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Seagrass restoration in the Berre lagoon (France): efficiency and optimisation of a wave absorption system, Laboratoire Hydraulique Saint-Venant LHSV, France
Context:   
The Berre Lagoon (France) is one of the largest Mediterranean lagoons (155 km²). It’s a shallow semi-confined ecosystem (mean depth 6.5 m) connected to the Mediterranean Sea via the Caronte channel. It receives freshwater from two main rivers and from Saint-Chamas EDF hydroelectric power plant. As a transitional system, there are many interactions between marine water, freshwater and meteorological forcings.  In the late 19th and early 20th centuries, the lagoon bottom was covered by very extensive Zostera sp.  meadows. These meadows perform numerous functions like stabilizing the sea bottom, providing food and habitats for other marine organisms. But, because of eutrophication, over-sedimentation, organic pollution through increasing agriculture and urbanization in river catchments, Zostera sp. meadows declined in the Berre Lagoon. Transplantation experiments have been achieved, with some positive but always limited results (survival, expansion). This could be due to strong hydrodynamic stresses (currents, waves): uprooting and damages on roots and leaves could prevent their expansion.  
Scientific work has already been achieved. Seagrass characteristics have been monitored in situ. Hydrodynamics and sedimentology in a bay of the lagoon has been studied during a PhD work at the CEREGE (European Center for Research and Teaching Geosciences and Environment, France) (Paquier, 2014).  A hydrodynamic numerical model (waves – currents coupled model considering effect of vegetation on hydrodynamics) has also been developed in a post-doctoral work at the LNHE-LHSV Lab (National Laboratory for Hydraulics and Environment EDF R&D and Hydraulics Laboratory Saint Venant, France) to better understand hydrodynamic processes involved.  As a result of these studies, temporary use of artificial seagrass mats could be considered as a soft engineering solution to reduce wave and induce favorable hydrodynamic conditions for natural seagrass expansion. The post-doctoral position is a preliminary step for a possible future mat (artificial seagrass) deployment as a demonstrator: it will assess the efficiency of such a system in real site conditions. The final objective of this work is to propose demonstrator specifications in terms of material, sizing, positioning, etc.   
Objectives:   
A numerical « waves – 3D currents » coupled model (TOMAWAC and TELEMAC-3D softwares) covering a Berre lagoon specific area exists and will be used during this research work. It calculates waves and currents characteristics resulting from hydrodynamic forcings and takes into account vegetation effect on hydrodynamics (implemented in the model by means of its geometrical characteristics and a drag coefficient). The proposed research work is both experimental (wave propagation experiments in a flume) and numerical (waves and 3D currents modelling). The 3 main tasks are:  
1. First step is based on numerical modelling. Wave generation and propagation in the studied area will be simulated for a period of about 3 years. Simulation results will be analyzed and the wave regime characterized in order to define the features of the waves for the flume experiments.    
2. Experimental part of the study will rely on LNHE physical modelling facilities and skills. First, the postdoctoral researcher will contribute to identification of some materials that can be used as artificial vegetal mats, depending on technical and environmental specifications. The candidate will also contribute to the definition of an experimental protocol (methodology, metrology…). Experiments in a wave flume will then be carried out with one or several types of artificial mats. The aim is to study their effect on waves attenuation depending on different parameters (materials geometric characteristics, water depth, wave characteristics…). Based on the measurements, the quantification of a “drag coefficient” will be used to estimate ability of the tested materials to attenuate waves.   
3. The numerical model will be used to compute hydrodynamics in the area of interest when considering artificial seagrass mats installed on the lagoon bottom. These mats are supposed to limit hydrodynamic stresses in some target areas in order to favor natural seagrass development. Numerical simulations will contribute to artificial seagrass mats sizing and positioning optimization and thus technical specifications for a possible in situ demonstrator deployment will be proposed.        
Conditions:   
12 months fixed-term job. PhD is required.   Skills: Hydrodynamics, numerical modelling. An experience in experimental field would be appreciated as well as some knowledge on Fortran programming.    
Laboratory:   
Laboratoire Hydraulique Saint-Venant LHSV 6, Quai Watier – 78400 Chatou  FRANCE 30 km west Paris  http://www.saint-venant-lab.fr/saint-venant   
Contact:  
Nathalie Durand (LNHE – EDF R&D) : nathalie-2.durand@edf.fr – (+33)1.30.87.77.50 
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