Madrid is bracing for more heavy rainfall, and a massive, largely unseen infrastructure is working overtime to prevent widespread flooding. Hidden beneath a golf course in the Madrid Country Club, the Arroyofresno storm tank—one of the largest in Europe—currently holds almost 400,000 cubic meters of water, a volume eight times greater than that of the pond in El Retiro Park.
A Hidden Fortress Against the Storm
The Arroyofresno tank is a critical component of Madrid’s flood defense system, quietly safeguarding the city from increasingly frequent torrential rains.
- The Arroyofresno tank is the largest storm tank in Europe, capable of holding 400,000 cubic meters of water.
- January was the rainiest month in Madrid since 1996, with February continuing the trend.
- The tank collects water from four districts: Moncloa, Fuencarral, Chamartín, and Tetuán.
- Madrid’s sewer system is “unitary,” meaning rainwater and sewage are collected in the same pipes.
The sheer scale of the Arroyofresno facility is striking. It’s currently receiving runoff from four districts—Moncloa, Fuencarral, Chamartín, and Tetuán—water accumulated over recent weeks. January broke records as the rainiest month since 1996, and February is already showing 3.5 times more rainfall than the same period last year, double the 20-year average. A visit to the tank reveals a stagnant, foul-smelling pool filled with debris: bottles, wipes, branches, and even mattresses.
Marta Lopez, head of the City Council’s sewer service, explained that Madrid operates a “unitary” sewer network. This means that both sewage and rainwater flow through the same pipes. When rainfall overwhelms treatment plants, the excess is diverted to storm tanks like Arroyofresno. Once the rain subsides, the stored water is pumped back into the network for treatment before being released into the river. “This surplus, if not stored, would be dumped into the riverbed and would be much more polluting,” Lopez emphasized.
Mayor José Luis Martínez-Almeida visited the Arroyofresno infrastructure alongside Borja Carabante, the delegate of Urban Planning, Environment and Mobility, and José Antonio Martínez Páramo, the delegate councilor of Cleaning and Green Zones. Martínez-Almeida stated that, “In situations like the ones we are experiencing right now, they give us an operational and strategic advantage to avoid damage to the city.” Officials believe these tanks are essential to prevent floods similar to those experienced in Paiporta following a DANA (Isolated Depression at High Altitudes) event.
The tank’s benefits are twofold: improving water quality by reducing pollution in the river and enhancing security by minimizing flood risks along the Manzanares River. It’s a largely invisible defense, working tirelessly beneath the surface to protect the Spanish capital.
