Catalan cities storm-proof: what has been done and what remains to be done?

BarcelonaThe storm this past week has tested the resilience of cities like Barcelona. Between October 3rd and 6th, the Catalan capital recorded 213 liters per square meter, a third of the rain it usually collects in a whole year. The downpours pushed the city's drainage systems to the limit and raised a key question: are large cities prepared for a new model of more aggressive storms? What are they doing and what can they do to protect themselves?

In the case of Barcelona, since the 90s, the commitment has been to improve the sewage system to minimize rivers of running water through the streets. Playing a key role in this objective are the fifteen storm tanks in the city, a network that has been reinforced and has continued to grow in recent years. Currently, together they have the capacity to retain nearly 470,000 cubic meters. And yet, the storms of the last few days have been on the verge of overflowing them.

"At Diagonal and Villarroel, the water was only 23 centimeters away from overflowing, and at Ronda Sant Pau and Paral·lel, only 30 cm away," explains Alejandro Ortiz, general director of BCASA, the public company of the Barcelona City Council that manages the water cycle in the city. "They were days of suffering and sleeplessness," he adds. In total, the tanks were filled and emptied two and a half times.

All in all, it helped to minimize in a large part of the city the images of flooded streets that until not so long ago were common, for example, on Avinguda del Paral·lel. The opening of the new Vilà i Vilà collector, which after years of construction work has come into operation to prevent this avenue from collecting the excess water from the Diagonal-Barceloneta basin, one of the most strained in the sewer network, has helped.

"Barcelona is a city that is increasingly resilient to the effects of the climate emergency," defends the city's first deputy mayor, Laia Bonet. However, she admits that the context of the climate emergency makes it necessary to "accelerate investments." In this regard, she lists some of the future reservoirs that are already being built—such as the one at Prim—or are in the planning stages, such as those at La Sagrera, Vallcarca, and Bac de Roda. The horizon is to build 29 more.

Towards a "sponge city"

Geography professor at the Autonomous University of Barcelona (UAB) David Sauri warns that storage tanks are a key tool, but they cannot be the only one. "A large part of the rain we collect in a year will come from intense episodes like those of the last few days. The challenge is to manage it so that this water can be used," he maintains. Therefore, he argues that urban planning must move towards a "sponge city" model.

What does this mean? Sauri explains it graphically. Currently, the water stored by rainwater tanks ends up in the sewage system and is lost to the sea, and it is necessary to encourage the ground to absorb it before it goes into the sewer network, having reserves to release it when convenient. To achieve this, the path is to intensify greenery.

This is another one of the strategies that cities like Barcelona are trying to follow, with what have been called sustainable urban drainage systems (SUDS). A category that includes flowerbeds, tree pits, or permeable pavements that allow rain to infiltrate the ground instead of overwhelming the sewage system. In this classification, there are also floodable gardens, large green areas designed so they can remain flooded with water for a few days and help retain rain.

Sauri points out that in a city as dense as Barcelona, it is difficult to find land where such projects can be carried out, but he notes that in the State there are success stories like the Marjal park in Alicante, where the confluence of torrents had caused havoc in the past and which is now a large floodable green area. In the Catalan capital, on a small scale, there is also the Tretze Roses park, in the Marina del Prat Vermell, which has this function.

"We are moving from the hard city with impermeable asphalt to reproducing, in the heart of the city, solutions that emulate nature, that prevent flooding, that recharge aquifers, and that green and cool our streets,” summarizes Bonet. The deputy mayor stresses that there are currently 104 SUDS spaces in Barcelona which, added together, reach 17.3 hectares, the equivalent of fourteen Eixample blocks.

Plans for mountain neighborhoods

The heavy rains of the last few days, however, have shown that the rainwater tanks and the SUDS are not, for now, enough to protect the mountain neighborhoods, where the main damage has occurred. Alejandro Ortiz explains that they are working on a protection strategy for the mountain neighborhoods that minimizes the risk of heavy rains causing floods and damage in urbanized areas.

On the table, he says, there are projects to create some lamination ponds in the Collserola mountain range that act as small staggered reservoirs and help to slow down the speed of the water and minimize debris flow. A system that helps to avoid episodes like the one experienced these days in la Font del Gos, where the flood damaged two houses and forced six families to be evacuated by boat at dawn.

Despite all the interventions already carried out and underway, Ortiz remarks that there will always be a level of rain where nothing can be done and water will flood some spaces. He points out that one line of work is to detect which areas will flood and build, in the style of Alicante, parks where the water can be retained. He also highlights the need to better sensor the city to have earlier data on which streets are filling with water and where it is necessary to cut off traffic. Also on the Ronda de Dalt, which in recent weeks has been one of the weak points and which, he points out, could be used in an extreme case as a rainwater reservoir if traffic is cut off in time during a downpour.