The seawall protects the coast and the hinterland from the sea. When determining the type and the elevation of the seawall in the Master Plan, safety was the key focus, but the designers also looked for the most economical and sustainable solution. The Maasvlakte 2 seawall will be a combination of hard (sand, rock and concrete) and soft (dunes and beach), with a total length of approximately 11 km.
In the north-western corner, where the waves are highest due to the depth of the water and the north-western location, the land will be protected by a hard seawall. In total, the hard seawall on the northern edge of Maasvlakte 2 will be 3.5 km long, with a crown height of +14 m New Amsterdam Water Level (NAP). The design, which is called a ‘stony dune', is unique in the Netherlands. The core of this seawall consists of sand. On the seaward side, it will be covered in stones with a diameter of 5 to 10 cm. At the foot, the contractor will position enormous concrete blocks in the sea, weighing 40 tonnes each (2.5 x 2.5 x 2.5 m). These blocks and a share of the stones come from the block dam of the existing Maasvlakte. This solution is both economical and sustainable. The landward side will be covered by grass on a layer of clay. In principle, the hard seawall will not be accessible to the public. The ‘stony dune' is more dynamic than a standard seawall, since the stones move along with the current and the waves. The natural cobble beaches that can be found in France and England have proven to be very safe. The ‘stony dune' will require limited maintenance. The design, a result of a ‘design and construct' approach, was developed by the contractor PUMA.
The western and southern sides of Maasvlakte 2 will feature a 7.5-km soft seawall of dunes with a beach. The beach will be nearly two times as long as the former beach of the existing Maasvlakte. The seawall will be constructed with a coarser grain of sand. This allows for the construction of a steeper foreshore with less sand. The row of dunes varies in elevation from +10 to +13 m NAP. Planting the dunes with marram grass and thickets will prevent the sand from being blown away. The row of dunes and the beach will for the most part be accessible to the public.
Multifunctional outer contour
The outer rim of Maasvlakte 2 will serve a number of purposes - which is why it is also called the multifunctional outer contour. Its primary function is to protect the area from the sea. In addition, it can accommodate a second road and a bike path. Thanks to its height, the rim will offer a fine view of the port activities. And people can engage in recreational pursuits around the outer contour, such as sunbathing and swimming at a bathing beach and surfing, kitesurfing, parapenting and fishing on an activity beach. At the bathing beach, a dune landscape will develop with a more ragged shape and natural dune vegetation. It will be possible to install wind turbines both on the hard seawall and along the activity beach.
Design of the outer contour
Maasvlakte 2 has a sophisticated form. On the one hand, the design is geared towards development being as efficient as possible, with minimal construction and maintenance costs. On the other hand, a stable coastal form needs to ensure that the project has a minimum impact on the existing Dutch coastline. Furthermore, the form of the port expansion is not allowed to negatively affect the shipping in the Maasgeul fairway.
2002 variant study
In 2002, researchers explored a range of alternative approaches to the land reclamation project. The options and effects of the alternatives were examined with regard to the access for ocean-going vessels, the orientation of the soft seawall, the accessibility for inland shipping and the development of the port and industrial sites.
The variants could be divided into two main groups: designs with and without their own direct entrance from the sea. The variants with their own seaport entrance turned out to require long breakwaters to move the port entrance outward. In the calmer waters created by these breakwaters, ships entering the port can safely reduce speed to navigate the bend leading into Maasvlakte 2. A negative effect of these long breakwaters was unfavourable current conditions for the shipping traffic during certain tidal phases. In addition, construction and maintenance would be costly. That is why it was decided to realise a variant that did not have its own direct entrance from the sea, but could be accessed via the existing Maasvlakte. This is called the cut through variant, since it means cutting through the existing Yangtzehaven.
Design principles for the seawall
The structure of the seawall was calculated to run a risk of collapsing once every 10,000 years and on the basis of the philosophy ‘soft where soft is possible and hard where hard is required'. This in consideration of the high cost of a hard seawall. The design of the hard seawall in the north-western corner was optimised in a further development of the Master Plan. The hard seawall presently connects to the existing hard seawall at a location more to the south. This optimisation results in lower construction costs.
The change in the maximum rate of flow just off the coast of Maasvlakte 2 is considerable. This effect is understandable in light of the changed geometry of the coastline. Near Maasvlakte 2's western point, the flow profile of the tidal stream is reduced (contraction). In front of the neighbouring coastal areas near Voorne, Goeree and Delfland, there will be no changes in the maximum rate of flow. There will be little to no changes to the tidal range and the wave height. The transport of sand by the current and the waves will alter around the land reclamation. Changes will occur near the land reclamation and in the entrance to Haringvliet, but the morphological effects (the moving of sand, water and silt) on these neighbouring coastal areas are extremely limited to negligible. The effects on silt transport extend from the entrance to Haringvliet all the way to the western Wadden Sea. The consequences of the changed silt transport are negligible, however. Like other locations in the Netherlands, the sandy coast of Maasvlakte 2 needs to be maintained by regularly depositing sand from the sea on the beach. This regular maintenance of the Maasvlakte 2 coastline will be hardly any more work than the maintenance of the Maasvlakte's existing beach. For the adjacent coastal areas, this coastal maintenance is more or less identical to the work carried out in a situation without Maasvlakte 2.
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.
The current patterns in front of and along the coast largely determine both the coastal morphology (the interaction between, among other things, moving water, sand and silt transport) and the safe and smooth movement of ships from and to the Rotterdam port. Over the years, there has been extensive research geared towards predicting the current conditions in and around the port. A current model has been developed that simulates the current in the existing and future situations. This current model has been validated on the basis of an extensive set of field measurements in and around the Maas Entrance. The current model was used in the nautical study to simulate the current patterns in a large number of land reclamation variants. The model was constantly adapted to the forms of the different designs.
The current model was also used to analyse and quantify the effects of the presence of Maasvlakte 2 on the current patterns in the Maasgeul and the Maas Entrance in the construction phases.
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. This allowed for constant improvement of the design, in which guaranteeing safety and keeping construction and maintenance costs as low as possible served as the points of departure.