Hydrodynamique EXtreme du largE à la COte – HEXECO

Objectives : Nearly half the world population lives within 200 km of the coast, and this proportion is expected to reach 75% by 2025 (source: UNESCO). The littorals are also more intensively used by the shipping industry, growing with the globalization of world trade, mass tourism, and energy industries: oil, gas, and renewable energies. Infrastructures and human lives are lost on a regular basis to the fury of hydrodynamic forces unleashed by extreme weather events. These include exceptionally strong waves, and coastal flooding, with devastating effects. Thus the planning and management of human activities in the nearshore requires knowledge of the marine environment with a level of detail on a par with these growing social and economic stakes. This project aims at enhancing our knowledge of the hydrodynamics of extreme events, and translating that knowledge in a usable form for marine meteorological forecasting and coastal engineering. Today, the modelling of the hydrodynamics of extreme events is still insufficient. For example, the forecasting of storm surges is largely underestimated in large storms in the Mediterranean (source: Meteo France), leading to the underestimation of potential damage. As a result risk planning uses empirical laws that do not take into account the specifics of one event or site. The HEXECO project will lead to a deeper understanding of hydrodynamic processes and feedback to operational forecasting in France. The project methodology is centered on the loop than connects applications and basic research, with the constraints of operational models taken into account in the basic research, so that processes are translated into usable parameterizations. Tools applied in this research include in situ observation, theoretical and numerical modelling, and laboratory experimental validation. Challenges: The hydrodynamics of extreme events is translated into a chain of numerical models from offshore to the shoreline. Processes at play and the most approriate resolution methods change with deacreasing water depth. - offshore, in larger scale models, the wind forcing is poorly known and represented : high winds are not well observed, and the parameterizations of wind stress that are in use today are not appropriate and only correspond to the weaker winds to which they have been tuned. - In the coasal zone, higher resolution models are usually nested in the large scale models which provide boundary conditions. In extreme weather the shallow water effect on the wind stress is still poorly known and the likelihood of extreme waves in these conditions is not known except for direct observations. Further, the variation with depth of wave energy dissipation and bottom friction are not well known in spite of their importance for the wave-induced set up. This requires a better representation of wave nonlinearity and dispersion in the numerical models. The wave amplitude modulation on the scale of groups generates strong infragravity waves close to shore with large variations of the water level on the scale of a few minutes. These random effects are still poorly modeled. - In the nearshore, yet smaller scale models covering up to the swash zone may be used to investigate the run up on the shore. Recent studies suggest that the bottom friction is important, but generally underpredict observed set-up. The wind stress clearly plays a role, and the infragravity waves often dominate the run-up. An important challenge is to link these mean and random water level changes to the wave forcing, going beyond today's empirical run-up estimations from the significant wave height and period and the slope of the beach. Actions : We will address these challenges with a combination of observation and analysis techniques, numerical modelling, laboratory experiments and the development of parameterizations in simple models. Parameterizations will be developed in academic cases and tested in real situations against field data. In particular, several hurricanes and storms for which rich field data sets are available will be hindcasted, and the various factors influencing the storm surge will be examined in detail from offshore to the swash zone. The work will focus on the observation of high winds with satellite remote sensing, the formation of extreme waves in the coastal zone in the presence of strong wind forcing, and the parameterizations of the wind stress at high winds and wave-induced radiation stresses from detailed breaking wave kinematics. Finally, the HEXECO project will feed into operational results since project partners include most the agencies involved in a better understanding and prediction of extreme hydrodynamic events at sea, namely Météo-France, IFREMER, SHOM and CNRS.