Considerations on the flood risk map: should we scout the light blue areas?

In my own scouting experience, I became quite frustrated that some of the more promising areas of Brussels (many industrial buildings!) are also marked as potential flood zones by the Region’s map of flood risks. The mighty @Lee took the initiative to write to Bruxelles Environnement to ask for more clarifications, and so I took a break from my break to read their reply and the map’s methodological note with some care. Here is what she and I learned:

  1. The map is a map of probability of flooding, abstracting from the risk associated to the impacts of floods on human life.
  2. Most floods in Brussels are pluvial (rain overflows the sewers network). Fluvial floods (the Senne overflows from its bed) are low probability.
  3. The colors on the map are roughly and unscientifically correlated with frequency of floods as follows: dark blue = floods on average every 10 years (if you plan on living in The Reef for 50 years, you are getting flooded at least once with probability 99.5%) ; bright blue = floods on average every 25 to 50 years (living in The Reef for 50 years, you are getting at least one flood with probability 63%); light blue = floods on average every 100 years (living in The Reef for the same 50 years, you are getting flooded at least once with probability 40%).
  4. The light blue area is by far the largest.

These numbers are clearly unacceptable.

However, the risk map states that risks are computed without taking into account local, building-level protective measures.

So, the algorithm for scouting for me should go like this.

  1. Ask the architects: what would it take to protect a building in the light blue area so that the effects of the 100 years flood will be mitigated in such a way that only one in 10 such floods (so, 1,000 years floods) will cause damage? What about one in 100 (so, 10,000 years floods)? To give you a point of reference, in the Netherlands the flood control measures are required to provide for 1 in 100,000 years to 1 in 4,000 years. With a 1 in 1,000 years protection level, the probability of living in The Reef for 50 years and getting flooded at least once is about 5%. With a 1 in 10,000 years protection, it is 0.5%. This should probably err on the side of caution, because the map is built on the basis of (mostly) pre-climate change data. We can look forward to more frequent and likely floods in the near future.
  2. If such waterproofing is technically possible and financially realistic, we can then scout the light blue area.
  3. Otherwise, flood areas should probably be out.

Going back to my break!


Hi, we live in a flood zone (light blue) and every year water enters our basement garage from the groundwater table. The same thing happened recently opposite in the cellars of the coffee shop (Phare du Kanal).