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Focus: map of the assessment and management of flood risks

The flood hazard map of the Brussels Region, which has been available since the end of 2013, shows the scale of flooding for three types of hazard (low, medium, high). As an essential information and management tool in the fight against floods, it should help to encourage inhabitants, business owners, industrialists and public authorities whose buildings or land are at potential risk of flooding, to better protect themselves against floods, but also limit constructions in flood-prone areas. This would make it possible to reduce the associated damage. The flood risk map shows that this problem potentially concerns 21% of the regional surface area, or nearly one inhabitant in three or slightly less than one worker in two.

The sewage system, the main cause of flooding

In the Brussels Region, the recorded flooding usually occurs during two main types of rainfall event: the brief but intense rainfall associated with storms in spring or summer (the flooding then affects both the areas situated at the river basin head and at the bottom of the valley and is characterised by its abruptness) or the long rains in autumn and winter (the flooding then primarily concerns the bottom of valleys and is gradual). In view of the historical overview of floods (illustrated for the period 2004-2012 below), it is more frequently the first type of rainfall event (storms) which are the cause of floods and which result in the most damage.

Occurrence and scale (in amount of flooded address points) of the last major floods which took place in inhabited areas of the BCR (2004-2015)
Source: Brussels Environment, based on surveys taken by the emergency services (SIAMU) (2004-2015), files of the Federal Natural Disaster Fund (2004-2009), data from VIVAQUA (2004-2012)

The floods generally observed were due to the sewage system being saturated (or of an unsuitable size), either directly by the overflow or backflow of the transiting water, or indirectly by the accumulation of run-off rain water which could not be evacuated. As a reminder, the sewage system is combined: not only does it evacuate waste water but also run-off rain water. However the proportion of run-off rain water is far from being negligible since it accounts for half of the water collected on average, according to estimates made by network managers and water treatment plants. It is easy to understand then that a significant increase in run-off rain water during severe rainfall events results in flooding. Moreover, in an urban environment, this increase is both in terms of volume and flow rate. Firstly, the volume of run-off rain water is high given the high rate of sealing of surfaces where it flows. Secondly, the acceleration of the flow velocity also results from the rate of sealing but also from the artificialisation of the environment (the flow is more rapid in conduits than on a surface), and even more so if the topography is pronounced.

Other types of floods have also been observed, although these are rare. These can be rising groundwater (at the bottom of the valley), or episodes of flooding from waterways.

What is the impact of climate change?

The combined historical data, due to inaccuracy or incomplete time coverage, do not provide an opportunity to identify a given trend of the occurrence of floods in the Brussels-Capital Region between 1900 and the present day.

The probable impact of climate change in the coming years will equate to an increase in flooding in the Brussels Region, both during the winter (river floods) and in the summer (sewer backflow) (Factor-X, Ecores, TEC, 2012). However the models converge on a decrease in heavy rainfall in summer. But they also show a trend towards an increase of temperatures, which could lead to increased atmospheric instability and consequently more intense storms. As a precaution, the flood risk is anticipated to rise during the summer.

The (potential) flood hazard map

The flood hazard map of the Brussels Region was completed by Brussels Environment in December 2013. It shows the sections of the territory which are potentially at risk of flooding, for three categories of scenarios or hazards:

  • An extreme scenario, with a low probability of occurring (low hazard thus), related to a flood event with a return period of 100 years (in other words likely to occur once every century).
  • An occasional scenario, or medium hazard, related to a flood event with a return period of between 25 and 50 years.
  • A frequent scenario, or high hazard, related to a flood event with a per-decade frequency (return period of 10 years).

In a flood-prone area, there is a risk of flooding, for which the occurrence and especially the consequences correlate to the intensity of the hazard: the higher the risk, the higher the hazard. However, the risk of flooding cannot be ruled out in other areas: unforeseeable events (such as a blocked gulley, pipe break, etc.) can lead to local flooding.

To demarcate flood-prone areas, four susceptibility factors pertaining to flooding have been mapped out and collated: the first one takes into account the relative altitude compared to the bottom of valleys, the second, "topographical humidity" (index which links topography and hydrological processes), a third, the drained urban surface (which includes the degree of sealing of the soils) and the fourth, the soil itself. The result obtained was cross-checked with the flooding files submitted to the Federal Natural Disasters Fund (1992-2009): these files provide information on the intensity of a given flood. The perimeter of the hazard areas could then be defined. The hazard map produced was then subject to a double check: by comparing it with other data records (SIAMU, Vivaqua) on floods which occurred between 1992 and 2012, then allowing experts to adjust it. Certain post-treatments were also applied, taking into account the remarks of the municipal services who have good knowledge of their territory. We should specify moreover that the protection offered by collective works (stormwater basins and levees) was included and helped to lower the intensity of the hazard in the areas concerned. For detailed information on the method used to develop the map, the reader is invited to consult the methodological sheet pertaining to it.

Flood hazard map
Source: Brussels Environment, version 2013

Note: map on 1/10,000 scale
See also the map 2.20 of the second water management plan which identifies the historical floods (data from the Federal Natural Disasters Fund (1999-2009), from SIAMU (1997-2009) and from VIVAQUA (1992-2012))

The flood plain of the Senne accounts for most of area which is classed as potentially at risk from flooding

A striking observation is apparent from this map: the areas potentially at risk from flooding seem to be directly correlated to the presence of the hydrographic network, as it is now or even its historical state (such as the Maelbeek valley). This is despite the fact that the primary cause of flooding is not the overflowing of waterways, but is due to the sewage system. However, this observation is not surprising given that the valley bottoms are the preferred flow and convergence routes of run-off rain water, and are also the location of numerous waste water collectors.

Moreover, the topography logically affects the size of the area which is potentially at risk of flooding. A relatively flat valley bottom is generally characterised by a vast submersible area: this is what we usually observe for the flood plain of the Senne (the broadest of its kind in the territory) or that of the Linkebeek. Conversely, the Woluwe valley, which is more enclosed, has a narrower area potentially at risk of flooding.

Soil sealing also plays a fundamental role. A valley bottom draining a predominantly green surface where water infiltrates into the soil naturally, will not be affected, a priori, by a flood. Some examples are the river basins upstream of the Molenbeek (in the presence of the Laerbeek wood), the Neerpedebeek and the Vogelzangbeek. This observation remains valid even when the drained surfaces have a notable gradient, like the river basin upstream of the Woluwe or its tributary, the Roodkloosterbeek, thanks to the presence of the Sonian forest. In contrast, a valley bottom draining highly sealed surfaces is situated in a hazard area. This is therefore the case in the western part of the city centre, as it is in the Maelbeek valley. The fact that the Maelbeek waterway flows in an underground pipe undoubtedly exacerbates the phenomenon.

The protection provided by the stormwater basins is "visible" at some points, like in the flood plain of the Senne. In the city centre, the use of the old underground pipes through which the Senne flows as stormwater basins provides protection for this area: it is classed as low hazard, but would be high hazard without this protection. The same goes for the flood-prone area at Forest, thanks to the stormwater basins of Baeck-Merill and Audi.

21% of the regional surface area is situated in a high or medium hazard area

1% of the regional territory is potentially situated in a high hazard area and is consequently affected by flooding on a recurrent basis, at least once every 10 years. And 4% of the territory is situated in a medium hazard area, and is therefore affected by flooding once every 25 to 50 years. For inhabitants and operators situated in these areas (particularly in high hazard areas), it is therefore advisable to protect themselves against these floods. As for the low hazard area, this covers 16% of the regional surface area. As a reminder, the floods concerned are only likely to occur once every century.

Percentage of the regional territory (surface area) situated in a flood hazard area
Source: Brussels Environment, based on the flood hazard map version 2013

Note: Each flood hazard area includes almost 30% of non-building land (i.e. rail tracks, green spaces, water zones).

An examination of land use carried out by the Regional Plan for Land Use (PRAS) revealed that 20% of the regional surface area which is already urbanised or could be, is situated in a hazard area. Half of these are residential areas, one quarter is high-mix areas, and one quarter is economic activity areas (encompassing urban industry, administrative, facilities of collective interest, and public service areas).

It should be specified that the flood hazard map does not provide guidance in terms of the potential water levels that could be reached in the event of flooding: there is currently no combined hydraulic model of the sewage system and hydrographic network which would make this calculation possible. Nevertheless, the water levels observed in submerged areas during past floods remained at around a few inches, and were in general lower than 1 metre (compared to ground level). In a certain sense, they can be characterised as "moderate", in comparison with other regions or countries. However, the impact can be considerable for cellars or underground infrastructures (e.g. metro stations, tunnels).

An evolving tool

The flood hazard map is an evolving tool, which needs to be updated regularly. This update may turn out to be necessary if a future flood submerges areas which are not listed as being in a hazard area or if the situation on the ground has undergone major changes, either positive or negative (significant use or sealing of soils, commissioning of a new stormwater basin, realisation of alternative management facilities, etc.). It is useful to implement the update when new data are available, such as hydraulic simulations for the hydrographic network and/or for the sewage system. The next update for the map is planned for 2016, given that the minimum update cycle stipulated in the regulation is 6 years.

Moreover, any inventory of areas affected by flooding episodes is useful to validate the map. There is a genuine need here since, as already indicated, the historical data currently available only cover a dozen or so years of observations. In this regard, the data from fire service interventions are regularly sought. In addition, a data capture tool for municipalities, Vivaqua and Brussels Environment has been designed to input the data pertaining to flooded areas (including the possibility of demarcating the area on a map), the presumed cause of the flood, a photo of the resultant water level, etc. Private individuals and associations do not have access to this tool, but they can always communicate their observations via their municipality or Vivaqua.

Assessment of flood risks

The flood hazard map has helped to identify the flood risk for certain targets (inhabitants, workers, buildings, sensitive establishments, certain economic activities or public spaces, activities, built or environmental assets, etc.). This assessment of the risks is based essentially on the location of these targets, the target being identified as "at risk" when it is partially or fully situated in a hazard area. It does not take into account flood protection measures possibly implemented at the local level, with the notable exception however of stormwater basins (as a reminder, the flood hazard map does take this into account). For more detailed information, the reader is invited to consult  the interactive map of the flood risks on the Brussels Environment website, and refer to the accompanying methodological sheet.

The potential exposure of the population to flooding has been estimated according to each class of hazard: in terms of the number of inhabitants and workers potentially affected at the neighbourhood level and municipal level respectively. Given the assumptions regarding the distribution of individuals in buildings and the non-inclusion of local protection measures (stormwater basins excluded), it is a question of potential exposure of individuals and not effective exposure. Nearly one inhabitant in three and slightly less than one worker in two are potentially affected by flooding. Although the vast majority of them are situated in a low hazard area. Nevertheless, 6% of inhabitants and 7% of workers are potentially situated in medium hazard areas; 3% of inhabitants and 4% of workers are situated in high hazard areas.

Potential exposure of the population and sensitive establishments to flood risk according to the class of hazard at the regional level
Source: Brussels Environment, based on flood risk maps version 2013

The data source for the population date from 2010 (BISA for the number of inhabitants, SITEX database for the breakdown of housing), data for workers from 2011 (National Employment Office - ONEM), data for schools and hospitals from 2013 (UrbMap), data from rest homes from 1999 (SITEX database). Educational establishments situated on the same site (same address) were recorded as one single school.

Vis-à-vis economic activity, the sites affected can be businesses or industries. As previously indicated, areas used for economic activity or high-mix situated in flood-prone areas each represent 5% of the regional territory. But by specifically examining the surfaces of industrial areas, we can observe that this percentage is clearly higher (37%). The residual industrial basin of the BCR is essentially on the Senne-Canal route.

In the case of certain industrial establishments, economic decline is accompanied by a risk of accidental pollution in the environment, given the presence of dangerous substances on the site or malfunctions in the installations (for example of the efficiency of the treatment process at the waste water treatment plants). There are 6 IPPC installations (out of the 11 within the BCR) (from the name of the directive on Integrated Pollution Prevention and Control), of the 4 Seveso sites and 2 water treatment plants. All of these sites are situated in the flood plain of the Senne.

Although the protected area for drinking water catchment at the Bois de la Cambre, the Sonian forest and the Natura 2000 network are essentially situated outside of the hazard area, the same cannot be said for other Natura 2000 sites: in the event of flooding, these are likely to receive polluted water either by overflowing of the sewage system or by contaminated run-off rain water.

Risk maps have also been drawn up to evaluate the negative consequences on mobility, in the event of flooding in sections of the transport network (road, rail, tram, metro), train stations, metro stations, or underground car parks. Unsurprisingly, the most sensitive infrastructure is underground sections and those at the valley bottoms.

Finally, the risk evaluation also looked at classified monuments and sites, as well as vulnerable infrastructure such as fire stations, police stations and high voltage cabins.

Better armed against floods

Both the flood hazard map and the risk map are tools which help to raise awareness among the population in the fight against floods. Inspired by the old adage "forewarned is forearmed", inhabitants, business owners or industrialists potentially affected by flooding have every interest in taking protective measures at the level of their building, if this is possible. In this regard, the flood guidance service of Vivaqua has been accompanying inhabitants who have encountered rising water problems in their homes since 2012. Additionally, a brochure by Brussels Environment for inhabitants on good practices before, during and after floods will be published in 2016.

In order to really be useful, these measures at the individual level need to go hand in hand with collective measures. Many of these feature in the flood risk management plan (integrated into the water management plan, which is in the process of being adopted), which will replace the rain plan. They cover the entire management cycle of a flood: prevention, protection, preparation, crisis management and redress. An alert and crisis management system therefore needs to be implemented. Furthermore, the emphasis is placed on prevention with one main objective: restoring the natural water cycle (water infiltration, water buffering in natural retaining basins, separation of rainwater/run-off water from the sewage system, etc.).

Date de mise à jour: 11/06/2020