According to a study published in Nature, the phenomenon known as “hydroclimatic volatility” describes sudden and extreme shifts between prolonged dry spells and intense wet conditions. These fluctuations result in “weather whiplashes,” such as torrential rains that trigger devastating flash floods—an increasingly frequent occurrence as global temperatures rise due to human-induced climate change.

Scientists have documented significant changes in rainfall patterns over recent decades. While total annual precipitation has remained relatively stable or increased slightly in many regions, rainfall events are becoming shorter but significantly more intense. This shift means fewer rainy days overall but heavier and more concentrated downpours when rain does occur, exacerbating flood risks.

The destructive potential of these extreme weather events was evident in Valencia, Spain, on October 29, 2024, when catastrophic flooding saw 1,000 cubic meters of water per second surge through the area. The disaster claimed 224 lives and resulted in severe material and economic damage.

While it is nearly impossible to fully prevent such events, engineering has long provided solutions to mitigate their impact. Among them, storm tanks stand out as a critical defense mechanism against catastrophic flooding.

How Storm Tanks Work

A storm tank is a massive underground concrete structure designed to prevent rainwater from overwhelming the sewer system during heavy rainfall.

Its operation is both simple and effective. During intense storms, storm tanks temporarily retain excess runoff and then release the stored water gradually, preventing wastewater treatment plants from exceeding their maximum capacity.

For a storm tank to be effective, its storage capacity must be sufficient to handle rainfall at a rate of 10 liters per second per hectare for at least 20 minutes. This level of rainfall can carry street debris, sediments, and pollutants into drainage systems, which, without proper intervention, would flow directly into waterways.

Before entering the tanks, stormwater passes through filtration systems that capture pollutants such as fuel or oil spills, cigarette butts, leaves, soil, plastic waste, bottles, cans, and even animal waste. This process helps prevent contamination of rivers, lakes, and other natural water bodies.

Protecting Rivers and Water Quality

Storm tanks play a pivotal role in managing large volumes of water from natural streams and urban runoff, regulating their release into the sanitation network in a controlled manner. This not only helps reduce flooding risks but also prevents polluted discharges from reaching rivers and other water bodies, ensuring environmental sustainability and water quality.

For storm tanks to be effective in their dual function, they must be strategically placed within the drainage network, particularly near discharge points where water is released back into the environment. Their internal flow regulation is also critical—vortex valves manage low flows, while gates control high flows during extreme weather events.

Additionally, key operational factors must be considered: whether the tank will be emptied by gravity, with pumps, or a hybrid system, what cleaning method will be implemented (manual or automatic), and whether remote monitoring and control technologies will be integrated to optimize efficiency and maintenance.

Madrid Has the Largest Storm Tank in the World

Cities with advanced infrastructure will be better equipped to handle climate change-induced flooding, including extreme weather events.

Spain has positioned itself at the forefront of rainwater management, with 470 storm tanks across cities such as Seville, Valencia, Alicante, and Bilbao. However, the Community of Madrid stands out—not only for having one of the most extensive networks of storm tanks worldwide but also for being home to the largest storm tank on the planet.

Located in Arroyofresno, this 35,000 m² facility reaches a depth of 22 meters. The water collector was constructed using a tunnel boring machine, featuring an inner diameter of 6.70 meters and extending 3 kilometers in length. Built beneath a golf course, the tank can hold up to 400,000 m³ of water—eight times the volume of Madrid’s iconic El Retiro pond, where tourists rowboats through the city’s most famous park.

Together, the Arroyofresno tank and the Butarque storm tank, also located in Madrid, store and redistribute approximately 8 cubic hectometers of rainwater annually, significantly reducing the risk of urban flooding.

Tokyo: A Sophisticated System to Contain Water

Tokyo’s highly advanced drainage system serves as a model for cities struggling with flood prevention. At the heart of this system is its massive storm tank, a key component of the Metropolitan Area Outer Underground Discharge Channel (MAOUDC).

This vast network of tunnels and cylindrical chambers, spanning 6.3 kilometers and located 50 meters below the surface, plays a crucial role in protecting northern Tokyo from severe flooding.

Recognized as the largest underground water diversion system in the world, the project required an investment of over $2 billion. However, its impact has been remarkable, reducing flood-related damage to homes by 90%.

Beyond its engineering brilliance, the facility has also become an unexpected tourist attraction, offering guided tours through its massive tunnels and awe-inspiring underground vaults.

London and the Challenge of Harnessing the Thames

London has taken a different approach to tackling storms and flooding from the River Thames. The Thames Barrier, a system of 10 massive steel gates—each as wide as the opening of Tower Bridge—serves as a flood defense, protecting an area of 125 square kilometers.

However, with sea levels projected to rise by around one meter by 2100 and storms becoming more intense, London authorities are already planning additional measures. These include raising retaining walls by half a meter and constructing downstream reservoirs to store excess floodwater, ensuring long-term protection for the city.

Towards Sponge Cities

Storm tanks are a cost-effective, low-impact, and environmentally beneficial solution for managing excess water from urban drainage systems. However, their maintenance is demanding due to high humidity, corrosive gases, and sludge accumulation, making them prone to operational challenges.

For instance, in the Butarque storm tank in Madrid, large quantities of fibers and rags clogged the drainage pumps, leading to inefficiencies. To address this, the BioCut Vogelsang system was implemented—a macerator designed to shred waste and protect the pumps and overall infrastructure from blockages and damage.

As cities seek long-term, sustainable solutions, the concept of sponge cities is gaining momentum. These urban areas incorporate nature-based infrastructure to absorb excess water through rain gardens, bioretention systems, and floodable parks. One of the most ambitious examples is found in Wuhan, China—known as the city of a hundred lakes—which has developed 389 sponge city projects spanning 38.5 square kilometers.

China has been at the forefront of this movement, with over 640 sponge city projects in 250 municipalities since 2015. The ultimate goal is that, by 2030, 70% of rainwater in these cities will be absorbed rather than accumulating in the streets. As Kongjian Yu, landscape architect and professor at Peking University, wisely states: “You can’t fight water; you have to adapt to it.”