The work of hydraulic engineers, chapter 3

2019-09-01T13:44:13+00:00August 31, 2019|Monaco, News|

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 solutions applied

The layout of the belt composed of 17 caissons on the waterfront did not leave much freedom for its designers.

It follows the underwater current that flows through this area. Hence its sinuous and streamlined shape.

On the other hand, to dampen the force of the waves, absorb the power of the water, break the reflection of the wave and reinforce the stability of the structure, solutions had to be worked on to adapt to the specific conditions that characterise ‘100-year waves’.

The backfill was not designed as a mere vertical wall acting as an obstacle to waves. It is a bank with a base 50 metres deep and which extends upwards to -20 metres on a gradual slope. Its top is a plateau on which the caissons are placed and then permanently ballasted. The ballasting operation doubles their weight to about 20,000 tonnes per caisson.

Ballasted caissons. Photo © Bouygues TP MC

But the main device to counter the effect of the waves is located on the front side of the caissons.

7 metre high compartmentalised openings (3 metres of which are below sea level), are now covered with black shutters (see photo below), making the structure semi open. In the event of a conventional swell or a storm, the waves will rush through these openings and dissipate their energy in two successive chambers. This device known as the ‘Jarlan chamber’ is a proven technique.

It has been optimised for its operation in Monaco. Tests were conducted in a ‘wave flume’ to precisely calculate the size of the openings. These chambers, visible over the entire caisson belt (approximately 500 metres long) can absorb up to 45,000m3 of water with each wave. Mechanically, this water withdraws with the backwash.

The Jarlan chambers. Photo © SAM L’Anse du Portier

The caissons that emerge seven metres above the water surface have integrated wave-return walls. As their name suggests, these devices make it possible to stop the ascent of large waves and protect the seaside promenade.

This is a tried and tested technique. However, its final design has undergone several changes to optimise its effectiveness in the context of the Monaco offshore extension.

Wave-return walls. Photo © SAM L’Anse du Portier

Verifying the sizing through sophisticated
calculations and 3D tests in a tank

Complex hydraulic calculations have been carried out to understand and manage the interaction between the structures and sea power, both in calm and agitated periods. Basin tests were also necessary to simulate what the calculations could not represent.

Two-dimensional tests in a wave flume allow the reflection coefficients of the caissons and the wave overtopping capacities to be determined. These tests, carried out at the Oceanide laboratory in La Seyne-sur-Mer (France), made it possible to finalise the shape of the Jarlan chambers and the double wave energy dissipation chamber inside the caissons.

Hydraulic calculations based on different scenarios, carried out using the REFONDE software, determine possible agitation in the port d’animation and make it possible to set upsolutions to limit their effect.

Finally, what could not be calculated was measured through 3D tests carried out in Wallingford, England, in the largest hydraulic laboratory in Europe and one of the largest in the world for this type of simulation.

The REFONDE software makes it possible to visualise the stirring zones. In red, the most exposed parts. Photo © SAM L’Anse du Portier

The 3D tests have two objectives:

  • Checking the stability of the backfill and its breakwaters in the case of extreme swells.
  • Checking and measuring water overtoppings above the caissons and in the port d’animation in the event of an extreme storm.

The tests are carried out on a 1:60 scale model in a wave tank. A scanner allows what happens on the backfill to be observed.

The Wallingford Basin Test Centre is the largest in Europe. Photo © Bouygues TP MC

Water collection tanks are also positioned in specifically chosen locations. They measure the amount of water that passes in these particular conditions.

With these results, it is possible to establish what type of device should be used to guarantee the stability of the structure, such as the weight of the rockfill that protects the backfill (blocks of 1 to 3 tonnes).

Pressure sensors are also installed to measure the forces that will be applied to the structures.

Cameras placed all around the model record the different situations. The images offer the possibility to check the consistency between 3D models and digital models.