Hydrodynamic simulation of anti-tsunami structures
Protection against tsunamis has always been important, but it is becoming an increasingly common topic given the growing population and land density of all coastal regions around the world. We know that these natural disasters can, through the effect of flooding and submersion, cause massive destruction and loss of life, and the design of structures to halt or reduce the impact of a tsunami wave is therefore crucial and urgent.
Coastal Areas at Risk of Tsunami
Tsunamis pose a major threat to areas in the seismically active regions found on the coasts of the Pacific, Atlantic and Indian Oceans. The presence of active seismic faults and underwater volcanic activity are the causes.
Japan, Indonesia, Chile and Alaska are notable for their history of devastating tsunamis. Japan, for example, is located on the Pacific Fire Belt, an area of high seismic activity. Indonesia is also frequently affected by underwater earthquakes that generate tsunamis.
The risks of tsunamis in the Mediterranean
The Mediterranean, although less known for its tsunamis than the Pacific, has a remarkable history of these events. Tsunamis have been recorded since antiquity, caused by underwater earthquakes or volcanic eruptions. For example, the eruption of Santorini in 1600 BC, near Crete, generated a devastating tsunami. The risk present in the western Mediterranean, on the Ligurian and Maghreb coasts is also worth noting.
Mediterranean tsunamis are generally of lower wave heights than those in the Pacific or Indian oceans, but the wave height can be several metres. Thus, the density of population and coastal infrastructure increases the potential for destruction and requires equally stringent protective measures.
Tsunami Wave Resistance Devices
Dams and dikes are commonly used to protect coastlines from tsunamis. Their design is based on hydrodynamic principles aimed at dissipating wave energy and reducing their impact by stopping the wave’s progression. They differ quite significantly from breakwaters developed for the fight against extreme swells, the reason being based mainly on physical characteristics quite different between these two phenomena, in particular the wave period.
The effectiveness of these structures depends mainly on their height and thickness. Given the relative rarity of these events, the statistical approach on wave height potential must absolutely be associated with an ultra-majored safety coefficient. Obviously this was not the case for the design of the Fukushima nuclear power plant wall, the wave having passed over.
Natural and man-made mitigation structures
Mangroves and coral reefs are natural barriers that can mitigate the impact of tsunamis. Mangroves, with their dense roots, absorb wave energy, while coral reefs break waves before they reach shore. Preserving these ecosystems is essential for protecting the coasts.
By extension, artificial reefs, enabled by recent innovations in materials and construction techniques, could provide additional protection to reduce the impact of tsunamis.
Numerical Simulation of a Tsunami and Hydrodynamic Efforts on the Structure
Numerical simulation is crucial to understanding and predicting the impacts of tsunamis on protection structures. It allows to anticipate the hydrodynamic forces and to optimize the design of the devices.
CFD simulation (Computational Fluid Dynamics)
CFD is used to model tsunami waves and allows very fine assessment of hydrodynamic stresses on existing or under development protection structures. It implements the Navier-Stokes equations applied to transient calculations on the air-water mixture.
A high requirement is implemented on the capture of phenomena at key points, especially at the interface between water and air but also close to the wall of the structures. Simulations are similar to physical experiments that can be found in wave channel-type installations, with the difference that it is much easier and much less expensive to generate a tsunami wave in simulation than in a channel.
The results of the simulations provide valuable data on hydrodynamic forces, turbulence, and flow velocity. They also provide access to the hydraulic coefficients of the structures, which allows a rational and scientific assessment of their effectiveness on the transmission, dissipation and reflection of the wave and energy of the tsunami.
Sizing and Design of Protective Structures
The design of tsunami protection structures must follow a rigorous and precise methodology respecting basic principles. It is often the result of interdisciplinary collaboration between engineers, designers and researchers. Each discipline brings unique expertise, allowing to create innovative and effective solutions. This emulation is essential to develop structures that will protect the coasts while being sustainable and economically acceptable.
Integration of simulation results into the final design
The results of numerical simulations are incorporated into the final design of the protective structures under certain conditions. They first have to demonstrate their reliability, particularly by correlating with cases of experimental tests. Then their contribution is magnified by a digital experiment plan to test different scenarios and predict the performance of structures under various wave conditions. Ultimately, the integration of these results ensures that structures are well-optimized to withstand the most powerful tsunamis.
