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MULTIWAVE Report Summary

Project ID: 290562
Funded under: FP7-IDEAS-ERC
Country: Ireland

Final Report Summary - MULTIWAVE (Multidisciplinary Studies of Extreme and Rogue Wave Phenomena)

MULTIWAVE was an interdisciplinary project at the frontiers of mathematics, physics and engineering that aimed to bring new insights into the processes driving the formation of giant rogue waves on the ocean’s surface. Given their destructive power, it may seem impossible to perform any systematic studies of rogue waves at all, but the key concept of MULTIWAVE was to use an analogy between waves on the ocean and waves of light to carry out experiments in an optics laboratory before transferring the results to the real-world environment of the ocean. The work was carried out in the frame of a co-investigator project between Professor Frederic Dias (UCD, Dublin) and Professor John Dudley (CNRS, France), and the project has seen a number of international partnerships develop with experts in optics and hydrodynamics from Russia, Australia, France and Finland.

MULTIWAVE resulted in a number of important theoretical and experimental developments. One fundamental development has been to place the analogy between nonlinear wave propagation in hydrodynamic and optical systems on a firm theoretical footing. In particular, although propagation of nonlinear waves is described by various forms of a complex model known as the “nonlinear Schrödinger equation,” we have now established firmly the particular parameter regimes for experiments in both hydrodynamics and in optics where this equation rigorously describes the emergence of analogous extreme rogue wave events, even when such events emerge from a noisy turbulent background sea. This work has been an essential step in allowing results in the experimentally model system of optics to be transferred to the more complex hydrodynamics environment.

As part of our studies of rogue wave events in Ireland we prepared the first map of rogue wave events that had been reported, from antiquity to the present day, and this map has been very widely distributed throughout Ireland. This has stimulated much interest from the public in coming forward with new records, and has even motivated other cataloguing efforts in other European countries to begin.

In optics, experiments were carried out studying light propagating in optical fiber, confirming the existence of particular wave growth and decay dynamics described by the nonlinear Schrödinger equation model. Work in optics has also applied novel experimental techniques to measure directly the instabilities that can occur during light propagation. These results in optics have led into the development of an extended model for water waves which has shown how nonlinear propagation effects in water waves can be important under some situations, especially during one dimensional propagation. The nonlinear water wave model has been confirmed experimentally by comparing its predictions with experiments studying changes in wave speed at the point of breaking. In addition, complete modelling of ocean waves including nonlinearity predicts that waves approaching the coast can increase their height six times more than expected based on propagation models that neglect nonlinearity. This work is expected to impact on coastal structure design.

On the other hand, work in both optics and hydrodynamics has confirmed without any ambiguity that although nonlinearity can amplify input noise to generate rogue wave events, a nonlinear process is not a necessary condition to observe rogue waves and linear mechanisms can play an equally important role. The precise contribution of linear and/or nonlinear effects depends on the particular environment under study. But when considering the most important objective of MULTIWAVE in attempting to gain new insights into real-world rogue waves on the ocean, perhaps our most important result has been to show that three examples of real-world ocean rogue wave data can be reproduced without including the third-order nonlinear process of modulation instability. Rather, the large wave amplitudes arise from the second-order nonlinear superposition of three dimensional random wave fields. This indicates clearly that any approach to forecasting rogue waves must be based on a full spatial analysis (amplitude and direction) of any given sea state.

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