Drops of knowledge

Water in Cities

What will it cost to protect your city against the next flooding?

Exploding Water Main Causes Flooding

Expert interview with Ole Mark

Sea levels are rising and it’s a growing challenge that many cities have to deal with. We asked our expert on climate change adaptation what needs to be done.

Please introduce yourself!

Hello, my name is Ole Mark. I’m Director at the Research Center at DHI. I’ve spend 10 years working with climate change, where I mainly focus on the impact of climate changes and how the impacts can be mitigated – with special emphasis on urban flooding.

What main challenges are cities facing when talking about climate change?

The main challenges cities are facing when we talk about climate change is the fact that either they have not been flooded for a long time, so they are ignoring the flood risk they have today, or there’s no legislations in their country telling them that they need to prepare for the future and for climate change.

Things they need to know would be, a hundred-year storm and a hundred-year rainfall: How strong will they be in the future? A storm surge: How much will the mean sea level rise? How much will the storm surge then increase in levels, and what will the impacts be on the cities?

This information is available in some format. Maybe it just needs to be re-computed into a format, which is applicable to the local city.

What is the single most important piece of advice you have for cities adapting to climate change?

The single most important piece of advice I have for cities all over the world who want to look into the future and adapt to climate change is that they need to map their risks. They need to know how much flooding will happen in the future compared to today. How much will the damages be? And then they need to figure out how much will it cost to mitigate these flood damages, which the future will bring. So they basically need to do a cost-benefit analysis between the risk in the future and the investments they need to make in order to reduce the risk.

In your experience, why do certain cities not have a climate adaptation plan in place already?

Most cities don’t have a plan. And it might be, I would say, basically for two reasons. The first reason: There’s no law, there’s no framework, no legislation, telling them that they need to make one. Because as soon as you have some legislation, then you’ll also see people make one.

On the other hand, you will see cities that are ignorant and unaware of the flood risk they have today. All cities – all over the world – have a flood risk. Somebody has designed their embankments, their pipe systems, whatever it is, for a certain return period. So if somebody has designed the flood infrastructure for a return period of ten years, well, they have a 10% risk every year of getting flooded. But many cities around the world are not aware of the fact and they simply ignore it, because they have not been flooded in a long time.

Is it possible to adapt to climate change in a way that also contributes to the community positively in the long run?

Many cities all over the world, and many people, see adaptation to climate change as something that will have a huge cost to the society. They forget the fact that when you do climate adaption in an intelligent way, there are also many positive benefits. For instance when you establish green areas to reduce the rainfall-runoff in the city, the people get a nice public park, and at the same time it reduces the flood risk and flood damages. So when you are dealing with climate change, you basically have a possibility to start climate change projects that have a good return on investment to society.

One example is in Denmark, where they had a couple of houses. The houses were flooded on a frequent basis, and then the municipality figured out that if they had to get rid of the flooding by use of concrete piles and traditional measures, then it would cost a fortune. Instead, they decided to build a small playground associated with the school. The playground had basins to store the water in, and at the same time, the basins acted as recreational environment for the kids to play in within the playground. So in this way, they constructed a playground, and that was the only construction cost to reduce the flood risk.

Which findings in your research work have left the strongest impression on you?

When it comes to my research, some of the most interesting findings are that all cities, all over the world, well, they have a flood risk. If you have a city that have designed the drainage system for a return period of ten years, well, then they have a 10% probability every year to get a flood.

What is very interesting in my research is that nearly everywhere – all over the world, this flood risk, which always will be there due to design, is completely ignored! People don’t look into the fact. We design for 50-year storms and there is a 2% probability that we are going to be flooded every year. But they don’t take that risk into account when computing the flood damages and see what they have at risk.

Can we construct flood mitigation measures, so we will never be flooded?

When we design flood mitigation measures, we need to be aware of the fact that there will always be some risk left. We can design pipes, embankments and other structural measures, which will take away a part of the flood risk. We need to be aware of the fact that there will always be some flood risk left. The last flood risk, we need to handle. And the way we do this, is for instance by building real-time information systems, so that people will have the information and emergency responses can be prepared. And in this way, we can reduce the flood damages.