Formes de fond (bedforms) : cet axe de recherche porte sur l'étude de l'évolution des dunes sous-marines. La compréhension de l'évolution des dunes sous-marines est un enjeu important pour prédire la circulation des écoulements, les flux de sédiments et les variations bathymétriques dans les environnements sous-marins. Bien que les dunes de sable représentent un grand intérêt scientifique et opérationnel (ouvrages offshore, navigation..), leur évolution est encore mal connue du fait de leur comportement complexe. D'importantes questions ouvertes demeurent, en particulier concernant les mécanismes physiques conduisant à l'équilibre des dunes sous différents types de conditions hydrodynamiques, et les dimensions des dunes à l'équilibre. Différents paramètres semblent entrer en jeu, qu'il s'agisse du confinement de la colonne d'eau, de l'inertie du transport des sédiments en suspension ou encore de la nature des sédiments. Pour étudier l'effet de ces paramètres, nous proposons d'appliquer une méthodologie ambitieuse mais réaliste mêlant modélisation numérique, études en laboratoire et mesures in situ. Enfin, nous développerons un modèle numérique pour simuler l'évolution des dunes sous-marines, qui est un outil indispensable pour assister les autorités côtières et les acteurs internationaux du secteur de l'énergie. Voir les détails dans l'onglet consacré au projet DUNAMICS.

Proposal sent to the Marsden Funds comitee in New Zealand for restoration of native reefs in the Bay of Auckland (May2023)

Sea level rise (SLR) is accelerating, and extreme storm events will occur more frequently by 2050 under all IPCC RCP* scenarios[1]. Annual coastal flood damages will increase by 2–3 orders of magnitude globally[2], affecting property, infrastructure, and damaging cultural and environmental sites[3]. In New Zealand (NZ), SLR is increasing faster due to land subsidence, up to a factor two [4]. Wave climate is changing rapidly, threatening NZ shorelines stability[5]. In response, ecosystems are suffering a loss of species habitat, biodiversity and functioning[6,7]. Anthropogenic stressors are responsible of a sharp decline of shellfish reefs, such as those created by oysters and mussels[8,9,10]. The reef-building green-lipped mussel endemic to New Zealand was almost completely destroyed by 1980 and have since not recovered[11,12]. Shellfish restoration projects are nowadays hindered by a scattered knowledge and a lack of evidence-based best practice [13,14,15]. An improved understanding of drag parameterization on reefs would benefit coastal managers exploring reef restoration as a strategy to protect shorelines from erosion [16].

The aim of our research project is to test the hypothesis that complex reef geometry -internal porosity and resistance elements- have a significant influence on wave and flow momentum fluxes dissipation that commonly used bulk drag coefficients underestimate. The outcome is to increase knowledge about nature-based methods (NBM) to support their use. We will rely on a strong NZ based and international team of experts and incorporate the iwi vision mātauranga of native ecosystems.

'Grey' engineering solutions in response to protecting coastal communities are becoming economically and ecologically unsustainable[17]. NBM is the creation or restoration of coastal habitats for hazard risk reduction[18,19,20,21]. Restored reefs are effective in protecting the shoreline and stabilizing the seafloor, besides providing benefitsthat support marine life[22,23,24]. Their restoration enhances water purification, biodiversity and carbon sequestration[25]. Pilot scale studies validated the efficacy of using small-scale experiments to test habitat suitability and optimize location selection to maximize efficiency of larger-scale restoration efforts [26,27].

We will adopt a three step-innovative methodology. First, we will perform flume measurements at the Fluid dynamics laboratory (FDL) of the University of Auckland (UoA) to test reefs under various hydrodynamic conditions. Reefs and species will be pre-selected among shellfish restoration projects that UoA is currently running with two iwi Ngāti Whātua Ōrākei and Ngāti Manuhiri. We will derive specific drag coefficients from flow measurements above and within 3D printed prototypes, using high resolution cameras synchronized with lasers, followed by Particle Image Velocimetry (PIV) analysis[28]. Second, we will introduce derived parametrizations into a RANS** model (XBEACH) to study effects of reef configurations on water levels and wave energy taking into account the ecological niche of shellfish species[22]. Time series of wave parameters will be used in a shoreline evolution model (Shorefor) tosimulate impacts on short- and long-term coastal equilibrium. We will work closely with Ngāti Whātua Ōrākei and Ngati Manuhiri iwi to implant tailored reef structures offshore an urban beach of significance in Auckland. UoA has experience of running the consent process for pilot restoration studies[25]. The shoreline evolution and reef development will be continuously monitored through regular beach surveys, to measure the impacts in terms of ecosystem restoration and shoreline stability. By unveiling fundamental hydrodynamics within complex reef structures, our research will generate unprecedented knowledge on reef-flow interactions. Our findings will have the potential to reinvent the paradigm of NZ shoreline management practices by providing a nature-based toolbox for coastal planners and authorities based on ecological reef design to improve coastal resilience.

* Representative Concentration Pathway

** Reynolds-Averaged Navier-Stokes

References :

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