Many of the roofs in Amsterdam have been converted into giant sponges that will make the city more resilient to heavy rains. The Dutch have always been famous for their ability to find innovative ways to control water, as much of their homeland is below sea level. Now their expert water management skills are being used to transform the city skyline of Amsterdam from terracotta, concrete, and shingles into green grass and brown earth. It’s part of a new climate-resiliency trend in architecture and civic planning known as the ‘sponge city concept’. Rooftops filled with a garden of water-loving plants, mosses and soil, absorb excess rainwater and then feed it into the building as needed for use in flushing toilets or watering plants on the ground. When heavy rains are predicted, a smart valve system empties the stored rainwater into the municipal storm drains and sewers in advance of the weather. This allows the roof to soak up the coming rain and reduce flooding in the city. You can see how “sponge cities” got that name, as the rooftops of buildings can be wrung out and filled up just like a sponge. In Amsterdam about 50,000 square meters of flat metropolitan rooftops have already been fitted with these systems, and their sponge capacity is over 500,000 litres. 

Experts in this field point out that the usefulness of the sponge city concept is not limited to areas where there is too much rain. They point out that it is adaptable to a variety of urban areas with different weather patterns. The best example of how it can take on a different role is its use in drought-prone regions. For instance, in parts of Italy and Spain (like Ibiza) where water is scarce, this concept can be used for rainwater catchment. Water absorbed by rooftops during heavy rains can be used for municipal purposes, and this reduces the strain on the underground aquifers. Excess water in the sponge roof can also be used to cool the building in hot weather. Perhaps this is something that Ibiza could look into as part of the solution to the massive flooding that occurred in Ibiza Town this October.