Abstract
Understanding the hydrodynamics of warm-water coral reefs and their lagoons is crucial for ecosystems and coastal populations in the context of global change. As coral reefs are particularly exposed to global change, long-term prediction of their hydrodynamics remains difficult. However, the long-term trajectories of the hydrodynamics of these systems (currents, waves, mean sea levels, etc.) remain difficult to predict. Furthermore, the existence of sudden disruptions to hydrodynamic trajectories (cyclones, coral bleaching episodes) calls into question the legitimacy of studies based on scenarios that exclude all accidents. Alternatively, since the 1970s, resilience has been defined by Holling (1973) as the capacity of a system to absorb changes without shifting to a different state. Although this concept has become an important key for representing the possible futures of disturbed natural systems, it has never been applied to coastal hydrodynamics, in particular to address the question of the hydrodynamic trajectories of reef-lagoon systems (including atolls). This thesis thus asks: ‘To what extent is the notion of resilience applicable to hydrodynamics to discuss the future of atolls?’To answer this question, this thesis focuses on a type of reef-lagoon system that has been little studied, which is representative of small quasi-circular and microtidal reef-lagoon systems, observable for example in Fiji, inside the Great Barrier Reef or in the Tuamotu archipelago. A typical simplified atoll (defined on the basis of around fifteen parameters) is defined in the manner of an experimental benchmark in a wave basin. The hydrodynamic behaviour of this simplified system is then studied using a large number of simulations carried out with a coupled wave-currents numerical model.The analysis of hydrodynamic simulations allows for discussion of the hydrodynamic behaviours of these atolls, especially concerning the open or closed behaviour with respect to ocean waters. A reduced list of parameters responsible for the major hydrodynamic changes is then isolated. Using these parameters to define long-term trajectories of existing, though simplified, atolls highlights significant hydrodynamic changes that can disturb ecosystems or local populations. Finally, the extensive exploration of these parameters reveals a first link between reef hydrodynamics and the notion of a “resilience landscape”, which is the heart of the original definition of the resilience concept.