With the increasing global warming, extreme marine phenomena such as waves and storms could become more frequent and intense. The water level also rises and this is of major importance on the phenomenon of submersion. This sets a major challenge to the protection of coastal areas, where infrastructure must be designed to withstand increasingly harsh conditions. The importance of coastal protection against extreme swell will be considerable for the next decades, with the costs of submersion estimated at 240 billion euros per year in Europe by 2100.
Increased risk of extreme swell and submersion
Impact of global warming
Global warming is certainly causing sea and ocean levels to rise, as well as increasing precipitation, with an atmosphere warmer by 1°C containing 7% more humidity. In the presence of storms and hurricanes, the risks associated with extreme swells are therefore even greater. Sea levels are rising, rainfalls are increasing, so the impacts on coastal areas will be even more devastating in the future.
Risk of destruction and consequences
Coastal infrastructure is particularly vulnerable to extreme swell. Potential damage includes the destruction of buildings, port infrastructure and dykes. Landslides also contribute to the catastrophic dimension and, combined with submersion, radically affect the destruction of residential areas. Loss of property and disruption to local economic activities have lasting impacts. The economic and social impacts are therefore often very significant, with reconstruction and repair costs becoming prohibitive.
Using CFD (Computational Fluid Dynamics)
CFD (Computational Fluid Dynamics) is a numerical simulation technique used to analyze fluid flows. In the context of extreme swells, CFD models and predicts complex interactions between waves and coastal protection structures. Numerical simulation allows a good understanding of the forces generated by the waves and a the possibility to study a large quantity of parameters leading to an economically controlled and efficient technical development of the structures design.
Importance of a fine resolution of the Navier-Stokes equations
Use of the Navier-Stokes equations
The evaluation of the phenomena impact on the structures requires to be done at a fine scale for the interaction between the water agitated by the swell, its pressure and the friction forces. For this, the implementation of a numerical model describing the motion of the fluid, the geometry of the structure, the height of the waves and their period is necessary. Among the most accurate fluid models, the Navier-Stokes equations provide a very good approach to viscosity and pressure terms, while allowing the integration of air-water mixture and turbulence models adapted to the flow regime. In the context of extreme waves, these equations allow to accurately simulate the interactions between waves and coastal protection structures.
Importance of the precision in hydrodynamic simulations
The accuracy in hydrodynamic simulations is crucial for estimating the forces applied to the structures walls as well as for estimating the air-water exchanges at the interface where these two fluids mix. We know that a fine resolution of the Navier-Stokes equations allows us to capture the details of turbulence and recirculation phenomena, essential for understanding the impacts on structures. However, a fair compromise must be found between how fine the calculation details are, this directly related to the size of the smallest meshes where the equations are solved, and the cost of simulation as seen in the project context.
Cost-objective compromise of the simulation
A good economic approach to assess the performance of a hydraulic structure by numerical simulation of the Navier-Stokes equations can be described as follows:
- 2D ,two-dimensional, representation of a section of the structure and the swell, similar to the experiments conducted in wave channels.
- Scaling of geometry and phenomena to limit the size of the domain to be simulated.
- Correlation of academic cases or test data to ensure good simulation behaviour for different types of waves under unsteady conditions.
- Calculation of the forces on the structure and of its hydraulic transmission, reflection and dissipation coefficients.
This approach can be integrated into a digital experience plan with the aim of developing and designing the best technical and economic solution.
What solutions to overcome extreme swells?
Multiple and varied systems
A wide range of solutions exists, appropriate to the typology of the sea and to the phenomena locally encountered. It includes conventional solutions such as rock dikes, hydraulic piles, artificial reefs, more advanced solutions such as wave and currents attenuators equipped with Jarlan slots, and even more ingenious solutions such as removable dikes against marine submersions.
Thanks to CFD simulation, it is possible to push the limits of the design of the future protection structures by including the impacts of the future situations where the sea level will have risen. The determination of extreme stresses on structures will lead them to be dimensioned with the appropriate safety factor to guarantee longevity and safety over several decades.
