Coastal Dynamics 2013
SALINITY AND SUSPENDED SEDIMENT DYNAMICS IN RESPONSE TO FORCING
CHANGES IN A SMALL MACROTIDAL ESTUARY (CHARENTE, FRANCE)
Florence Toublanc1, Isabelle Brenon2, Thibault Coulombier3, Olivier Le Moine4
Abstract
A three-dimensional numerical model was developed to reproduce the hydrodynamics and salinity dynamics of the
macrotidal Charente estuary, located on the French Atlantic coast. The model uses horizontal structured grids and
vertical sigma layers. Tidal elevations from a series of nested models and river flows are considered for the open
boundary conditions. The roughness length and the turbulent scheme were adjusted in order to better fit the tidal
elevation, currents, and salinity observed on different locations. The simulations give good results, with a decreasing
accuracy when moving upstream. The validated model was used to simulate the impact of the river flow and the
spring/neap tidal cycle on the salinity intrusion and the residual circulation. Sediment fluxes were also evaluated from
currents simulated in realistic conditions of weather and river flow, associated with turbidity in-situ measurements.
Key words: Estuary, saline intrusion, vertical stratification, sediment dynamics, numerical 3D modeling.
1. Introduction
Estuarine tidal circulation, salinity and sediment transport processes are the result of complex interactions
between coastal ocean and upland rivers. Water surface elevation and current velocities within the estuary
result from non-linear interactions between the river flow and the incident open sea hydrodynamics at the
mouth. The resultant hydrodynamic behavior of the estuary determines the salinity structure and the
sediment transport. A classical two-layer circulation emerges from the combination of seaward transport
induced by the river and a periodic tidal seaward/landward transport. In a macrotidal estuary, tide-induced
turbulence can be large enough to mix water bodies and impede the formation of a stratified water column.
Previous studies investigated numerically the incidence of forcing variations (tides, river flow and wind) on
saline intrusion and vertical stratification. Prandle (2004) explained that for partially mixed estuaries, an
increase in tidal currents provoked a decrease in density related mixing, but an increase in mixing due to
tidal straining. Gong et al. (2011) showed that the salt intrusion length depends greatly on the river
discharge.
Sediment transport and suspended sediment concentrations are strongly dependent on the tides and the
river discharge. Allen et al. (1980) demonstrated that the turbidity maximum in the Gironde estuary is
moved upstream for low river discharge, and downstream during high discharge. This behavior was
confirmed by several studies in the Gironde (Sottolichio et al., 2000), or in other macrotidal estuaries
(Brenon et al., 1999; Uncles et al., 2006). Higher suspended sediment concentrations are expected for the
highest tidal ranges, against low concentrations for neap tides. Strong vertical mixing due to fast tidal
currents occurs during spring tides, eroding more sediment. During neap tides, tidal currents are weaker
and the sediment is settling (Dyer, 1997). Combination of high river runoff and spring tides usually leads to
high suspended sediment concentrations and seaward export (Castaing and Allen, 1981; Lopes et al.,
1
UMR 7266 CNRS-LIENSs, University of La Rochelle, 2 rue Olympe de Gouges, La Rochelle 17000, France.
[email protected]
2
UMR 7266 CNRS-LIENSs, University of La Rochelle, 2 rue Olympe de Gouges, La Rochelle 17000, France.
[email protected]
3
UMR 7266 CNRS-LIENSs, University of La Rochelle, 2 rue Olympe de Gouges, La Rochelle 17000, France.
[email protected]
4
IFREMER-LERPC, Center of La Tremblade, Ronce-les-bains, La Tremblade 17390, France.
[email protected]
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2006). Under these conditions, the turbidity maximum is located downstream and tidal induced mixing is
enhanced, redistributing sediments in the water column that can be exported by high surface velocities
(Castaing and Allen, 1981).
Understanding the salinity and turbidity dynamics of the Charente estuary represent an important issue
since the river is a source of fresh drinkable water for the area. Economic interests are also considered
since local leisure activities and oyster farming are greatly dependent on the suspended sediment
concentrations and the outflows directions (Ravail et al., 1988; Modéran et al.; 2010; Modéran et al.,
2012). A field study, including a large set of in-situ data, was conducted, highlighting the strong effect of
tides and river runoff on the turbidity maximum extension and position, and on the suspended sediment
concentration (Coulombier et al., 2013). The aim of this study is to determine the impact of tides and river
flow on the vertical stratification and horizontal distribution of salinity, on the residual circulation, and on
the sediment fluxes at the river mouth, using a 3D-numerical model validated through in-situ data.
2. Material and methods
2.1. Study site
The Charente estuary (45°96'N, 1°00'W) (Figure 1) is small, shallow, and under the influence of a semidiurnal tide. Tidal range can reach 6.5 meters at the river mouth, with a 5 meters mean level, which
indicates a macrotidal regime. Mean river flow is estimated at 70 m3/s, reaching maximum values of 600
m3/s over the last decade. A dam is located at Saint-Savinien, 50 km from the mouth, and the river flows
into the Marennes-Oléron bay, in the southern part of the Pertuis Charentais. The tidal dynamics of the area
are characterized by an inversion of the tidal asymmetry over the spring/neap cycle: flood duration is
shorter than ebb duration during spring tides and longer during neap tides (Toublanc et al., 2012).
The sediment encountered is cohesive and the estuary is highly turbid, with a great influence on the
sediment distribution of the Marennes-Oléron Bay (Ravail et al., 1988; Modéran et al., 2010; Modéran et
al., 2012).
Figure 1. Map of the Charente Estuary and its adjacent coastal area. The locations of the stations used to calibrate and
validate the model are indicated.
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2.2. 3D numerical model
The 3D numerical model MARS3D is a finite differences model with sigma coordinates, solving the
Navier-Stokes equations with the classic Boussinesq and hydrostatic hypotheses. It has been fully
described by Lazure and Dumas (2008).
The model is forced by atmospheric conditions (wind, air temperature, atmospheric pressure, cloud cover
and relative humidity) over the entire domain, provided by the Météo-France model ARPEGE (0.5° and 6h
resolution). Sea surface elevation is imposed at the open boundaries and is obtained from a series of five
nested models of decreasing extensions and increasing spatial resolutions. The finest grid is represented
horizontally by a 30 m uniform mesh of 1405 x 766 points, and vertically by 5 sigma levels. Daily
discharges of the Boutonne and the Charente rivers are considered. High resolution bathymetric data were
provided through several organizations (SHOM, Ifremer, EPTB Charente) with datasets from 2003, 2007
and 2010.
A two equations k-kl turbulence closure model was chosen (Warner et al, 2005a). This model, close to the
Mellor-Yamada level 2.5 scheme (1974, 1982), solves the vertical eddy viscosity and diffusivity by
computing the turbulent kinetic energy and the turbulence length scale. The horizontal eddy viscosity and
diffusivity are computed by taking the local shear conditions into account, using the Smagorinsky method
(1963). Bed roughness height is set to 0.0001 m.
2.3. In-situ measurements
Model validation is performed using in-situ data recorded for different periods and at different locations
detailed on Figure 1. Continuous tide gauge data from Rochefort and the Aix island (from the REFMAR
portal) and ADCP data (1200 kHz, 5 minutes bursts, February to April 2011) at the river mouth (Lupin)
were provided for sea surface elevation and current speed validation. Simulated salinity levels were
compared to datasets obtained from multi-parameter probes (YSI 6600V2) located at the mouth, surface
(February to March 2011, 10 minutes sampling) and bottom (October to December 2012, 5 minutes
sampling). In order to estimate sediment fluxes, turbidity data were also recorded at the mouth
(approximately 2 meters from the bed) with YSI 6600V2 probe (6026 optic turbidimeter, 5 minutes
sampling). Salinity and turbidity are laboratory calibrated using 12880Us.cm-1 and formazine 1000 and
4000 NTU standard solutions.
3. Results
3.1. Model validation
Model validation was conducted over different periods of time, depending on the in-situ data available.
Modeled water surface elevation, velocity currents and salinity concentrations were compared to observed
data. For each variable, the mean absolute error (MAE) and the root mean square error (RMSE) were
calculated. A skill parameter developed by Willmott (1981) and recently used in different estuarine
dynamics modeling studies (Li and Boicourt, 2005; Ma et al., 2011; Warner et al., 2005b; Xing et al., 2012)
was also considered to estimate the accuracy of the model:
= 1 − ∑| − |
∑| − | + | − |
(1)
This parameter allows to take into account modeled and observed deviations around the observed mean to
estimate the performance of the simulation. Perfect agreement between model and observations
corresponds to a skill parameter of 1. All calculations are presented in Table 1.
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Table 1. Model/data comparison statistics for water surface elevation, current velocity and salinity.
Variable
Location
Water surface elevation
Ile d'Aix
0.15 m
0.21 m
0.9937
Lupin
0.23 m
0.29 m
0.9904
Rochefort
0.37 m
0.45 m
0.9746
Lupin (averaged)
0.15;0.08 m/s
0.18;0.10 m/s
0.9762;0.9205
Rochefort (bottom)
0.11;0.14 m/s
0.14;0.18 m/s
0.9075;0.9283
Current velocity (u;v)
Salinity
MAE
RMSE
Skill (equation (1))
Lupin (surface)
3.24 psu
4.06 psu
0.9432
Lupin (bottom)
3.51 psu
4.64 psu
0.9261
3.1.1. Tidal elevation and currents
Tidal elevation is compared to tide gauge data from two locations (Ile d’Aix, Rochefort), and ADCP data at
the mouth (Lupin). Comparison for Lupin is presented in Figure 2. RMS errors and skill parameters (Table
1) suggest that the model reproduces well the water surface elevation, with a larger error for the most
upstream location (Rochefort). However, double high waters are often not reproduced by the model. High
waters are more accurate than low waters, both for the Lupin and Rochefort locations. At the mouth, this
error can be related to the position of the ADCP, which was not fully immersed at low water during spring
tides. Concerning the Rochefort station, Walther et al. (2007), showed that the presence of fluid mud could
lead to a significant decrease of low water levels that is not reproduced by hydrodynamic models. This
phenomenon would be consistent with the sediment dynamics of the Rochefort area where high suspended
sediment concentrations lead to fluid mud deposition (Coulombier et al., 2013).
Figure 2. Comparison of water surface elevation between model (solid) and observations (dots) at Lupin
Comparison of ADCP and modeled depth-averaged currents at Lupin are presented in Figure 3. Some data
are missing since the ADCP was not fully immersed at all times, particularly at low water during spring
tides. Model results agree with observed data reasonably well, but the maximum currents are generally
underestimated, especially for the meridional velocity. This observation is confirmed by the skill parameter
(0.92), inferior to the one calculated for the zonal velocity (0.97). Bottom ADV and simulated velocities are
compared at Rochefort. In this case, meridional velocities are better reproduced than zonal velocities (0.93
skill parameter against 0.91). Maximum current velocities are again globally underestimated, especially for
the zonal velocity. For the two locations, simulated along-channel currents are then better simulated than
lateral currents. Both for current velocities and for water surface elevation, simulated and observed data are
very well in phase.
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Figure 3. Comparison of zonal (U) and meridional (V) depth-averaged current velocities between model (solid) and
observations (dots) at the river mouth.
3.1.2. Salinity
Surface salinity concentrations at the mouth are presented in Figure 4. Both the RMS error (4 psu) and the
skill parameter (0.94) show that the model reproduces well the surface salinity (Table 1). The peaks due to
low or high water are almost perfectly in phase for the whole simulation. Transitions between spring and
neap tides are the dominant source of error. Between the 26th of February and the 2nd of March, surface
salinity is particularly overestimated by the model. This could be explained by the precipitations that
occurred in the area at this time, combined with a slight peak in wind velocities (around 30 km/h). These
atmospheric conditions were not observed for the second neap tide (around the 13th of March). Bottom
salinity at the mouth is also well reproduced with a skill parameter of 0.92 (Table 1).
Figure 4. Comparison of surface salinity concentrations between model (solid) and observations (dots) at the river
mouth
3.2. Response to tidal and river forcing changes
3.2.1. Saline intrusion and stratification
After calibration and validation, the model is used to estimate the impact of tides and river flow on the
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Depth (m)
salinity dynamics of the estuary. Two river flows were considered, over a spring/neap tidal cycle: 5 m3/s
and 400 m3/s. For both simulations, realistic meteorological conditions were considered, respectively
representative of summer and winter periods. Transect salinity distributions along the estuary are presented
in Figure 5 for low and high waters of the strongest spring tide and the weakest neap tide encountered
during the simulated period.
LW-NT
LW-NT
HW-NT
HW-NT
LW-ST
LW-ST
HW-ST
HW-ST
Distance from the mouth (km)
Figure 5. Salinity transects along the estuary, 5m3/s (left) and 400 m3/s (right).
(LW-NT): low water, neap tide. (HW-NT): high water, neap tide. (LW-ST): low water, spring tide. (HW-ST): high
water, spring tide.
Vertical and horizontal salinity distributions are evaluated in Table 2, in function of the tidal and river
forcing. Stratification at Lupin and Rochefort are presented as the difference between bottom and surface
salinity. The salinity intrusion limit is given by the distance from the mouth at which the bottom salinity is
becoming inferior to 1 psu. Calculations are made for the same conditions as for Figure 5.
Table 2. Salinity stratification and salinity intrusion in function of tidal and river forcing.
River flow
(m3/s)
Bottom to surface salinity
difference (psu) : Lupin
Bottom to surface salinity
difference (psu) : Rochefort
LW
34.36
≈0
7.9
HW
34.72
≈0
12.1
LW
≈0
≈0
0.4
HW
25.41
≈0
12.1
LW
0.66
1.57
33.8
HW
0.32
4.21
41.1
LW
0.05
1.60
40.2
HW
0.01
2.13
43.8
Tidal conditions
Neap
Tide
400
Spring
Tide
Neap
Tide
5
Spring
Tide
Salinity intrusion
limit (km)
As expected, saline intrusion length is maximum for the highest tidal range and the lowest river flow
(Figure 5, Table 2). For a 5m3/s river flow, the movement of this intrusion between low and high water is
higher for neap tide (7.3 km) than for spring tide (3.6 km). The intrusion variation between neap and spring
tides is about 10 km. Salinity stratification at the mouth is inferior to 1 psu both for spring and neap tide.
Transects also show the channel weak stratification for low river input (Figure 5).
Under high river flow conditions, salinity intrusion is moved downstream (12.1 km from the mouth for
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neap and spring tides). The intrusion limit is almost pushed outside of the estuary at low water for spring
tide. Its extension is higher for spring tide (11.7 km), but smaller for neap tide (4.2 km). Salinity
stratification is very strong at the mouth, except during spring tide at low water. This is simply because
there is only fresh water in the water column in the channel, which is also the case at Rochefort for all tidal
conditions. Simulations show that salinity can locally be higher (up to 5 psu) on the sides of the channel at
the mouth, probably because of the lower dynamics of the tidal flats. Strong vertical stratification for high
runoff is confirmed by transects presented in Figure 5. For both runoff conditions, neap tide stratification is
equivalent or higher than spring tide stratification.
3.2.2. Residual circulation at the mouth
Tidally averaged current velocities are calculated at the river mouth to determine the tidal and fluvial
influence on the residual circulation. Under low river flow conditions, residual velocities are small and
decrease almost linearly from surface to bottom. They do not exceed 8 cm/s for neap tide and 19 cm/s for
spring tide. Values decrease very rapidly in the first part of the water column: 72% and 75% drops,
respectively for neap and spring tide, between the first two layers.
With a 400 m3/s river flow, surface residual velocities reach 53 cm/s during spring tide and 48 cm/s during
neap tide. Vertical distribution is more linear, especially for spring tides. For all cases, residual circulation
is oriented seaward at the surface. At the bottom, residual velocities are all inferior to 8 cm/s.
3.3. Sediment dynamics
Turbidity data at the Lupin station are available from October to December 2012. In order to estimate
sediment fluxes at this station, the current velocities were simulated, with real atmospheric and runoff
conditions. Two floods are observed over this period of time, with respectively 100 m3/s and 350 m3/s
runoffs. Turbidity data are converted, through calibration, to obtain suspended sediment concentrations in
g/L. Current velocities are extracted from the model to obtain suspended sediment fluxes for each time step
(flood 1: Figure 6a, flood 2: Figure 6b). Negative values indicate sediment exporting, positive values
indicate sediment importing. Net sediment transport is calculated over a tidal cycle. Since the turbidity
variability (vertically and horizontally) observed on the field is significant, the choice was made to not
integrate these values on the water column or on the channel cross-section. It is also important to notice
that calculated fluxes remain estimations since the turbidity sensors can only measure up to 4000 NTU. As
described by Coulombier et al. (2013), this limit is exceeded very often, more particularly during high
runoff and spring tides.
(a)
(b)
Figure 6. Suspended sediment fluxes (kg/m²/s) and net transport per tide (ton/m²/tide) at the mouth in function of the
river flow and tidal range.
(a): flood 1 (≈100 m3/s). (b): flood 2 (≈350 m3/s).
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Spring tide conditions provoke higher sediment transport, due to higher current velocities. For both cases,
higher net sediment transport occurs during spring tide, respectively 12.9 and -16.1 tons/m²/tide.
For flood 1 (Figure 6a), four phases can be observed. First, with spring tides and a very low river discharge
(≈20 m3/s), net sediment transport is negative, indicating seaward transport (15th-18th of October). As the
river flow increases (20 to 100 m3/s) and the tidal range decreases, sediment is imported (19th-23rd of
October). Net transport is approaching zero under neap tide conditions, with the river discharge reaching its
maximum of 105 m3/s on October 24th and dropping to 76 m3/s on October 26th. Finally, tidal range
increases and river flow decreases, leading to landward sediment transport.
For flood 2 (Figure 6b), such phases are more difficult to observe since net sediment transport values are
less linearly distributed. Between the 13th and the 17th of December, sediment tends to be exported, under
spring tide conditions and average river flow (≈60 m3/s). Tidal range decreases, and river discharge
increases (60 to 210 m3/s) between the 17th and the 20th, leading to net sediment import. Sediment transport
is then directed seaward, as the river flow keeps on rising and the tides approach neap conditions (20th-23rd
of December). At the runoff peak (December 23rd to 26th) and during neap tide, sediments seem to be
imported again.
In both cases, net sediment transport is sensitive to river flow variations, but is strongly modulated by the
spring-neap tidal cycle. According to field observations, salinity concentrations at the same location
remained driven by the tide, meaning that the river flow was not large enough, or not for a sufficient period
of time, to eject the turbidity maximum over several tidal cycles. However, salinity variations between high
and low tide are higher during flood 2 (26 psu against 18 psu for flood 1), suggesting that the turbidity
maximum is moved downstream, and could be oscillating at the mouth, in function of the tides.
4. Discussion
4.1 Model validation
A three-dimensional finite differences model was developed to reproduce the hydrodynamics of the
Charente estuary, and to assess the impact of tidal and runoff variations on estuarine dynamics. The model
performance was first evaluated by calculating the mean absolute error (MAE), the root mean square error
(RMSE), and a skill parameter developed by Willmott (1981) describing the correspondence between the
observed and predicted deviations around the observed mean. This series of statistics (Table 1) shows that
the model gives a good representation of the water surface elevation, the current velocities and the salinity
concentration in the Charente estuary (skill > 0.9 for all parameters). However, modeled low water levels at
Rochefort are not very well reproduced (Table 1). This error could be linked to the presence of deposited
fluid mud observed in the area (Coulombier et al., 2013), inducing very low bottom friction. Taking into
account the variation of friction due to sediment dynamics seems then essential to improve the model
accuracy, as shown by Walther et al. (2007). Preliminary results obtained from a test with very low friction
showed an improvement of errors on the water level at Rochefort (from 0.45 m RMSE to 0.34 m).
Bathymetric uncertainties also need to be considered as a source of error.
The salinity response to atmospheric forcing (Figure 4) could be enhanced with higher resolution data. Li
et al. (2005) also showed that turbulence mixing schemes often fails to reproduce correctly salinity
stratification under high runoff conditions. To improve model predictions, they suggest that the background
diffusivity should not be a constant but a time and stratification dependent variable.
4.2 Estuarine circulation and mixing dependence on fluvial and tidal forcing
The effect of the spring-neap tidal cycle and the river runoff on salt intrusion and stratification is
investigated in the Charente estuary (Table 2). As shown previously by several studies (Liu et al., 2007,
Azevedo et al., 2010, Gong and Shen, 2011), river flow influences greatly the saline intrusion length in the
estuary. For a very low runoff, the intrusion limit (defined as the 1 psu limit) is localized between 34 and
44 km from the river mouth, highlighting the low dynamics of this configuration, already observed in other
studies (Bowen and Geyer, 2003; Gong and Shen, 2011). Under high runoff conditions, salt is exported
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towards the Marennes-Oléron Bay and stratification at the mouth can be very high (up to 34 psu). Residual
circulation calculations confirm this behavior since surface seaward velocities can be six times higher than
with low runoff. The surface layer is dominated by the river flow. Salinity at the Aix island can decrease
below 10 psu, confirming the strong influence of the river on the surrounding area and its ecosystem.
Spring tide salinity stratification is equivalent or weaker than neap tide stratification, especially with high
runoff. Calculated residual velocities are vertically well distributed under these conditions, showing that
tidal mixing induced by higher velocities is more important during spring tide. During neap tides, tidal
straining decreases and velocities are reduced, preventing the turbulent mixing from developing (Lewis,
1997; Thain et al., 2004).
The movement of the salinity intrusion limit between low and high water is higher for spring tide and high
runoff, but also higher for neap tide and low runoff. For the weaker fluvial forcing, this limit is located far
upstream (around 40 km), where the tidal influence is lower than when the river flow is higher and the
intrusion limit is located close to the mouth. However, it is necessary to notice that tidal range difference
between the spring and neap tides considered is higher for the 400 m3/s case than for the 5 m3/s case. A
stronger spring tide would probably lead to a deeper upstream penetration of salinity and a larger difference
between low and high water.
4.3 Response of sediment transport to tides and river flow
As expected, sediment transport is enhanced by the highest tidal ranges, which provoke higher velocities.
Sediment fluxes are then more important for two reasons: higher velocities means more transport per unit
of time, but also more straining on the river bed, leading to erosion and higher suspended sediment
concentrations. Turbulence and mixing are also more important, whereas neap tide weaker currents allow
sediments to settle (Dyer, 1997). Sediment fluxes semi diurnal and fortnightly variability observed in
Figure 6 demonstrates that sediment dynamics are strongly determined by tidal currents.
Net sediment export from the estuary requires a river flow large enough, and over a significant period of
time. According to field observations and results shown in Figure 6, these conditions were not satisfied
during the two periods simulated, since the turbidity maximum does not seem to be ejected from the
estuary. Both runoff peaks also occurred during neap tides, when suspended sediment concentrations are
lower. As shown by Geyer et al. (2001), net sediment export can be more determined by the timing
between high runoff events and the spring-neap tidal cycle than by the intensity of the river flow. However,
salinity variations at the mouth, over a tidal cycle, are higher for the strongest river flow (26 psu for 350
m3/s against 18 psu for 100 m3/s), suggesting that the turbidity maximum, which is generally linked to the
salinity intrusion (Uncles and Stephens, 1993, Lopes et al., 2006), is moved downstream under high runoff
conditions.
For both cases, net sediment export is observed for spring tides and low (20 m3/s) or average (60 m3/s)
river flow. During spring tides, the flood is shorter than the ebb but flood current velocities do not exceed
ebb velocities by more than 10 cm/s in most cases. Ebb currents can even be stronger. Since the ebb is
longer, this could lead to higher sediment export. Such export under spring tide and low runoff conditions
was also observed by Kitheka et al. (2005). However, since turbidity probes saturated both during ebb and
flood, suspended sediment concentrations could be underestimated, more particularly during floods,
leading to errors in sediment transport estimations.
Sediment import observed from the 23rd of December (Figure 6b) could be explained by the strengthening
of flood tidal currents observed at this time. Since the turbidity probe is located near the bottom, sediment
erosion from the bed due to stronger currents, combined with higher flood velocities, could explain this
behavior. Near-bed dynamics at the mouth are mostly determined by the tides, explaining why this
phenomenon would occur, even if the river runoff is high. Coulombier et al. (2013), showed that more than
half of temporal variations of sediment transport at the mouth is explained by the tidal regime.
Further investigation is needed to determine when the turbidity maximum is exported in the MarennesOléron Bay, possibly affecting oyster farming. Incorporation of sediment transport in the model is
essential, because of the vertical and horizontal variability of velocity currents and suspended sediment
concentrations, but also because of the 4000 NTU limit of probes.
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5. Conclusion
The 3D numerical model developed for this study gives a good representation of the Charente estuary
hydrodynamics, especially at the river mouth. Different fluvial and tidal conditions can be tested to
determine their effect on the salinity dynamics and the residual circulation. Results show that the estuary is
more stratified during neap tides, due to weaker velocities. Particularly high runoff enhances stratification,
the surface layer of the estuary being dominated by the river. Saline intrusion is maximum for the highest
tidal range, when the river input is small. Sediment transport is strongly related to the semi-diurnal and
spring-neap tidal cycle. Net sediment export can occur for high river flow, more especially if this event is
consistent with spring tides. Sediment transport will be included in the model for future research, allowing
further investigations on the turbidity maximum formation and movement in function of various forcing.
Acknowledgements
The authors gratefully acknowledge funding from the Conseil Général of Charente Maritime, the CNRS,
the FEDER and the University of La Rochelle. The REFMAR portal is also acknowledged for the data
provided, as well as Ifremer for the bathymetry and the MARS3D code. This study would not have been
possible without the technical and data support from the Ifremer-LERPC Laboratory. The authors thank
particularly Nicolas Lachaussée, Philippe Pineau, Christophe Arnaud, Jean-Michel Chabirand, Florence
Cornette, Philippe Geairon, Stéphane Robert and Jean-Luc Seugnet for their precious help on the field.
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