Hydrodynamics of a hyper-tidal estuary influenced by the world's second largest tidal power station (Rance estuary, France)

Highlights

• Quantitative analysis of hydrodynamics and tidal patterns of Rance estuary, influenced by a tidal power station.

• In the absence of the plant, changes in bathymetry does not have significant influence on the estuarine hydrodynamics.

• The plant decreases tidal range, currents and tidal prism inside the estuary.

• The tidal power station increases the estuarine low-water level and elongates period of slack water and residence time.

• Tidal patterns analysis suggest that the tidal power plant does not impact the origin of sediments present inside the basin.

Abstract

The Rance estuary is a relatively small low-discharge steep-sided ria, located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5 m. Taking advantage of this hyper-tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth and has been in operation since the 1960s. Despite the well-known effect of damping of estuarine water levels, little attention has been given to quantifying the influence of the plant on the propagation and asymmetry of the tidal wave inside the estuary. In this study, hydrodynamics and tidal wave patterns were analyzed in this anthropogenically influenced estuarine system. A two-dimensional depth-averaged numerical model of the Rance estuary was developed. Two scenarios without the tidal power plant involving the dam's pre- and post-construction bathymetry (1957 and 2018 respectively) and present-day conditions scenarios were designed, to highlight the impact of bed evolution and the tidal power station on hydrodynamics and tidal asymmetry. Numerical results showed that, without the structure, bathymetric evolution did not substantially influence estuarine hydrodynamics. Nevertheless, on the estuary-side of the dam, the presence of the tidal power plant induced (i) a decrease in both tidal range and tidal prism, (ii) an increase of low water levels, and (iii) a decrease in both flood and ebb currents. Contrastingly, the region close to the structure reacted differently to plant operating modes, with an increase in flood currents (ebb currents) upstream of the sluice gates (downstream of the turbines). For both the natural condition and the artificially-induced hydrodynamic forcing due to the presence of the plant, numerical results showed that the Rance estuary mainly exhibits flood-dominant behavior, with a longer duration of falling than rising water and stronger peak flood currents than ebb currents. Spanning a period of approximately 60 years, this study presents a quantitative analysis of the influence of the tidal power station on the hydrodynamics in the Rance estuary, and its possible consequences for sediment dynamics. This approach is novel for this particular enclosed water body, characterized by the presence of a dam at its mouth and a lock at its uppermost limit.

Introduction

Hyper-tidal estuaries exhibit large tidal range (i.e., mean tidal range>6 m) and strong tidal currents, making them ideal for tidal renewable energy projects. Tidal energy is a form of hydro-power with potential as one of the future sources of renewable energy. However, a tidal power project can modify local hydrodynamics significantly, with impact on sediment dynamics, water quality and ecosystems (Xia et al., 2010; Cornett et al., 2013; Kirby and Retière, 2009). Therefore, understanding the impact on hydrodynamics induced by tidal projects is crucial for predicting possible environmental impacts.

In estuaries, hydrodynamic behavior is influenced by several factors (Stark et al., 2017a, 2017b; Thurman, 1994; Sumich, 1996): (i) the gravitational forces of the Moon and the Sun combined with the rotation of the Earth; (ii) the estuary's morphology, and (iii) the freshwater input discharge. In macro-tidal estuaries, hydrodynamics is mainly governed by tides which, have a profound impact on residual sediment dynamics and consequently on morphological evolution (Zhang et al., 2018). As examined by several authors (Aubrey and Speer, 1985; Speer and Aubrey, 1985; Friedrichs and Aubrey, 1988; Nidzieko and Ralston, 2012; Guo et al., 2018), tidal asymmetry plays an important role, causing residual sediment transport in estuarine systems (Wang et al., 1999; McLachlan et al., 2020; Mandal et al., 2020), and can be computed from flow velocity and water elevation (Friedrichs and Aubrey, 1988; Nidzieko and Ralston, 2012; Bolle et al., 2010). The former identifies the nature of the asymmetry: i.e., ebb- or flood-dominance in the estuary. The latter compares the durations of rising and falling tides. This indicates the predominant direction of residual transport of coarse sediment (gravel and sand) carried by bedload and of fine sediment (silt and clay) carried by suspension. Asymmetry in low and high slack water duration is also relevant to the net transport of the finer sediment fraction in the water column (Dronkers, 2005). A human intervention such as a dam located at a seaward boundary modifies the hydrodynamic regime and significantly alters non-linear tidal interactions (Aubrey and Speer, 1985; Speer and Aubrey, 1985; Vellinga et al., 2014; Hoitink et al., 2003), which can be relevant to sediment transport and accumulation in highly anthropized estuarine systems.

Located on the Brittany coast of northern France (Fig. 1a), the Rance estuary is a relatively small steep-sided, 20 km long ria (Evans and Prego, 2003). Its maximum perigean spring tidal range reaches 13.5 m at the mouth (Saint Servan, Fig. 1b). Taking advantage of this hyper-tidal regime, the first ever tidal power station in the world was built at the estuary mouth (Fig. 1b). The plant has been in operation and managed by Electricité de France (EDF) since 1966 and is currently the second largest operational tidal power station in the world (Pelc and Fujita, 2002). With a peak (mean) output capacity of 240 MW (57 MW), it supplies 0.12% of the power demand in France, which is equivalent to a medium-size city such as Rennes (c. 225,000 in habitants) (EDF, 2020).

Experimental and numerical studies were conducted, mainly focusing on qualitative analysis of sediment dynamics or ecosystem evolution in the estuary, without prior investigation of how hydrodynamics was influenced by the tidal barrier (Kirby and Retière, 2009; Bonnot-Courtois et al., 2002; Guesmia, 2001; Guesmia et al., 2001; Thiebot, 2008). Despite a well-known effect on estuarine water levels (Bonnot-Courtois et al., 2002), little attention has been given to quantifying the influence of the plant on the propagation, and vertical and horizontal tidal asymmetry of the tidal wave. The first numerical hydrodynamic model of the Rance estuary was developed in 2001 (Guesmia et al., 2001). It consisted in a two-dimensional (2D) model used to separately study the sea-side and estuary-side regions of the dam. The aim was to determine hydrodynamic parameters for morphological simulations (Thiebot, 2008). Although it provided good results with respect to measurements (Guesmia et al., 2001), the approach did not evaluate the influence of the power plant on flow characteristics and tidal asymmetry, which could have significant implications for sediment dynamics and morphological changes in the estuary. In 2018, 2D and 3D numerical models were developed, to evaluate bacteriological impact in the estuary (Chevé and Le Noc, 2018). The study area included both the basin and the offshore region. However, mesh resolution was constant over the computational domain, which was insufficient to capture flow structure close to the plant and between Mordreuc and Chatelier lock (Fig. 1b). The main conclusions of this study were based on the 2D model results, without any further hydrodynamic analysis.

Both 2D and 3D numerical models are used to assess hydrodynamic impacts of existing or planned tidal power plant projects. With the third highest tidal range in the world (15 m maximum in spring tide), the Severn estuary (United Kingdom) would be an optimal location for tidal power projects. A 2D numerical model was developed by (Xia et al., 2010) to estimate the impact of three renewable-energy projects: Cardiff–Weston, Fleming lagoon and Shoots dams. The basic dam operation regime adopted was ebb generation only. It was concluded that the Fleming Lagoon project would have little influence on hydrodynamic processes in the Severn estuary, however dam construction would have significant environmental impact (Xia et al., 2010). Young et al. developed a 2D hydrodynamic model to highlight the impact of the world's largest tidal power station, the Sihwa Lake tidal power plant in South Korea (Young et al., 2010). The tidal energy scheme of this plant is a single flood-generation mode. The study established that limiting water surface elevation would modify the estuary's ecosystem (Young et al., 2010). Another optimal location for a tidal power plant would be the Bay of Fundy, located on the Atlantic coast of North America, where tidal range can exceed 16 m during spring tides. 2D and 3D hydrodynamic models (Cornett et al., 2010, 2013) simulated a range of hypothetical development scenarios with three different operating modes: ebb generation only, flood generation only, and ebb-flood generation. It was concluded that operating mode had considerable influence on local velocities near the lagoon, and particularly near the powerhouse, but seemed to have little influence on the magnitude of far-field hydrodynamic impact (Cornett et al., 2010, 2013). One common feature of these studies was that they were all conducted ahead of plant construction, and consequently the impact assessments were only estimations.

The main objective of this paper is to analyze how the hydrodynamics of the Rance estuary is influenced by the world's second-largest tidal power station through basic flow characteristics and tidal asymmetry (Nidzieko and Ralston, 2012; Friedrichs and Aubrey, 1988). For this, a two-dimensional depth-averaged model was developed corresponding to both ebb generation and flood-ebb generation schemes (section 3). The numerical model was first calibrated and validated on measurement datasets (section 4.1) and then employed to assess present-day hydrodynamic conditions in the Rance estuary (section 4.2). Application of the numerical model on diverse scenarios involving past/present bed elevations and presence/absence of the dam analyzed the impact of the plant on flow patterns and tidal asymmetry (section 4.3). Finally, section 5 discusses the impact of the Rance tidal power station on hydrodynamic processes and its potential impact on sediment dynamics.