Flood zones in Belgium and Wallonia: how to assess the risk for your home

A home can be exposed to several forms of flooding: a watercourse overflowing, rainwater runoff, network backflow or a temporary rise in groundwater. Knowing the risk is the first step to adapting a building and its equipment.

The flood hazard map is the reference tool for assessing risk in Wallonia. It takes into account natural overflow of watercourses and runoff concentration axes. The hazard is classified in four levels: very low, low, medium and high.

Flood zone maps show different return-period scenarios, while the hazard map combines the recurrence and the magnitude of the event. A plot can appear on a map without ever having flooded in living memory: the mapping indicates a possibility, not only events already observed.

During heavy rain, water concentrates on roads, sloping land, dry valleys, car parks and impermeable surfaces. A house far from any watercourse can therefore be affected. Wallonia provides data on runoff axes.

Flood risk matters before a build, a renovation or fitting out a basement. In Wallonia, the hazard map is used in certain planning permission procedures, and hazard levels can have insurance consequences.

Prevention depends on the building and the origin of the water: rainwater management, separation of networks, securing low access points, protecting technical equipment, a suitable lifting system and a level alarm.

In a basement or an area below the drainage level, a lifting station removes water arriving in a tank or sump. It must be sized for the potential flow and the head — and a lifting pump alone is not a complete flood protection solution.

During a major event, a sewer can be under pressure. A suitable design includes, depending on the network layout, devices preventing water from flowing back. Sizing and equipment choices take real site conditions into account.

Weber Water analyses an existing pumping installation or studies a lifting solution to protect a building against water accumulation in low areas. Official mapping remains the reference for determining a plot's hazard level.