Brussels sewage system cannot handle torrential rainfall, say experts
In the wake of the recent disastrous and deadly floods in Spain – particularly in the Valencia region where more than 200 people lost their lives - experts in Brussels say the capital’s existing sewage and drainage system would not cope with such a p
In the wake of the recent disastrous and deadly floods in Spain – particularly in the Valencia region where more than 200 people lost their lives - experts in Brussels say the capital’s existing sewage and drainage system would not cope with such a phenomenon.
The "water bomb" meteorological phenomenon that caused the heavy rain is an area of low pressure filled with cold air at high altitude. It is more frequent in Spain, but the country has never before suffered so much rainfall, also prompting reactions that the tragedy was caused by climate change.
Ann van Griensven, professor of hydrology and hydraulic engineering at VUB warned that this phenomenon, while not with the same disastrous consequences, could also happen in Brussels.
“There are currently several research institutes studying the water bomb model in Western Europe,” she said. “This shows that they are becoming more frequent and more intense. There is a link with climate change.”
VUB urban hydrologist Boud Verbeiren agreed that while the risk of Brussels’ Senne river bursting its banks or flooding the canal is not very high, given the river’s "diversion channel" activated at high tide, the capital could not cope if it did.
Verbeiren, who also works for water management company Hydria, said most floods in Brussels are caused by an overloaded sewage system or by too much rainwater remaining in the streets.
“Brussels has a combined sewage system that drains both wastewater and rainwater,” he said. “But with excessive rainfall, the sewer system becomes saturated. The water no longer enters the sewer system, or is expelled from the sewer system because the pressure is too high.”
This is partly due to the city’s paving stones, which prevent the natural cycle of water infiltration into the ground and causes rain to enter the sewer system.
According to figures from Brussels Environment, 53% of the region’s soil is impermeable. But in the Pentagon (inner city), Saint-Josse and Saint-Gilles, the percentages are 93%, 91% and 90% respectively and the degree of impermeability is constantly increasing.
According to Verbeiren, the current Brussels sewage system, in combination with buffer basins and spillways - channels for surplus water - can handle rainfall with a force that could occur once every 10 years.
“But that’s nothing compared with what happened recently in Spain or in 2021 in Wallonia,” said Michaël Antoine, hydrologist at Brussels Environment.
“On 24 July 2021, 80mm of rain fell in one hour in Namur and Dinant. This is a rain shower that occurs once every 10,000 years - just as intense as the rainfall in Spain, but fortunately much shorter. Brussels’ current sewage system is no match for it.”
Brussels Environment has used the Namur and Dinant water bombs and the Vesdre valley flooding in the Verviers region, also in July 2021, to create simulation scenarios for Brussels.
“These flood maps give an idea of the consequences of a possible water bomb,” said Antoine. “Because what happens in such an event? The streets become rivers.
"It’s particularly dangerous on the city’s many sloping streets. There, the water flows at lightning speed and takes the cars with it.
"We identify the streets that are most susceptible. Take Rue Gray in Ixelles, for example, which becomes a swirling river. This has already happened during severe weather in the 1950s, resulting in one death. We also find out where the fire brigade can go in such cases.”
General flood zone maps are available to the general public, but not those for extreme risk scenarios. “We don’t want to frighten people unnecessarily,” Antoine added.
“The latter maps are intended solely for local authorities and safe.brussels, the regional safety authority. It uses them to draw up the Special Emergency and Intervention Plan (PNIB) in the event of an imminent flood.”
Brussels Environment issues storm warnings in the event of bad weather and warns safe.brussels - the capital's security management system - if there are extreme weather conditions. Safe.brussels can then issue a BEAlert safety warning, said Antoine, while adding: “So far, we’ve never had to do that for flood risks.”
BEAlert is the Belgian government’s alert system. In emergencies, a mayor, governor or the interior minister may send you a message describing what has happened and what can be done to get to safety.
You can receive this message via text, e-mail or voice call. In some places, you can also pick up BE-Alert messages on other screens, such as digital information boards on public streets.
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