Climate change and human activities impact the volume and timing of freshwater input to estuaries. These modifications in fluvial discharges are expected to influence estuarine suspended sediment dynamics, and in particular the turbidity maximum zone (TMZ). Located in southwest France, the Gironde fluvial-estuarine system has an ideal context to address this issue. It is characterized by a very pronounced TMZ, a decrease in mean annual runoff in the last decade, and it is quite unique in having a long-term and high-frequency monitoring of turbidity. The effect of tide and river flow on turbidity in the fluvial estuary is detailed, focusing on dynamics related to changes in hydrological conditions (river floods, periods of low discharge, interannual changes). Turbidity shows hysteresis loops at different timescales: during river floods and over the transitional period between the installation and expulsion of the TMZ. These hysteresis patterns, that reveal the origin of sediment, locally resuspended or transported from the watershed, may be a tool to evaluate the presence of remained mud. Statistics on turbidity data bound the range of river flow that promotes the upstream migration of TMZ in the fluvial stations. Whereas the duration of the low discharge period mainly determines the TMZ persistence, the freshwater volume during high discharge periods explains the TMZ concentration at the following dry period. The evolution of these two hydrological indicators of TMZ persistence and turbidity level since 1960 confirms the effect of discharge decrease on the intensification of the TMZ in tidal rivers; both provide a tool to evaluate future scenarios.
Macrotidal estuaries are highly variable systems as a result of the strong influence of both tides and river discharge. In particular, dynamics of suspended particulate matter (SPM) and the occurrence of a turbidity maximum zone (TMZ) are complex and difficult to predict (Fettweis et al., 1998; Mitchell and Uncles, 2013). Different processes can induce the formation of the TMZ (for details see Allen et al., 1980; Dyer, 1988; Jay and Musiak, 1994; Talke et al., 2009). This highly concentrated zone plays an important role in estuarine morphodynamics. Sediment depositions from the TMZ may generate gradual accretion of bed and banks (Pontee et al., 2004; Schrottke et al., 2006; Uncles et al., 2006). Therefore, many estuaries require regular dredging against ongoing siltation events to maintain the depth of navigation channels.
Quite recently, considerable attention has been paid to evaluate the effect of climate change (Fettweis et al., 2012) and human interventions (Schuttelaars et al., 2013; Winterwerp and Wang, 2013; Yang et al., 2013; De-Jonge et al., 2014) on natural distribution of SPM in estuaries. There is numerical evidence linking freshwater abstractions to an increased potential for up-estuary transport (Uncles et al., 2013). Nevertheless the effects of shifts in freshwater inflow on sediment regime are not yet fully understood (Mitchell and Uncles, 2013). The longitudinal TMZ migration as a result of seasonal variability of runoff has been well described in many estuaries (Grabemann et al., 1997; Uncles et al., 1998; Guézennec et al., 1999). However, the effect of floods or long-term hydrological variability on sediment dynamics is scarcely documented – Grabemann and Krause (2001) showed differences in SPM concentrations of the TMZ in the Weser Estuary between a dry and a wet year, although the gaps in data hamper a detailed analysis. The transitional periods of upstream migration and downstream flushing of the TMZ and of its associated mobile mud in fluvial sections have also not been detailed. These limitations are partly due to the absence of relevant long-term data sets, which are not so common in estuaries (Garel et al., 2009; Contreras and Polo, 2012).
The Gironde fluvio-estuarine system (SW France) is unique in having a long-term and high-frequency monitoring of water quality. This estuary presents a pronounced TMZ well documented in the lower and central reaches (Allen and Castaing, 1973; Allen et al., 1980; Sottolichio and Castaing, 1999). The Gironde watershed has the largest water structural deficit in France (Mazzega et al., 2014). Warming climate over the basin induces a decrease in mean annual runoff, a shift to earlier snow melting in mountainous areas and more severe low-flow conditions (Hendrickx and Sauquet, 2013). In addition, according to data of the agricultural census, irrigated areas have duplicated its surface in several regions of the watershed between 1988 and 2000, promoting strong water storage and abstractions. This context makes the Gironde estuary a good example to evaluate how changes in freshwater regime may affect the estuarine particle dynamic.
The goal of this work is to analyse the response of fine sediments to hydrological fluctuations, based on a 10-year high-frequency database of turbidity in the fluvial Gironde estuary, in order to do:
document the trends of SPM at all representative timescales, from intratidal to interannual variability; analyse the role of floods on the sedimentary dynamic of the tidal rivers; analyse the influence of hydrological conditions on TMZ features
(upstream migration, downstream flushing, concentration, persistence); discuss the effect of the long-term decrease of runoff in the upstream intensification of the TMZ.
With a total surface area of 635 km
The tidal asymmetry toward upstream and the subsequent tidal pumping coupled
to density residual circulation develop a turbidity maximum zone (TMZ). The
high tidal ranges and the great length of the estuary promote a highly
turbid TMZ (Uncles et al., 2002). In surface waters of the middle estuary,
SPM concentrations range between 0.1 and 10 g L
The Gironde fluvial-estuarine system:
In contrast to the middle estuary, the tidal Garonne and Dordogne Rivers are still poorly documented. Measurements over a maximum of 3 days (Romaña 1983; Castaing et al. 2006) and satellite images (Doxaran et al., 2009) revealed the seasonal presence of the TMZ during the summer–autumn period. Brief field observations in September 2010 (Chanson et al, 2011) showed the presence of low consolidated mud deposits upstream of Bordeaux. In the following, and in accordance with Uncles et al. (2006), the term mobile mud is used for these low consolidated mud deposits that are easily erodible, and likely to shift seasonally with the TMZ.
The Gironde estuary counts on an automated continuous monitoring network,
called MAGEST (MArel Gironde ESTuary), to address the current and future
estuarine water quality. The MAGEST network includes four sites (Fig. 1):
Pauillac in the central estuary (52 km from the mouth); Libourne in the
Dordogne tidal river (115 km from the mouth); and Bordeaux and Portets in
the Garonne tidal river (100 and 140 km from the mouth respectively). The
automated stations record dissolved oxygen, temperature, turbidity and
salinity every ten minutes at 1 m below the surface. In addition, an
ultrasonic level controller measures the water depth in the stations of
Bordeaux, Portets and Libourne. The turbidity sensor (Endress and Hauser,
CUS31-W2A) measures values between 0 and 9999 NTU with a precision of
10 %. The saturation value (9999 NTU) of turbidity sensor corresponds to
about 6 g L
The first implemented station was Pauillac on 15 June 2004. Acquisition at Portets and Libourne stations began on 16 November 2004 and at Bordeaux station on 1 March 2005. Operation of Portets station was stopped on 11 January 2012. The severe environmental conditions, electrical/mechanical failures and sensor malfunctions could cause missing or wrong data. Therefore the database needed a cleaning for erroneous values in turbidity. For example, 9999 NTU corresponds to saturation values, but also to sensor errors, and these latter need to be removed. A routine in Matlab was developed to retain only turbidity values corresponding to saturation. The validated database of turbidity corresponds to 1 223 486 data points recorded between 2005 and mid-2014. This corresponds to a rate of correct operating of 71, 70, 70 and 57 % for Bordeaux, Portets, Libourne and Pauillac stations respectively.
In addition, two tide gauges, managed by the port of Bordeaux (Grand Port
Maritime de Bordeaux), record tide height at Pauillac and Bordeaux every 5 min.
Hydrometric stations record every 1 to 24 h discharges of the
Dordogne River (Pessac; Lamonzie Saint Martin) and of the Garonne River (La
Réole; Tonneins) (Fig. 1). Data are available on the national website:
Turbidity was analysed as a function of river flow and water height at
different timescales. To better identify intertidal trends, we calculated
tidal-averaged turbidity with its corresponding tidal range. In order to
avoid biased averaged values, we only consider the tidal averages
corresponding to at least 70 % of measured values for the considered
period of time. Since management directives are often based on daily values,
tidal and daily averages were compared. Figure 2 compares both turbidity
averages and shows a very good agreement between the two calculations
(
We performed statistical analysis on the tidal-averaged data. We compared
turbidity values according to stations (Portets, Bordeaux, Libourne and
Pauillac), period (months, and tidal range), and their interactions (e.g.
station within period), by performing analysis of variance. We used
parametric tests (
Comparison of tidally averaged turbidity and daily averaged
turbidity for Bordeaux station showing the correlation coefficient
(
The Gironde estuary drains a watershed of 81 000 km
Tides are semidiurnal (the main harmonic component is the M
Figure 4 presents examples of high-frequency (10 min) data recorded at Bordeaux under two contrasted conditions of fluvial discharge. Continuous measurements reveal turbidity patterns related to tidal cycles, and to changes in fluvial discharges. Only such a continuous record can capture the turbidity signature of a flood that often occurs for a few hours.
The first selected data set (Fig. 4, column I) corresponds to a low-water period: the
Garonne discharge was below 120 m
The second selected data set (Fig. 4, column II) represents the turbidity signal
related to a spring flood with a discharge peak of the Garonne River at
1730 m
Table 1 collects maximum discharge value and its associated maximum turbidity (when recorded) of each flood event at Bordeaux and Portets stations. Flood events are identified in the time series of river discharge in Fig. 3a. The associated turbidity peaks were calculated as the maximum of the turbidity values at low tide (fluvial signature) in order to consider only the sediments transported by river flow. As shown in Table 1, turbidity maxima during flood events are 5 to 30 times lower compared to TMZ maximum values (50 % of the recorded floods present a maximum turbidity < 1000 NTU).
The 10-year time series of tidal averaged turbidity (Fig. 3) reveals short oscillations related to neap–spring tide cycles and seasonal trends induced by hydrology. Maximum turbidity values are recorded during spring tides, since higher current velocities favour the resuspension of sediments (Allen et al., 1980). The highest turbidities occur during low discharge periods (usually between July and November) in the up-estuary waters (Fig. 3d–f) due to the upstream displacement of the TMZ. Turbidity is usually minimal in spring after the flood period. In the middle estuary (Fig. 3c) seasonal changes are more moderate and show an inverse trend. This is due to the existence of a permanent TMZ in this estuarine zone, which is possibly related to a mud-trapping zone (Sottolichio and Castaing, 1999).
Discharge and turbidity characteristics of flood events for the period 2005 to mid-2014 in the tidal Garonne River (Bordeaux and Portets stations). Flood event are numbered by f plus a number according to Fig. 3. Hysteresis loops are classified as: [C] clockwise; [CC] counterclockwise; [M] mixed; [No] no trend. Mixed loops with a clear clockwise [M(C)] or counterclockwise [M(CC)] predominance are specified. Flood without turbidity record are included to facilitate the interpretation of the hysteresis succession.
Examples of 48 h raw data of
Mean (red cross), median (red bars), percentiles 25–75
(blue bars) and minimum–maximum (black bars) values of tidally averaged
turbidity depending on the season (months of February and August) and the
tidal range (TR) in each MAGEST station. The minimal, mean and maximal
values of river flow in February (2005–2014) are 176, 566 and 2994 m
Duration of the TMZ presence per year at the three tidal rivers stations. Striped bars designate the duration of the TMZ when it appears in winter: 17, 18, 9 and 39 days respectively in the years 2005, 2008, 2011 and 2012 at Bordeaux; 6 days in the year 2012 at Libourne.
Figure 5 summarizes the main characteristics (mean, percentiles) of
turbidity to compare the four stations during high (February) and low
(August) river discharges and tidal ranges. In the fluvial stations
(Bordeaux, Portets, Libourne), turbidity in August is significantly
(
The observation of the entire data set of tidally averaged turbidity
evidences a strong interannual variability in SPM in the fluvial Gironde
estuary. Figure 3 makes it possible to appreciate marked differences in the
concentration and in the duration of the TMZ for the monitored years at
Bordeaux, Portets and Libourne. The maximum turbidity values exceeded 7200
NTU in the years 2010, 2011 and 2012 at Bordeaux and in the years 2010 and
2012 at Libourne. By contrast, during the year 2008 tidal-averaged turbidity
was always below 6700 and 4400 NTU at Bordeaux and Libourne respectively.
Portets station is less documented: tidal-averaged turbidity maxima ranged
between 4730 and 6880 NTU (years 2009 and 2006, respectively). The durations
of the TMZ occurrence (Duration
The presence of TMZ (duration, turbidity level, hibernal occurrence) is more marked and better documented in Bordeaux waters. The following discussion is dedicated to the tidal Garonne.
Relationship between Garonne discharge and turbidity at Bordeaux, and corresponding hysteresis patterns for the successive floods occurring since the downstream flushing, in December 2012, and the following upstream migration, in August 2013, of the TMZ; f plus a number refers to the flood events according to Fig. 3 and Table 1.
Schematic representation of suspended sediment dynamics in tidal rivers associated with the different types of hysteresis (clockwise, mixed and counterclockwise) during river floods.
Tidally averaged turbidity
Turbidity as a function of tidal range (2-day running
averages) for three neap–spring–neap cycles (see the cycles in Fig. 3d)
during a period of
River floods expel the TMZ (and its associated mobile mud) from fluvial to
middle estuary as shown in Fig. 3. Mitchell et al. (2012) related this
downstream flushing to a lack of settling at high slack water during high
river discharge. According to Castaing and Allen (1981), the repetition of
strong flood events, along with spring tides, favours the flushing of a part
of the TMZ toward the sea. Floods also transport eroded sediments from the
watershed that contribute to the TMZ. Identifying both processes is
important to discuss the role of floods on the sedimentary budget of tidal
rivers. The literature proposes hysteresis-based analysis to search for
specific patterns of sediment transport in rivers (Williams, 1989; Klein,
1984; López-Tarazón et al., 2009). The relative position of sediment
sources within the catchment is analysed through the flow sediment
hysteresis shapes (clockwise or counterclockwise). In short, counterclockwise
loops correspond to a transport of sediments from upstream distant sources,
while clockwise loops occur when the sediment source is the channel itself
or adjacent areas. Based on the MAGEST turbidity database, flow sediment
hysteresis shapes were systematically analysed for the 26 floods recorded at
Bordeaux (13 at Portets; Table 1). Only the values at low tide were used to
trace the loops in order to preserve the fluvial signal and to avoid the
impact of local resuspension by tidal currents on the levels of turbidity.
The succession of hysteresis shapes over several years follows a seasonal
pattern in the Garonne tidal river (Table 1, illustrated for the year 2013
in Fig. 7). In the case of Bordeaux:
The first floods that occur at the end of the low discharge period
and expel the TMZ down-estuary show clockwise (C) hysteresis loops (e.g.
f3, f8, f11, f24, Table 1; f24 in Fig. 7). This indicates the advection
of resuspended sediments from the close bed and banks. When the TMZ is
present in the fluvial section, there is an accretion of sediments that
remain after the TMZ downstream flushing. This mud is eroded by river flood. Winter and some early spring floods present mixed (M) hysteresis
curves, i.e. clockwise loops with a counterclockwise loop around the
flood peak (f25 in Fig. 7). Some events show a predominance of the
clockwise loop (M(C), e.g. floods f1, f4, f17, Table 1), or of the
counterclockwise loop (M(CC), e.g. floods f15, f18, Table 1). This
pattern suggests the presence of local sediments, probably remaining from a previous TMZ period, and also the transport of sediment from
remote areas. The predominant loop could be interpreted in terms of the
proportion of each sediment source. Spring floods follow counterclockwise (CC) hysteresis patterns
(e.g. floods f2, f7, f10, f28, Table 1; f26, f27, f28 in Fig. 7).
This means that sediments are mainly transported from upstream
areas; the TMZ-derived mud is expected to be totally expelled.
A similar seasonal evolution of hysteresis also exists at Portets, but it is
subtler probably due to its upstream position: the flow sediment
curves of the first floods are mixed and counterclockwise loops already
appear in winter (Table 1). For example, the flood f1 (31 January 2006) presented
a mixed, but predominantly C, loop at Bordeaux indicating dominant local
sediments, whereas the simultaneous CC loop at Portets traced a distant
origin of sediments. The TMZ-originated mud is less present locally and more
quickly expelled in the uppermost section.
Therefore, hysteresis curves are indicators of the presence of mobile mud in tidal rivers, as schematized in Fig. 8, and allow us to discuss its rhythm of downstream flushing for different hydrological conditions and positions along the tidal river axis. During the wet years 2008 and 2009 the mud disappeared from Portets and Bordeaux in the beginning of winter with the first floods. In contrast, mud was only expelled in May during the dry years 2007 and 2012. In the case of the period from January to May 2010, the observation of mixed patterns shows that mobile mud was not completely flushed out (Table 1): this is explained by the absence of major floods until the following upstream migration of the TMZ.
Examples of clockwise discharge/turbidity hysteresis curves during the transition periods of installation and expulsion of the TMZ (see these periods in Fig. 3).
Mean (red cross), median (red bars), 25–75th percentile
(blue bars) and 9–91st percentile (black bars) values of tidally averaged
turbidity per 30 m
Duration of the TMZ presence as a function of the number
of days per year where the river flow was below 250 m
This first detailed study of 10-year continuous turbidity records suggests that deposition of mobile mud also occurs in the tidal Gironde, as already reported in the central estuary (Allen, 1971; Sottolichio and Castaing, 1999). Two-thirds of the floods from 2005 to mid-2014 contributed to the progressive downstream flushing of mobile mud from Bordeaux. As turbidity values associated with floods are significantly lower than those in the TMZ, this demonstrates that floods play a more important role in flushing sediment downstream than in increasing the TMZ concentration.
The prediction of TMZ location is nowadays a need in the fluvial Gironde
estuary and of particular interest to improve regional sediment management.
The present work, based on turbidity measurements over the last 10 years,
reveals a seasonal occurrence of the TMZ at Portets, 40 km upstream of
Bordeaux. The position of the TMZ along the longitudinal axis depends mainly
on the freshwater inflow in major macrotidal European estuaries (e.g. Weser,
Seine, Scheldt, Humber, see Mitchell, 2013). To better understand the
relationships between turbidity and river flow in the tidal Garonne River,
Fig. 9 shows the tidally (A) and daily (B) averaged turbidity as a
function of river flow (3-day average). In Pauillac (central estuary) the
dependence on river flow is the weakest: turbidity is slightly lower when
the TMZ elongates to the upper reaches, but also when floods push suspended
sediments seaward. In the tidal Garonne River, turbidity increases with
decreasing river flow for discharges lower than about 1000 and
600 m
Determining a precise discharge threshold of the TMZ installation per
station is tricky, due to the large variability in turbidity, more than 1
order of magnitude at 200 m
Turbidity maxima of the TMZ as a function of the water
volume passed:
Evolution of the duration of low discharge period
(Duration
Differences in turbidity between the periods of decreasing and increasing
river flow are also notable in the fluvial estuary (Fig. 9b). In the tidal
Garonne, for same discharge intensity, the smallest turbidity values are
always associated with the TMZ installation (decreasing discharge) and the
highest values during the TMZ expulsion (increasing discharge). This
indicates that the discharge turbidity curve follows a clockwise hysteresis
over the transitional periods of installation and expulsion of the TMZ (Fig. 11).
For example, for a river flow of 500 m
A distinction in turbidity values corresponding to the periods of TMZ
installation or expulsion is then necessary to identify the discharge
threshold of the TMZ installation in tidal rivers. Figure 12 summarizes the
distribution of turbidity values as a function of river flow (intervals of
30 m
In the absence of historical turbidity data in tidal rivers, it is difficult
to judge the evolution of the TMZ. There are only a few limited available
data sets, issued from field campaigns. For example in September 1960, SPM
concentrations of surface waters at Bordeaux range between 1 g L
The 10-year data set of the MAGEST stations of Bordeaux and Portets was used
to evaluate the impact of hydrological conditions on TMZ (turbidity level
and persistence) in the tidal Garonne. The annual maximum turbidity value
(Turbidity Duration Vol Vol
The Duration
There is also a good correlation between Turbidity
In summary, the duration of the low discharge period mainly determines the
TMZ duration, and the freshwater volume during high discharge periods the
TMZ concentration. High river flows are efficient in flushing the TMZ in the
central estuary, even to the coastal waters, and expel higher quantity of
mobile mud, as seen in Sect. 5.2. In order to discuss the potential
evolution of the TMZ in the last decades, we calculated the Duration
According to recent streamflow simulations from 1976 to 2100 based on 22 European river basins, including the Garonne watershed, average discharges are projected to decrease in southern Europe, and extreme events to increase (Alfieri et al., 2015). In this context, the finding of straightforward river-discharge-based indicators of TMZ behaviour should be of great interest for future river basin management plans in the fluvial Garonne.
The effect of river discharge is assumed to be a major factor in the longitudinal shift of the TMZ. However, morphological changes (natural or anthropogenic) may also contribute to the TMZ intensification (Winterwerp and Wang, 2013; De-Jonge et al., 2014), by amplifying tidal asymmetry and hence enhancing trapping of fine sediments, as suggested by Sottolichio et al. (2011). The existence and importance of these changes is not documented yet and will be the subject of future research. The combined effect of changes in topography and in river flow on the TMZ evolution needs be analysed by numerical modelling.
The high-frequency and long-term turbidity monitoring provides detailed
information on suspended sediment dynamics in the fluvial Gironde Estuary
over a wide range of timescales and hydrological conditions. Tide, river
flow and sediment stock (mobile mud patches) induce large variability on
turbidity levels. Suspended sediment dynamics related to tidal cycles
(semidiurnal and fortnightly) follows the same cyclic processes in the tidal
section, as previously described in the lower estuary (Allen et al., 1977).
The TMZ occurrence in the tidal rivers is very sensitive to changes in
hydrological conditions. River discharge is a key variable to explain the
upstream migration, downstream flushing and concentration of the TMZ and its
associated mobile mud. River discharge thresholds promoting the installation
and expulsion of the TMZ at Bordeaux have been delimited, 250 and at least
350 m
The extrapolation of hydrological conditions suggests an intensification of
the TMZ occurrence in the fluvial Gironde during the last decades and could
be used to evaluate future scenarios. This can be very useful to water
management strategies in order to address the global change impacts as
Garonne 2050 (
Finally, this work will be useful to improve the calibration of numerical models coupling hydrodynamics and suspended sediment transport. Numerical simulations will allow evaluate the turbidity in the upper estuary for different hydrological and climate scenarios (naturals and anthropogenic), including the effect of morphological changes.
I. Jalón-Rojas thanks the Agence de l'Eau Adour-Garonne (AEAG) and the Aquitane Region for the financial support of her PhD grant. The MAGEST network is financially supported by the following organizations: AEAG (Agence de l'Eau Adour-Garonne); SMIDDEST (Syndicat MIxte pour le Développement Durable de l'ESTuaire de la Gironde); SMEAG (Syndicat Mixte d'Etudes et d'Aménagement de la Garonne); EPIDOR (Etablissement Public Interdépartemental de la Dordogne); EDF; GPMB (Grand Port Maritime de Bordeaux); CUB (Communauté Urbaine de Bordeaux); Conseil Régional Aquitaine; CG-33 (Conseil Général de Gironde); Ifremer; CNRS; Université de Bordeaux. The authors also gratefully acknowledge the support of the OASU (Observatoire Aquitain des Sciences de l'Univers) through the SOLAQUI (Service d'Observation du Littoral AQUItain) programme. MAGEST is a contribution to the CNRS observation programme DYNALIT.Edited by: A. D. Reeves