INGENIEURSBUREAU AMSTERDAM

CLIMATE ADAPTION ON CENTRUMEILAND IJBURG

Authors: Bram de Vries and Remko van Diepen

HOW INGENIEURSBUREAU AMSTERDAM DESIGNS A CLIMATE RESILIENT ISLAND

When the theme of climate adaptation needed to be elaborated for Centrumeiland, a forward-looking perspective was adopted by Ingenieursbureau Amsterdam, which manages the project as part of the municipality of Amsterdam. Anticipating higher water levels in the IJmeer, the island’s ground level was raised and the increased risks of extreme precipitation as well as extreme drought were also taken into account. As much as possible, rainwater will be retained and infiltrated into the ground, where it forms a valuable underground buffer for the island’s vegetation during drier periods.

In 1996, the city of Amsterdam decided to build IJburg; a residential district situated entirely on artificial islands in the IJmeer. This unique project was started with great enthusiasm, but then experienced some delays due to the 2008 crisis. Demand for housing increased again after the crisis and the property market stabilized. In 2016, the time was ripe to start the construction of Centrumeiland, named for its central position in the future archipelago. The island will form the connection between the already existing Haveneiland and Strandeiland, which is currently being raised from the bottom of the IJmeer by means of land reclamation.

Each of the islands that make up IJburg has its own distinct atmosphere, character and living environment. Centrumeiland is compact, with a strong contrast between the broad embankments and the sheltered interior. It will be given an urban look and feel, due to the predominant building height of four to six storeys and the prominent role of pedestrian zones in its public spaces. Centrumeiland is currently the most important self-build location in Amsterdam. Residents here have a big say in the design of their home and living environment. With regards to sustainable development, the municipality has identified two priorities: an energy-neutral island and climate adaptation.

FIG. 1 Experimental wadi in 2020.

As mentioned, Ingenieursbureau Amsterdam and the municipality adopted a forward-looking perspective on climate adaptation. Anticipating increased water levels in the IJmeer, the island was designed to have an elevated ground level and robust embankments; in addition, the increased risks of extreme precipitation as well as drought were both taken into account. It was decided not to install a conventional rainwater drainage network, but to retain as much water as possible and infiltrate it into the ground. In this way, rainwater forms a valuable underground buffer for the island’s vegetation during drier periods.

In public spaces, it was decided to establish a system of infiltrating green zones and wadis, connected to each other through a network of cascading gutters that cleverly utilise the existing height differences on the island. Near the access roads, where gutters tend to be incompatible with the high traffic intensity and public transport, the design provides for a perforated infiltration/transport storm sewer.

In private spaces, the municipality promotes the inclusion of green and water-retaining roofs. Self-builders install infiltration devices to infiltrate the water from their roofs into their own gardens. As a quite unique planning innovation, the municipality has included a clause on mandatory infiltration in the local zoning plan, ensuring the system will function effectively in the future too.

The Square of Trees

The realisation of the ultimate design was kicked off by the planting of a group of Turkey oaks on the highest and driest part of the island: the Square of Trees. Innovative technology has also been applied under the surface here: the trees are supplied with water retained in capillary sponges placed underneath the tree roots. This water storage system will be connected to the roof surface of the neighbouring primary school, which ensures the sponges are replenished regularly with fresh rainwater. Any excess precipitation will trigger an overflow mechanism that drains the water into infiltration pits located below the storage system. This way, not a drop of water will be wasted. The urban plan for Centrumeiland follows the principles of the municipal programme Amsterdam Rainproof, so that the island will be able to cope with extreme precipitation. The orientation of streets, building blocks and green zones has consciously been designed to lead rainwater to places where it does not cause any damage or inconvenience. The wadis will be planted with a wide variety of perennials, grasses and shrubs, which will benefit the biodiversity on the island.

FIG. 2 Water depth from a hydrodynamic model.

Hydrodynamic model

During the design process, it became clear that there was a need for deeper insight into the water levels and flows associated with extreme precipitation. The municipality therefore approached Nelen & Schuurmans during the planning stage, to see if it would be possible to build a model that visualises the effects of an extreme downpour on the prevailing urban plan. It is important to do such an evaluation at an early stage, so that the findings can easily be translated into modifications to the plan.

A hydrodynamic model for Centrumeiland was created with 3Di simulation modelling software. This model was used to calculate how water would be dispersed across the island in the future, and to identify the locations most likely to experience problems. The hydrodynamic model for Centrumeiland covers all hydrological processes; the rainwater that is retained on top of buildings, the water that flows from streets into gutters and on into the wadis and the water collected from the access roads to flow into the infiltration/transport sewer.

One of the challenges in building a model for an area yet to be developed is that a digital elevation map, which can be used in the model, is not available yet. Normally, the elevation map has to be created on the basis of design drawings. To facilitate the study, the municipality drew up a digital elevation map for the roads on the island, and added the anticipated ground levels of buildings, gardens and wadis. The model also included a component to cover infiltration and resistance, based on future land use. In addition, the overflows draining into the wadis along the access roads were added, with their connections to the infiltration/transport sewer. Waternet, the company responsible for managing the sewage system, provided the design for the infiltration/transport network.

New insights

The model was run for a normative rain shower (standard shower 09) and for an extreme downpour (60 mm in one hour). The normative shower was used to verify if the sewage system and wadis would meet the municipality’s requirements. The extreme downpour was used to analyse how the system behaves under extreme precipitation and to identify places where flooding would be most likely to occur.

The model generated new insights, which helped in making informed decisions about climate-proofing the design of Centrumeiland. It led to the optimisation of the sewage system design. The buffer capacity of the sewage system was not yet fully utilised, which would have resulted in flooding of a main road. Better utilisation of the sewage system’s buffer capacity has decreased the risk of flooding on main roads and has reduced the amount of water that will flow over into the IJmeer. The extreme downpour scenario was run through the model several times, each time with an improved urban plan. The first calculation showed that some wadis received too much water, while others received hardly any at all. One block of buildings was taken out of the plan, and one wadi was added, so that the distribution of water across the wadis was improved. The final calculation resulted in a modification of the road profiles, to make it less likely that any flooding reaches the facades of buildings.

The results also show clearly how water will flow across the island in the future. The water is channelled to the wadis through gutters. When the most elevated wadis are completely full, the water overflows into gutters that take it to lower-lying wadis. The lowest wadis are connected to a network of infiltration/transport sewer pipes that can overflow into the IJmeer during heavy downpours.

FIG. 3 Centrumeiland seen from the air in 2016.

Cooperation

The innovative design of the island can be regarded as the result of an effective collaboration between urban planners, landscape architects, hydrologists and ecologists. Testing the urban plan at an early stage gave Ingenieursbureau Amsterdam and the municipality the possibility to make the appropriate modifications to the plan relatively easily. Ultimately, this has led to a robust and climate-adaptive definitive design for Centrumeiland. The first buildings have now appeared on the island. The expectation is that the entire new residential district will be completed by 2027.