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 distinctive characteristics of the natural environment and the constraints of architectural aesthetics
It is well known that in the Mediterranean the tides are small (the average tidal range is only 40 centimetres), but there are significant fluctuations in water levels. The movement of the waves puts strain on the structures built to protect the district (the backfill, the caissons). These forces can have an effect on the stability of the structures and can even damage them.
The strength, intensity and frequency of the waves are related to the effects of wind (blowing towards the coast), storms and atmospheric depressions that can raise sea level by several decimetres.
There are also swell patterns to consider. The swell is a non-breaking wave characterised by an undulating movement generated by a wind far from the observed area. Two swell patterns affect the Anse du Portier area, one from the east and one from the south.
In addition, the legendary ‘Ligurian current’ circulates in the area of construction, a current that can neither be ignored nor hindered, as it is useful for oxygenating and renewing the seabed.
A day of swell on the construction site. Photo © SAM L’Anse du Portier
In addition to these local conditions, there are the effects of global climate change accompanied by an inevitable general rise in water levels.
When designing and developing the project, the engineers in charge of hydraulic studies collaborated with URBAMER1 and used the pessimistic hypotheses formulated by the Intergovernmental Panel on Climate Change (IPCC2) as a basis for calculation.
They agree on a two-step sea-level rise of 70 cm by 2100. This increase is expected to be around 30 cm by 2060 and a further 40 cm over the following 40 years. This rise in sea level will be accompanied by an increase in violent or extreme weather conditions that will increase the risk of erosion and flooding.
The low promenade borders the entire district. It is 6 metres above sea level. Photo © Principauté de Monaco – SAM L’Anse du Portier – Renzo Piano Building Workshop – Valode & Pistre Architectes – Michel Desvigne Paysagiste
There are also significant architectural constraints. For its designers, Anse du Portier is “an urban maritime structure” which means that constructions such as the coastal promenade and waterfront buildings have been designed as close as possible to the water surface, precisely six metres above.
This configuration allows the newly created space to be harmoniously integrated into the natural environment. This is also why buildings and facilities on land are only a few metres away from the marine structures, designed to counteract the effects of waves.
This method is the opposite of what has been done in the past. Traditionally, the rule
applied in offshore construction involves placing these structures as high and as far away as possible, to limit the effects of the sea on the buildings. Here, it is the opposite: the choice has been made to place the various developments as close as possible to the Mediterranean, thanks to an adapted design of the maritime infrastructure.