Floods: “Flood protection begins at the upper reaches of the rivers”

Many areas in Germany are affected by heavy rainfall. Huge areas are flooded. Aquatic ecologist Christian Wolter explains what could be done better in interaction with nature tagesschau.de.

tagesschau.de: Some regions in Germany are currently experiencing one low pressure area after another discharging. We see flooded areas, protective measures, sandbag walls. What goes through your mind?

Christian Wolter: Of course it is always very, very problematic for the local people. But of course I always think about the fact that something like this has happened in the past and that a lot of the damage and problems we have now were caused by the fact that we have gotten into the habit of building up the river floodplains over the years .

It is missing flood plains

tageschau.de: What is the problem we are dealing with?

Wolter: The problem on site is the precipitation. The heavily compacted soils can no longer absorb water so quickly. Then the moors accompanying the rivers, which were historically drained, are missing along the rivers.

And if, for example, you look at the floodplain status report and then read that more than 70 percent of the floodplains along the rivers are no longer functional, then of course that also means that more than 70 percent of the historical retention areas where the water could flow are no longer available. This certainly creates local problems with flooding. And then the water retention in the landscape is limited, or at least it clashes very strongly with human use there.

Frost could defuse the situation

tageschau.de: Is it possible that the frost, which is now slowly coming from the north, can help a little, for example with regard to the strength of the dikes?

Wolter: I cannot judge to what extent it is enough for dikes to freeze through or at least to freeze on the surface and that this promotes stability. In any case, the water on the meadows is fixed by the frost and no longer flows into the waterways and does not lead to an increase in the flood wave. Water will run off and the frost will ensure that less water runs down. So the flood situation should ease.

The straighter the river, the faster the water

tagesschau.de: What can you do to reduce the risk of such large amounts of rain?

Wolter: What is always forgotten is that flood protection starts at the upper reaches of the rivers. We have the problem that not only the major waterways have been straightened, but also many headwaters of rivers have been straightened in order to drain precipitation into the mountains as quickly as possible. And the straighter the small backwaters are, the faster a wave of water will of course be on the downstream side, i.e. in the areas downstream. And if this happens very quickly, then in the end the losers will hardly have a chance to react.

“You would have to slow down the water”

So far, attempts have always been made to counter floods with new, more sophisticated and higher dikes. But the right solution would actually be to slow down the water in the upper reaches where the first precipitation occurs, to hold it back, i.e. to meander these upper reaches again, to allow islands in the water and side channels again.

Historically, in our latitudes the narrative that a river only has one large course does not correspond at all to the state of nature. The bodies of water were all multi-bed channels, interrupted by large pools, so that the landscape acted as a water reservoir and the water drainage was significantly delayed. And flood waves did not build up as high and did not rush through as quickly, so that downstream residents also had the opportunity to react.

The river landscapes would actually have to be revitalized on a large scale so that the runoff is delayed, not through damming, but through natural river landscape elements such as floodplains, islands and side channels.

“Don’t dam the waters, but the infrastructure and the cities”

tagesschau.de: If we now say we want to restore the rivers to nature, what price would we have to pay for it? How would our lives change as a result? What would that mean for people then?

Wolter: If we first look at the use, there is shipping on the large rivers, which in the past has worked very hard to ensure that we have these embedding channels and that certain fairway depths are guaranteed for as long as possible. A second use is the agricultural use of the floodplains and a third, very rough use is various infrastructures, cities and so on, which have also emerged in the floodplains.

And in principle, the considerations were to allow more lateral erosion on certain waterways that are no longer so important for freight traffic, so that the river can be developed back into natural structures. Then there would be no commercial shipping in these waters.

If you let the floodplains flood more, for example, if you move dikes back, you would lose local agricultural use, although it actually – to put it bluntly – almost doesn’t matter whether I compensate farmers across the board for crop failures due to drought or because the fields are flooded.

Another option would of course require a rethink. We have now diked tens of thousands of kilometers of rivers in order to protect infrastructure in the floodplains that are built on there. It would definitely be worth considering revitalizing floodplains and not dyking entire water landscapes, but rather the infrastructure that needs to be protected, the cities.

Opportunities for biodiversity

tagesschau.de: What effects can the renaturation of rivers have, other than on flood protection? Would that be good news for species protection, for example for trout, whose population has recently been classified as endangered?

Wolter: So basically this is good news for the productivity of the waters, provided that there are also floodplains that are flooded, so that we have slow-flowing areas on the surface where fish species that are weak in swimming can survive. The trout would be an immense beneficiary of natural flood protection starting in the upper reaches. It is a type of upper river, and if they become more winding, more structured, have a lot more drainage obstacles, i.e. dead wood, large stones, deep scours, wide places, narrow places, small rapids, then that would be great for the trout which are really gaining habitat again.

The second aspect is the multi-bed channels, which we have often lost on large bodies of water. In their natural state, several river courses have different depths and widths and therefore also different flow speeds, so that even when the water is low, there are still areas of still water, but also areas where we have turbulent flowing water, which then affects species such as trout, which have high oxygen requirements , is very good.

Revitalization the body of water serves the water balance

And the last aspect that is important for the revitalization of water bodies: that it not only serves natural flood protection, but above all the water balance in the landscape, that more water is retained, groundwater reservoirs near the surface are replenished, and, if possible, moors are also revitalized . That in times of low water supply, when there is naturally less rain in summer, there is still water in the landscape that can then buffer such rain deficits.

The interview was conducted by Bernd Großheim, tagesschau.de.