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'Stony dune with block dam'

The hard seawall to the north of Maasvlakte 2 will provide protection along a 3.5-km stretch of land. This seawall has an innovative concept. The design is called a 'stony dune with block dam' and is unique in the Netherlands. The core of this seawall consists of sand. On the seaward side, the contractor will deposit fist-sized rocks with an average diameter of 8 cm. A total of 7 million tonnes of quarrystone is required, in a variety of sizes. Some 2 million tonnes of this rock will be recycled from the existing block dam. 5 million tonnes will be transported in from Norwegian quarries. At the foot of the structure, in the sea itself, the contractor will position 40-tonne concrete blocks that measure 2.5 by 2.5 by 2.5 metres. A unique crane, known as the Blockbuster, was developed for this specialist job.

Points of departure

The design of the hard seawall is a Dutch first. It was developed by the building consortium PUMA (Boskalis and Van Oord) on the basis of a sustainable cost approach: when determining the design, PUMA not only took the costs of construction into account, but also those of 50 years of maintenance. The structure of the total seawall has been designed to withstand a super-storm that occurs only once every 10,000 years and follows the philosophy ‘soft where soft is possible and hard where hard is required'. This is in consideration of the relatively high costs of a hard seawall.

Maritime safety

The location and the shape of the hard seawall have been specifically designed to avoid negative effects on the current in the port entrance. This will prevent unsafe situations from developing for the shipping traffic. In fact, the current and wave patterns in the port entrance will actually improve as a result of the seawall's development.

Model tests

To verify the stability of the hard seawall in everyday and extreme circumstances (water level and wave attack), researchers have conducted extensive model tests in various hydraulics laboratories in Europe. The results of these tests were used to optimise the seawall further and further, in order to keep total construction and maintenance costs as low as possible and at the same time guarantee safety.

1. Sand (yellow) The core of the hard seawall consists of different layers of sand. The designers decided to include a layer of relatively coarse sand under the cobbles (minimum grain: 370 mu), which prevents the finer sand deeper in the seawall (approximately 150 mu) from washing away.

2. Gravel (brown) It is also important to shut in the sand: which is why the sand under the block dam is covered by a layer of gravel (3-35 mm).

3. Cobbles (orange) The 'stony dune', i.e. the seawall's cobble beach, will soon consist of an approximately 4-m thick layer of cobbles with a diameter ranging from 20 to 135 mm. This cobble beach is more dynamic than a standard seawall because the cobbles move with the current and the waves. But to keep deformation within bounds, the cobble beach should not be too steep. That is why it was decided to construct a relatively gentle slope of approximately 1:7.5. This slope has more or less the same incline as the slope created by a heavy storm. The ‘stony dune' will only require limited maintenance thanks to the construction of a breakwater of concrete blocks (5).

4. Quarrystone (blue and dark yellow) The different layers of rock on the cobbles under the concrete blocks have been designed in such a way that each layer is shut in by the layer on top of it. If this were not the case, material would be washed away by the waves, which would ultimately result in the collapse of the block dam. Under the concrete blocks (5), one can find a layer of quarrystone with rocks of 150-800 kg (blue) and 5-70 kg (dark yellow).

5. Concrete blocks (grey) Approximately 20,000 blocks from the existing Maasvlakte block dam will be reused in the new seawall. This solution is both economical and sustainable. The block dam protects the cobble beach. Waves will only pass over the block dam and attack the cobble beach in the event of a storm.

6. Toe construction (green dots) The toe construction, which consists of rocks weighing one to ten tonnes, will close in the large concrete blocks so that they cannot roll or shift from their intended position. Without both toe constructions, the concrete block formation, which is under heavy pressure in hydraulic terms (due to their large dimensions, the blocks are less stable under water), would collapse in a heap during an extremely heavy storm.

7. Waters levels NAP = New Amsterdam Water Level
Average water level high NAP + 1.0 m. Average water level low NAP - 0.5 m. North Sea flood of 1953 NAP + 3.75 m Sea level rise 2060 NAP + 5.30 m*

The landward side of the hard seawall will be a layer of clay covered with grass.

* In 2010, the average water level was approximately NAP 0 m. The seawall has been designed to weather a storm that, statistically speaking, occurs once every 10,000 years. The Port of Rotterdam Authority has established this protection standard for the new seawall of Maasvlakte 2 in consultation with Rijkswaterstaat. A storm condition of 1:10,000 years can be paired with a water level of NAP + 5.0 m and a wave height of 8 m. The strength of the seawall was tested in the model study for the year 2060, with a storm water level of NAP + 5.30 m, taking into account a rise in the sea level of 0.30 m for the next 50 years. For the following 50 years, space has been reserved for raising the crown of the seawall by another 0.50 m.