Appearances can be deceptive: although it seems to be delicately placed, as if it were weightless, a few metres above the Mediterranean, the eco-district of Anse du Portier is not floating! It is firmly anchored on an underwater structure. This construction is essentially composed of two elements: the backfill and the belt of caissons. These structures are located in the open sea and are therefore subject to significant physical constraints. These constraints affect both the marine space and the submerged structures.
We are going to look at the work of hydraulic engineers who develop solutions adapted to the special features of the project in order to make it possible to build the maritime infrastructure. Their choices were made using calculations, digital simulations and tests on models in both 2D and 3D. The solutions chosen define the geometry, dimensions and positions of the backfill and the caissons. They also made it possible to meet the safety and durability requirements of the structures and to respect the functional and aesthetic design defined by the architects. In the end, these studies have paved an original and unprecedented path for the development of sustainable offshore urbanisation.
The size of the maritime infrastructure is defined by the understanding of the environment
Innovative solutions had to be found to overcome this complex situation. Hydraulic engineers first assessed the average swells and extreme waves likely to reach Anse du Portier over a 100-year period. They also relied on the knowledge of marine weather specialists. The latter have acted as ‘sea historians’ to reconstruct the climate of the last 20 years. These reconstructions have been correlated with satellite data and contemporary data on swell behaviour.
From this body of knowledge emerged what can be called ‘100-year waves’, i.e. situations that occur only once a century. These ‘100-year waves’ are extreme phenomena that are characterised by wave height, wavelength and orientation of the expected bad weather.
To these results, hydraulic engineers added a value of 10% to reinforce the capacity of the structures to cope with meteorological events with which they could be confronted.
This covers, in the most extreme cases, waves that could reach up to 8 metres in height.
The marine infrastructure is mainly composed of the backfill and the belt of caissons. Image © Bouygues TP MC
It is based on the description of these possible extreme conditions that the structures comprising the maritime infrastructure have been designed. They meet three criteria.
Stability, because no variation in construction (backfill, caissons) is acceptable, nor slope, nor erosion, even under the effect of the worst storms.
Overtopping. When a wave hits a vertical wall, it tends to ascend. If the wall is not high enough, there is a real risk of the wave overtopping the wall. This can cause floods or even structural damage to buildings in the district and can be dangerous for pedestrians.
Reflection. If the force of the wave ‘bounces’ off the infrastructure, the wave spreads to neighbouring spaces. However, the new district is located between two marine reserves and near to Monaco’s Hercules Port to the south of the project, meaning that these reflections must be avoided.