric for example) but also original and precise like the low altitude remote sensing, we could estimate the morphological and sedimentary impacts of the principal types of installation carried out by the man in the estuary of the Seine these last years. These impacts are variable and depend on the nature and the extent of work. They can be immediate, in the case of constructions of the dykes, with an erosion of the muddy zones, sensitive to the variations of currents and continue over 7 years before a new hydrodynamic balance is reached. These morphological and sedimentary impacts can also be differed in time and be different from that observed with dyke constructions. It is the case of the construction of the bridge of Normandy from which the effects on the growth of the salt marshes in the Northern mudflat were reached ten years after the beginning of work. Finally, at the present time, the whole of constructions undertaken induce a sedimentary assessment which shows that intertidal mudflats are drastically reduced during this period of 27 years. It is appropriate however to indicate that the constant rhythm of the work completed since 2002 in the estuary of the Seine complicates the study of the morphosedimentological impacts related to constructions. Indeed, the time between two works is shorter than the time of the morphological equilibrium. In addition, it is advisable to also take into account the role of the climatic variations that decrease the flow of the river in period of winter rising since 2001, and climatic changes that increase the marine level in the Seine from 1 to 5 mm per annum (Waeles, 2005). Lastly, if the morphosedimentological response can be estimated with more or less difficulties, one can put questions about the consequences even more complex on the habitats and the biological compartment. Acknowledgements This study could not have been completed without the collaboration of the members of La Maison de l'estuaire and the people in the dredging and the hydrography departments of both the Le Havre and Rouen Port Authorities. This work was carried out in the framework of the Scientific Seine-Aval Program coordinated by the Conseil Régional de Haute-Normandie. References Anthony, E.J., 2000. Marine sand supply and Holocene coastal sedimentation in northern France between the Somme estuary and Belgium. In: Pye, K., Allen, J.R.L. (Eds.), Coastal and Estuarine Environments: Sedimentology, Geomorphology and Geoarchaeology. Special Publication N°175. Geological Society, London. 435 pp.
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