Don’t trust flood maps blindly

23.01.2018 / Bjørgvin Thorsteinsson and Svein Ole Åstebøl

A significant number of people live in cities that were doomed to flood from the minute they were established. Other cities are caught by surprise when the weather disaster strikes. One thing is certain; in some areas of the world, the risk of flooding is increasing due to climate change. This calls for updated flood maps. 

Houston, Boston, Brisbane, Guangzhou, Mumbai, Copenhagen – the list of cities that have been affected by flooding goes on. Severe floods and extreme rain are hitting us harder and more frequently than ever before. 

In October 2017, respected medical journal The Lancet released a study showing there were 797 extreme weather disasters in 2016 – a 46% increase from 2010. 

With temperatures rising, we will keep seeing more extreme weather. In some parts of the world, the water table will fall and droughts will become more frequent. In other parts, rainfall and storm surges will increase in strength and frequency, storm water will increasingly fill our compact cities, and rivers and creeks will flood more often. Still, most cities are taken by surprise every time the water hits them.

In the United States, Houston is one of its hardest-hit cities. The problem started almost the moment the city was founded in 1836. Situated on low-lying marshy terrain, it is highly exposed to extreme weather conditions. It is also extremely flat so floodwater has nowhere to go. During its first century, the city experienced a hundred floods, giving new meaning to the widely used expression '100-year flood'.

In 2016, there were 797 extreme weather disasters - a 46% increase from 2010. 

Bjørgvin Thorsteinsson Senior Project Manager, COWI

Calibrate model up against reality

During the summer 2017, storm surges hit Denmark twice in two weeks. Subsequently, COWI's analysis showed that an extreme storm flood leading to a three-metre increase in water levels will occur every 300 years – not every 1,000 to 7,000 years as previously expected.

The trust we have in existing models is evident. Yet, it seems, we might trust them too much. 

Let us zoom out a bit. When you make a flood map, you start by producing a purely theoretical model based on other models and existing data. To ensure your assumptions and input are correct, you need to calibrate the model up against reality.

This is often where it stops; calibration incurs higher costs and requires a significant amount of time – you have to wait to see how different real life scenarios fit your initial theoretical model.

Outside the safe zone

Engineers and consultants are becoming more aware of the models’ limitations. For example, some models do not consider the storm water, leaving municipalities with a surprise when they realise that certain areas flood even though the flood map said they would not. 

This was the case when we were asked to control and improve the flood maps for a small city in Norway hit by extreme rain. We asked one local citizen whether the flood has affected his property - the flood map clearly stated that it was in the safe zone. He pointed at the contours of a dusty mud line in the basement of his house showing exactly how high water level had been. 

This is why it is crucial to calibrate the models and continuously update our existing flood maps. We cannot trust all data retrieved in the distant past. 

The muddy truth of storm water

When it comes to extreme weather, reality usually hits you harder than expected. It might hit you faster than expected too. In Norway, we will see a 50% increase in the intensity of rain in the next hundred years - but, as we have seen in Houston, a theoretical 100-year flood does not only come every hundred years. 

When the power goes, it might even become a bit cosy for a while – you can light up a candle and enjoy a moment offline with your family. When a flood hits, however, taking the clean water supply and electricity with it, there is no cosiness. The blue watermarks indicated on a flood map are rarely blue in reality – they are a mixture of our own wastewater, sewage, soil, garbage and, in many cases, dead animals and bacteria.

This is why we must stay one-step ahead of the weather prognosis. Thoroughly calibrated flood models may require significant investments of time and money, but it will surely cost more if we are not prepared when heavy rain and storm surges hit, and the consequences flood our cities.