Preventing catastrophic flooding

Where do things stand in relation to flood prevention?

30. June 2022

With severe flash floods, flooding in river basins and a tally in Germany alone of over 180 people who lost their lives in torrents of water, the flood disaster of July 2021 left a trail of devastation in its wake. A year later, the question arises of where things stand in relation to flood prevention in Germany.  An interview with hydrologist Bruno Merz from the Helmholtz centre in Potsdam.

#explore: One year has now passed since the flood disaster in Rhineland-Palatinate and North Rhine-Westphalia. Will we have to reckon with such extreme events more frequently in Germany in the future?

Bruno Merz: For sure! Our data show that the frequency and intensity of local heavy rainfall is increasing. This is occurring mainly in summer and leading to floods in cities and flash floods like the one which hit the Ahr Valley. The model simulations also show that these phenomena will continue to intensify. The reason for this is climate change: with higher temperatures, evaporation increases, and warmer air can both absorb and hold more water vapour. This means that more water vapour is present in the atmosphere for heavy precipitation – with the consequences that we’ve seen.

Are our early warning systems and disaster management good enough?

Forecasting, early warning and disaster management are the responsibility of the federal states, and how they are set up varies from state to state. For example, we saw last year that our early warning systems didn’t work very well. So, one thing we need to do is to develop those systems. We also need to change the way we go about making predictions. The current situation is that predictions are made about water levels. Let’s look at an example: at level X, a certain water level can be expected in six hours’ time. In the meantime, however, there are faster computer models which can be used to make statements about which areas will be flooded and when. This is valuable information, of course, when, for example, a decision has to be made about evacuating a local hospital. The rule of thumb is that warnings alone aren’t sufficient. People need to understand the warnings and know what to do when they come. This has been revealed by the surveys we conducted with the people affected in flood areas.

„In recent decades, the focus has primarily been on technical flood protection. At the same time, we also have to reduce the vulnerability of people, infrastructure and buildings.“

Bruno Merz, director of the Hydrology Section at the Helmholtz centre in Potsdam and Professor at the University of Potsdam

How could flood protection itself be improved?

In recent decades, the focus has primarily been on technical flood protection, i.e., dikes, flood protection walls, dams or retention basins. Of course, this makes a lot of sense, but it's not enough on its own. At the same time, we have to reduce the vulnerability of people, infrastructure and buildings – for example through private building precautions. People living in risk areas first need to be informed about this flood risk, and then they have to prepare for it with appropriate measures, such as watertight windows and doors or water barriers in their gardens. Flood protection for heating oil tanks is also very important. It’s not particularly expensive, but it makes a huge difference. This really matters because, if heating oil escapes and the contaminated water gets into the walls, it will then in most cases mean that the house can no longer be restored and will instead need to be demolished.

Can and should we redesign cities and infrastructures to protect ourselves better against such extreme events?

Our towns and villages are built far too close to rivers and streams. This is just as true of Germany as it is in the rest of the world. Due to the increase in extreme events, the associated risk is growing. In critical risk areas, such as some places in the Ahr Valley, it would therefore make sense to relocate houses and businesses. This is because, once you’ve reached a certain flood level and flow rate, even watertight windows won’t protect you any longer. In Vietnam and in the US, people have been resettled after flood disasters, in some cases on quite a large scale, but in Germany this has only ever happened sporadically. Of course, relocations are politically fraught. But they would be the most effective lever to significantly reduce the risk.

Might urban development offer ways to reduce the risk of flooding?

In centuries gone by, we used to focus primarily on draining water quickly where it was a nuisance to us: we used to drain fields and straighten rivers. However, this has increased the risk of flooding elsewhere; after all, the water has to go somewhere. The sewerage systems in our cities are also designed for rapid water drainage. But they aren’t designed to handle the volume of water that comes with extreme rainfall events – nor would that even be possible. This is why experts are now turning to the “sponge city” concept. This means building decentralised water storage systems in cities: greening roofs and creating more infiltration and retention areas, and also removing impermeable surfaces. The aim is to have a city that soaks up water like a sponge and slowly releases it again. This will also make our cities greener. And because water can evaporate from these green spaces on hot days, this will also improve the microclimate in urban areas.

From your point of view, are we making fast enough progress on flood protection?

From my point of view, there’s still a problem here. Since flood disasters like the one that hit the Ahr Valley, the public and policymakers have been very aware of the issue. In most cases, however, people are just trying to improve the specific situation on the ground to ensure better protection next time around. But that is too short-sighted a perspective. After all, the next flood event will be different again and occur in a different region. Which means that we need to approach this problem more systemically and ask ourselves some fundamental questions as a society: which risks can we bear and which not? Are we prepared to accept the consequences and costs associated with investments or relocations? Countries such as Switzerland and the Netherlands are pursuing exactly this kind of systemic approach and are already a long way ahead as a result. In my view, it would also make sense to aim at the more targeted protection of critical and sensitive infrastructures such as electricity substations, retirement homes or kindergartens.

Please could you explain the concept of “nature-based solutions”? What potential do these have?

Nature-based solutions mean, for example, the renaturation of rivers and streams or removing impermeable surfaces from land areas and making agriculture more environmentally friendly. This would have a significant effect on small and medium-sized floods. That effect would, of course, be much more modest in extreme weather events like the one which hit the Ahr Valley. But these nature-based solutions would in all cases improve the ecological state of the waters. And, most importantly, they would also help replenish groundwater. In addition to more frequent floods, we’re also increasingly struggling with droughts in the wake of climate change. Nature-based solutions could counteract this trend, but, of course, they need more space and, above all, a revolution in the way societies think.


About Bruno Merz:

Bruno Merz is the director of the Hydrology Section at the Helmholtz centre in Potsdam (German Research Centre for Geosciences) and Professor at the University of Potsdam. His research focuses on extreme hydrological events, flood risk management and risk analysis.