Flooding can occur on hardened surfaces during and after heavy rainstorms because the excess water cannot drain away into the soil and through the sewer system quickly enough. If very large volumes of rain fall over a short time, an enormous amount of water has to be collected and drained away. The sewage system is unable to handle this enormous amount of water, resulting in the flooding of streets and cellars. Capturing and draining the water faster prevents flooding. The rockwool elements can absorb 95% of their volume of water and have a throughput of 200 metres a day. A cubic metre of Rockflow system can absorb 950 litres of water in 8 to 10 minutes. "This is more than enough to create water management systems that meet local standards," says De Kubber. Once saturated, the rockwool element can allow the water to infiltrate the substrate or drain away to the sewer gradually over 24 hours.
De Kubber has been involved in the development of Rockflow from the very beginning. He says that buffering water with rockwool is a method that is widely used in greenhouse construction. "In greenhouses, it’s used for growing plants on. We have developed stone wool elements that are not only able to store huge amounts of water very rapidly, but are so strong that they can cope with the load of a full parking deck or being driven over by trucks.”
More and more municipalities are becoming interested in this innovative water management system. Within each municipality there tends to be an area that is regularly flooded. "Not only has the rain become more intense due to climate change, the current design of the sewer system also contributes to the problem. The sewers in the Netherlands were built eighty years ago and were not designed for the capacity that is required today. The problem of excess water is becoming increasingly urgent.” As De Kubber explains, “Rain showers are becoming more extreme due to climate change; the sewers can no longer drain away the water and the streets become flooded. At the same time, more and more surfaces are being hardened in the Netherlands, resulting in even more damage. Municipalities are often liable for the damage caused by flooding, and realise that they need to take decisive action in the short term to combat increasing flooding."
Replacing the sewers is an expensive process. It’s much cheaper to capture the surplus of relatively clean rainwater, and to gradually drain it away or allow it to infiltrate the soil. Many urban areas in the Netherlands suffer from a lack of water. "It’s a waste to drain this relatively clean rainwater away through the sewers to a water purification plant. Rockflow allows the collected clean rainwater to gradually seep into the soil, allowing the water balance to be slowly restored,” says De Kubber.
The basic material used in the system is rockwool, which is literally made of stone and therefore contains all the properties of stone: it is strong, porous, hardly deforms under pressure and is completely recyclable, thereby avoiding environmental damage. The system is made-to-measure, and adapts to the shape of an infrastructure project without losing its functionality. The water management system consists of modular rockwool elements that are installed below ground level. The slabs are buried under the built-up surface of the urban area, leaving the above-ground functions completely intact. During heavy rain, the water flows through the road drains to the lower part of the rockwool package. The hollow spaces between the rockwool fibres fill up with rain water. The road drains are connected to the Rockflow elements via a piping system. Because these elements fill with water from the bottom up, air can escape through the ventilation channel at the top, so that the buffer can optimally fill with water.
Royal Haskoning DHV has been closely involved in the technical development of the water management system. Marc van den Heuvel, hydrology consultant, explains the benefits of Rockflow: "A major advantage of this system is that the infiltration surface is much larger than the regular crate systems or lava boxes that are often used to drain water to the soil in built-up areas. The system is open on all sides in order to infiltrate water to the subsoil. The bottom surface of the crate system or a lava box should not be counted as an infiltration surface, because this system silts up with dirt. The water that infiltrates from the Rockflow is filtered, clean rainwater, free from small clogging particles, resulting in more rainwater infiltrating faster into the substrate.” In fact, the structure of the rockwool elements filters out any contamination from the inflowing water, so that the system does not become clogged with sand, sludge, algae and moss particles. If necessary, the water can be filtered in advance using filter pits. These filters are easy to replace and are also recyclable.
Van den Heuvel continues: "A further advantage of Rockflow compared to the crate system is that there are no filter cloths around the rockwool elements. The elements are installed directly onto the ground layer. Filter cloths quickly become damaged, for example by maintenance work, and soil then flows into the system. But this doesn’t happen with Rockflow.” An additional advantage is the flexibility of the material. "In the case of underground work, a slot can easily be made in the elements to lay a cable, without affecting the operation of the water management system,” he explains. "Maintenance of these systems only needs to take place during the annual standard maintenance of the sewers," says Van den Heuvel.
The engineering office is constantly investigating how the Rockflow systems can be used sustainably as a water-storage facility, taking the local permeability and the groundwater level into account.
This water management system is now also available in Benelux.