Research papersTracking the turbidity maximum zone in the Loire Estuary (France) based on a long-term, high-resolution and high-frequency monitoring network
Introduction
Regions of high suspended sediment (SS) concentrations, named turbidity maximum zones (TMZ), are key features of tidal estuaries (Allen et al., 1980). The spatial and temporal evolution of the TMZ govern the transport and deposition of fine sediments (Uncles et al., 2006b) and hence may cause significant morphological changes, such as the siltation in channels and ports (Pontee et al., 2004). The TMZ also influences biochemical processes, such as particulate transport of nutrients and pollutants (Turner and Millward, 2002, Etcheber et al., 2007) and alter light and oxygen conditions (Talke et al., 2009, Lanoux et al., 2013). While the TMZ formation is governed by universal basic mechanisms (Dyer, 1988), its concentration and persistence vary from one estuary to another under the control of local tidal and river regimes, morphology, and type and availability of sediments (Grabemann et al., 1997; Uncles et al., 2006a; Garel et al., 2009; Mitchell et al., 2012).
Observational studies of the TMZ have usually been carried out for short periods of time or for specific regions of an estuary (Guézennec et al., 1999, Grabemann and Krause, 2001, Mitchell et al., 2003, Uncles et al., 2006a, French et al., 2008). The long-term tracking of the TMZ in an entire estuarine system is not very common, despite it provides worthwhile information to advance our understanding of sediment processes (Mitchell et al., 2012). More specifically, the tracking of the TMZ becomes essential for two reasons: (i) the temporal evolution of the TMZ is a factor that can help to explain long-term morphodynamic trends in estuaries, typically shifts in sedimentation zones, changes in SS concentration or general infilling; (ii) a good knowledge of TMZ geometry from field data is necessary to validate numerical models, which are increasingly widespread and used to simulate estuarine processes that couple sediment transport, morphodynamics and water quality at annual time scales.
Currently, two main techniques are used to track the TMZ: typically remote sensing and in situ long-term monitoring. Remote sensing is an efficient tool to characterize the spatial distribution of turbidity in surface waters along estuaries (Doxaran et al., 2009, Cai et al., 2015). However, despite recent improvements in algorithms and sensors (Gernez et al., 2015), temporal resolution remains limited. In addition, in situ measurements of SS concentrations are necessary for calibration of the satellite signal, and the quality of the image is highly dependent on atmospheric conditions. The use of in situ long-term and-high frequency monitoring has demonstrated its efficiency to address TMZ dynamics from semi-diurnal to multi-year time-scales (Jalón-Rojas et al., 2015). The spatial representativeness can be limited, depending on the number of monitoring stations. At present only few estuaries throughout the world utilize this technique, mainly due to the financial and practical constraints (Buchanan and Ruhl, 2000, Etcheber et al., 2011, Contreras and Polo, 2012).
Situated on the French Atlantic coast (Fig. 1), the Loire Estuary, extending 100 km from the mouth, is one of the three largest French estuaries. This macrotidal and highly turbid system plays the double role of an ecologically important wetland and an axis of economic development. During the last two centuries, continuous interventions of channeling and deepening have heavily modified the morphology of the Loire Estuary (Sogreah, 2006). These engineering works have favored tide amplification and flood-dominant conditions, upon which fine sediments are pumped more upstream, reducing the effective hydraulic drag (Winterwerp and Wang, 2013, Winterwerp et al., 2013). As a consequence, the estuary has evolved into a self-maintaining hyperturbid state, characterized by a highly-concentrated TMZ. Gallenne (1974) defined this TMZ as the region of the Loire where SS concentration exceeds 0.5 g L−1, and explained its basic mechanisms of formation due to gravitational residual circulation, even if tidal processes are also important and probably dominant, as shown later by a 1-D numerical model (Le Hir and Thouvenin, 1994). However, since Gallenne's thesis to the present-day, there has barely been any study about SS dynamics based on field observations. Based on 7Be budgets, Ciffroy et al. (2003) estimated the residence time of TMZ suspended sediments to be 6–10 months in summer, and about 0.7 month during flood periods. A sediment transport model based on the TELEMAC-3D system has been implemented for the Loire Estuary, but until now the applications (Cheviet et al., 2002, Walther et al., 2012) were focused on improving the simulation of basic physical processes, such as salinity gradients and bottom friction in the presence of mud. More recently, the SS distribution in the entire estuary has been estimated for a two day period through satellite data (Gernez et al., 2015). However, TMZ dynamics in the Loire Estuary are still not detailed despite the TMZ's environmental impact.
This study aims to describe and understand, for the first time, TMZ dynamics in the Loire Estuary over all the relevant time scales. This work is based on 7-year (2007–2013) records of turbidity from an automatic, high-frequency monitoring network called SYVEL (SYstème de Veille dans l'Estuaire de la Loire, Watch system in the Loire Estuary). Firstly, we present in detail the turbidity dataset and describe trends. Secondly, we discuss TMZ dynamics in terms of the TMZ's position, persistence, concentration, rhythms of upstream migration and downstream flushing, and inter-annual variability.
Section snippets
Material and methods
The Loire Estuary relies on the six monitoring stations of the SYVEL network distributed from the mouth to 62 km upstream, near the limit of salinity influence (Fig. 1), in order to assess water quality (http://www.loire-estuaire.org). The stations were implemented in 2007, except for the station of Donges that was added in December 2010. Operation of the Cordemais station was stopped in December 2011. Each station measures four parameters at 1 m below the surface: dissolved oxygen, salinity,
Hydrological conditions between 2007 and 2014
The Loire Estuary drains the longest river in France (1012 km), with a drainage basin of 118,000 km2, covering about one fifth of the French territory (Benyoucef et al., 2014). The Loire hydrological regime is pluvial with a high seasonal variability of discharge. The mean daily river flow over the study period (2007–2013, Fig. 2A) was 775 m3 s−1, with considerable variation ranging from 103 to 3980 m3 s−1. The annual-average discharge ranges from 425 m3 s−1 (year 2011) to 1128 m3 s−1 (year 2013), with a
Upstream migration and downstream flushing of the TMZ
The dependence of the position of the TMZ on river discharge is well known in macrotidal estuaries (e.g. Allen et al., 1980; Uncles et al., 1998). However, only good-quality data collected for a number of years ensures the establishment of a reliable local relationship of turbidity against the whole range of possible discharges in a given estuary (Mitchell et al., 2012). The present turbidity dataset in the Loire Estuary allows an accurate tracking of the TMZ and its variability. Fig. 2D–F
Conclusions
Turbidity was measured over a 7 year period (2007–2013) in the Loire Estuary using a continuous monitoring network (SYVEL) formed of six stations distributed between the mouth and 62 km upstream. The analysis of turbidity data series together with water level and river flow records detailed the behavior of the TMZ. River discharge controls the position changes of the TMZ. During low river discharges, the TMZ migrates upstream over an average distance of 40 km, and its upstream boundary may reach
Acknowledgments
I. Jalón-Rojas thanks the Agence de l’Eau Adour-Garonne (AEAG) and the Aquitane Region for the financial support of her PhD grant. Authors acknowledge the Groupement d’intérêt publique (GIP) Loire Estuary for providing the SYVEL data. We wish to warmly thank the two anonymous reviewers for their pertinent comments and concrete contributions that helped improve the manuscript.
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