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Flash floods and Urban flooding Belgium


As the sewage systems were designed to cope with less violent downpours, heavier summer storms also will mean more flooding in urban areas.

The recent increase in the amount of heavy rainfall leading to floods within the sewer network, is caused by hydrometeorological conditions that are less or as extreme as those observed during the 1960s. Of course, land use has changed significantly in the meantime (for example, urban areas have become larger with large-scale sewer networks constructed). Consequently today’s hydrological impacts are very different from those of the 1960s (2).

Design parameters for urban drainage systems in Belgium have been revised (4). The revision involves extrapolation of the design rainfall statistics, taking into account the current knowledge on future climate change trends till 2100. Uncertainties in these trend projections have been assessed after statistically analysing and downscaling a broad ensemble set of 44 regional and 69 global climate model runs, based on three different emission scenarios (A1, A1B, B1). The simulation results were considered for the scenario period 2071–2100 and the control period 1961–1990. It was shown that the 10-year design storm intensity can increase up to about 50% by the end of this century. Or, systems currently designed for a 20-year return period of flooding, might flood with a mean recurrence interval of – in order of magnitude – 5 years by the end of the century. It is found that increase in storage capacity of 11–51% is required to keep the overflow frequency to the current level. Thus, sewer surcharge or flooding might occur up to about twice to four times more frequently than in the present climate (if no other environmental or management changes are accounted for). It is clear that this might have a significant impact on future urban water management and planning, and that adaptation strategies will be required.

The flooding of July 2021

Severe floods in mid-July 2021 killed at least 220 people in Germany and Belgium. The floods followed 2 days of heavy rain that caused several rivers in the region to burst their banks. The worst-hit areas were around the Ahr and Erft rivers in Germany and the Meuse in Belgium, which experienced the most rainfall in a 24–48-hour period since records began. According to a model study carried out by the ‘World Weather Attribution’ initiative, human-caused climate change had increased the rainfall intensity of such storms by 3–19%, relative to a pre-industrial climate 1.2 °C cooler than today (5). The study concludes that similar events can now be expected to hit any part of Western Europe, in a large region between the north of the Alps and the Netherlands,about once every 400 years. The authors state that the likelihood of such an event to occur today compared to a 1.2 °C cooler climate has increased by a factor between 1.2 and 9.

Adaptation strategies

In the Brussels region, the “Plan de prévention des inondations – Plan Pluie“ (PLUIE flood prevention plan), which received government approval in late 2008, adopts both a preventive and palliative approach. Preventive measures can help to ensure that the built environment is better adapted to increased precipitation, both in terms of improved soil infiltration and short-term water retention on plots of land. Various measures are being implemented to recover rain water (the installation of rainwater tanks is compulsory for new housing and promoted through regional grants for existing housing) and to increase infiltration and evapotranspiration (limit on built-up areas, choice of permeable materials, plantings and green roofs) (1). These green measures in cities are called ‘ecoysytem-based approaches to adaptation’ (EbA) or ‘urban green infrastructure’ (UGI) (3).

The principal palliative measure is the construction of a network of storm drains, generally connected to the sewer network. When rainfall is very heavy, these drains enable runoff water to be stored in order to regulate wastewater flow within the sewer system. The Region has more than thirty storm drains, some of which have a capacity of 30,000 m3. They are big enough to absorb floods likely to occur once every 10 years (1).

In Flanders, building plans, as well as any plan that might have consequences on water, are checked for their hydrological consequences. It applies in particular to zones prone to flooding, important for infiltration, or close to a drinking water catchment area (1). One way to deal with the huge uncertainties in the future climate change when building or revising urban drainage systems is to consider (in addition to the traditional range of return periods up to 5 years) a 20-year design storm for scenario investigation. Research has shown that a 20-year storm could become – in order of magnitude – a 5-year storm. If after a design for a 5-year storm, the 20-year scenario investigation would conclude that specific zones along the sewer system would have severe additional impacts, changes could be made to the design or flexible adaptation measures designed for the future (4).


The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Belgium.

  1. Ministry for Social Affairs, Health and Environment (2009)
  2. Results CCI-HYDR project, in: Ministry for Social Affairs, Health and Environment (2009)
  3. Gaffin et al. (2012)
  4. Willems (2013)
  5. Kreienkamp et al. (2021)