1 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences Archimer Article In Press https://archimer.ifremer.fr Acceptation date : 2020 https://doi.org/10.1098/rsta.2019.0492 https://archimer.ifremer.fr/doc/00641/75286/ The Alderney Race: General hydrodynamic and particular features Bailly du Bois P 1, *, Dumas Franck 2, Morillon M 1, Furgerot L 3, Voiseux C 1, Poizot E 3, 4, Mear Y 3, 4, Bennis Anne-Claire 5 1 Laboratoire de Radioécologie de Cherbourg, IRSN-LRC, Rue Max Pol Fouchet B.P. 10, 50130 Cherbourg en Cotentin, France 2 Service hydrographique et océanographique de la marine – Shom, Brest, France 3 LUSAC, Laboratoire Universitaire des Sciences Appliquées de Cherbourg, Univ. Normandie, Cherbourgen- Cotentin, France 4 Conservatoire National des Arts et Métiers, Intechmer, Cherbourg-en-Cotentin, France 5 Normandie Univ., UNICAEN, CNRS, UNIROUEN, Morphodynamique Continentale et Côtière (M2C), Caen, France * Corresponding author : P. Bailly du Bois, email address : [email protected] Abstract : This study presents an overview of the main hydrodynamic features of the Alderney Race strait based on in-situ measurements and 2D hydrodynamic model simulations. The strait encompasses a large amplitude of tidal properties (tidal range and tidal wave propagation) and particularly strong currents exceeding 5 m.s-1 with associated counter currents and gyres. Variations in depth, sea bottom roughness, coastal topography and current orientation around the La Hague Cape provide access to a large variety of original hydrodynamic regimes. Some are revealed as locations with a 0.4 m drop in the mean sea level associated with strong average currents. A resonance effect associated with the offshore currents can also be observed close to the coasts. The "St Martin whistle" occurs in a bay whose gyre center oscillates with a reversal of the measured current every 5 – 7 minutes. The Alderney Race represents a particular area of interest for coastal hydrodynamic studies. The available in-situ measurement datasets are rich with recordings of: sea levels; Acoustic Doppler Current Profiler (ADCP) current profiles; surface radar currents; waves; dye experiments; surface and in-depth dissolved tracer surveys. Combined with hydrodynamic models, the complexity of this area can be further understood and knowledge of hydrodynamic process and forcing parameters can be refined, which can be applied to other coastal areas. Keywords : Alderney Race, La Hague Cape, Current, Tide, Hydrodynamics, Model Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. 2 1. Introduction The Alderney Race is located between the La Hague Cape and the Alderney Island at the centre of the English Channel, which is part of the western European continental shelf. The English Channel (Figure 1) extends longitudinally and is connected to the Atlantic Ocean to the west via the Celtic Sea between northern Brittany and Lands-End (177 km in width). Seawater flows eastward into the North Sea through the Dover strait (32 km in width). Depths do not exceed 200 m and decrease westward to eastward with depths lower than 50 m in the eastern English Channel and lower than 30 m in the Channel Islands area. Two narrow depressions are located in the centre of the English Channel, north-west and north of the La Hague Cape. They are mainly oriented west to east with maximum depths of 160 m and 110 m ("Hurd Deep" and "Fosse de La Hague" also called “La Hague deep”, respectively). The Alderney Race shows a 30 m deep rocky platform south of 49.71°N, with a marked step in the middle of the strait. This geological fault extends 4 – 8 km west of Goury with a step ranging from 30 to 60 m in depth (Figure 2, 40 m isobath in bold). The coasts are mostly rocky with coarse and sandy mobile sediments in the bays (Ecalgrain, St Martin) and locally finer sediments deposits in the Goury harbour. Details of the Alderney Race morphologic structures are given in [1]. The general circulation of the English Channel flows eastward where it enters into the North Sea through the Dover strait. The tidal waves follow the same orientation from the west to the east with a delay of roughly seven hours (between the western entrance at the longitude of Brest and the eastern boundary at Calais): they are compatible with the propagation characteristics of a progressive long wave. The Alderney Race is located north west of the Cotentin peninsula, which can be seen at the tidal wave scale as a major obstacle significantly reducing the north-south section of the Channel into which the tidal waves propagate. Moreover, the Alderney Race is located on the south side of the English Channel where the tidal range is larger than the north side. Both these points schematically explain the intense hydrodynamic of this area. This study aims to present the main hydrodynamic processes of the Alderney Race based on bathymetry, sea level variations, current, dispersion observations and hydrodynamic model simulations. This inventory accounts for the interest of the Alderney Race as a natural laboratory for coastal hydrodynamic studies. The introduction presents a short description of the situation and an overview of previous studies concerning the Alderney Race. The next section describes the methods applied to obtain in-situ measurements and simulations. The results section presents the particular features observed concerning the tide and associated sea level and currents. The discussion section includes possible ways to improve the understanding of the hydrodynamics in energetic regimes and how the Alderney Race may be valuable as a toolbox to study extreme coastal hydrodynamic processes. Lastly, examples of research perspectives are presented. The conclusion syntheses the main results obtained and the value of the Alderney Race in improving knowledge of hydrodynamic processes and forcing parameters that could be applied in other coastal areas. Phil. Trans. R. Soc. A. 3 Figure 1.Bathymetry map of the English Channel with the main locations cited. The yellow rectangle shows the study area (Figure 2) Figure 2.Bathymetry map of the Alderney Race and measurement locations. Phil. Trans. R. Soc. A. 4 1.1.Scale model Before the nuclear reprocessing plant at La Hague was set up, a scale model was built by the Institut de Mécanique de Grenoble (currently the Coriolis platform operated at UMR n°5519 CNRS-LEGI) in a rotating plate representative of the Channel. It has been applied in the investigation of the currents around the La Hague Cape and the study of dispersion of dissolved releases (dye experiments [2]). It has also been applied in the study of the tidal propagation at larger scale in the English Channel [3]. 1.2.Dye experiments Several dye experiments were performed in 1962 and 1963 before the nuclear reprocessing plant at La Hague was set up using rhodamine β at the planned outfall for radioactive releases. These experiments were designed to catch the dispersion of the dissolved releases over days to weeks following the releases [4]. Recent dye experiments were conducted south of the La Hague Cape (DISCO project) and are still underway. 1.3.Radiotracer dispersion The Orano nuclear reprocessing plant at La Hague has performed controlled releases of dissolved radionuclides since 1963 in the Alderney Race (a black star shows the location of the release outfall in Figure 2). Many studies have been performed to investigate the behaviour of these dissolved radionuclide releases in the past thirty years. They involve several time and space scales, ranging from minutes and kilometres following an individual release [5, 6] and up to years and thousands kilometres [7-10]. As the dispersion of dissolved releases is governed by hydrodynamics, the measurements of radionuclide concentrations and plume dispersion reveal the cumulated effects of the current and diffusion effects at different scales. The methods applied to sample and measure the dissolved radioactivity are described in [5, 10]. At short scales, tritium associated to seawater (HTO) is the main radiotracer measured and 10 ml seawater samples were sufficient to determine the tritium concentrations. More than 22,000 measurements are given available in [11]. These measurements enable horizontal and vertical maps of the plumes of individual releases to be plotted of dispersion around the La Hague Cape. 1.4.Drifters Drifters were deployed in 1962 and 1963 during pre-operation studies of the La Hague reprocessing plant [4]. In March 2007 two ARGOS drifters were released [5]; these drifters are composed of a surface buoy with a GPS receiver and a transmission link with the ARGOS constellation. The buoys are linked with a cable to a holey sock that is weighted at its base and immersed 15 m below the surface. Their drift is representative of currents in this layer. These two drifters were released at a distance of 100 m apart at the same time in the Alderney Race and then tracked every 30 min over 13 days. They were separated by a distance of 54 km from each other at the time of retrieval [5]. Phil. Trans. R. Soc. A. 5 2. Methods 2.1.Bathymetry Precise knowledge of the bathymetry is a prerequisite for most marine hydrodynamic studies. This is particularly the case in the Alderney Race as the bathymetry determines the frame of the “pipe” where the seawater flows. It explains the current variations at short and large scales. Most bathymetric data are provided by the French hydrographic and oceanographic service (Shom) [12]. We supplemented these data with measurements obtained during campaigns on board the Genavir Haliotis vessel. A compilation of all available measurements from Shom and IRSN is provided in [1].