Amsterdam becomes rainproof with Rockflow

Features and challenges

Each location has its own features and challenges. For example, at the ‘Kruislaan’ location, one of the challenges was the provision of water compensation for the increase in roofed and paved surfaces. Because rainwater is no longer able to soak away into the soil due to this increase, it can cause flooding or overloading of the surface water drainage and/or sewer system. Waternet’s statute therefore states that for every 1,000 m2 increase in hardened surface, a minimum of 70 m³ rainwater storage must be generated in compensation. This requirement is considerably stricter than the normative T10 which, according to the drainage guidelines, occurs once in 10 years. The reason for this is that Waternet wishes to implement robust facilities and resilience measures in the city, so that it is prepared for a future of climate change.

The project is located in the ‘Watergraafsmeerpolder’, which was drained in 1629. The soil consists mainly of clay and peat, with a top layer of sand infill. This top layer determines the infiltration capacity of the soil. The average groundwater level, together with the average highest and lowest groundwater levels, were determined on the basis of measurements taken from Waternet’s network of monitoring wells and on the hydromorphological profile features (oxidation and reduction stains) from a previous study. In addition to this data, the available area within the project boundaries and the level in the surrounding waterways formed the most significant input for a study into the various alternatives relating to the buffering of rainwater.

Underground solution

Iv-Infra considered various alternatives, each one of which had advantages and disadvantages in relation to several factors such as space requirement, cost, maintenance and complexity. Given the limited space, neither the above-ground collection of rainwater nor the expansion of surface water was an option.

With permeable paving, the intensive maintenance required was too great a disadvantage and the collection and temporary storage of water on the roof of the building was financially infeasible, since this required too many alterations to be made to the temporary housing. An underground solution therefore needed to be found. The final choice was for an underground storage system in which the water infiltrates into the ground. This was the most suitable system, taking into account the capacity required and the groundwater level.

Robust & future-proof solution

In addition to making the city ‘rainproof’, Amsterdam is also committed to sustainability. The underground system therefore also had to be produced sustainably and be fully re-usable after a 10-year period; cradle to cradle. The Rockflow water management system suited these requirements perfectly. The basis of the system consists of stone wool elements with a very high capacity to absorb and store water. Stone wool is a natural material made from basalt and is sustainably manufactured. It is also fully recyclable.

The major advantage compared to traditional plastic or concrete crate systems is that the Rockflow system can be easily adapted. And if, for example in the future, it is decided that a hole needs to be excavated for a tree to be planted in the centre of the inner garden, which is the location of the storage system, it will not pose any problem. Even though some of the stone wool will have to be removed, the system as a whole will remain intact, albeit with reduced capacity. Cables can also run through it, should that be necessary. With a crate system, this would result in leakage, run-off and eventual subsidence as a consequence. It is a robust and future-proof solution, regardless of current and future plans. The Rockflow elements are lightweight and easy to install. Despite the fact that the elements are lightweight, with the right ground cover they can withstand heavy loads. At the ‘Kruislaan’ location, an inner garden is being created above the stone wool elements. Heavy loads do not occur here, so a ground cover layer of 30 to 50 cm in thickness will suffice.

The Rockflow system

Water flows from various drains and gutters to the lowest section of the stone wool elements via a system of pipes. While the empty space between the stone wool fibres fills with water from beneath, the air is driven upwards via ventilation pipes.

According to the supplier, the elements absorb approximately 95% of their volume in water within 10 minutes, even at low pressure. This is partly due to the throughput speed of the water in the stone wool elements of approximately 200 metres per day.

The system can be designed in such a way that it empties entirely within 24 hours and is then ready for the next rainstorm.

Pilot project

This is the first Rockflow system to be installed in Amsterdam and is therefore viewed as a pilot project. Despite the fact that there isn’t much of long-term experience of this type of system in the Netherlands, project leader Cees Voorburg and geohydrologist Baukje Dijkstra of the municipality of Amsterdam’s engineering department are convinced of the benefits of the product and the system. This is particularly because in areas with intensive land use a combination of usage functions must be sought, in this case for water storage and an inner garden.