what you need to know about the Rhine flood

The Rhine remains on orange alert to the risk of flooding this Thursday, July 15. A specialist in the French waterways (VNF) answered questions from France 3 Alsace.

With its 1,230 kilometers long, the Rhine is one of the largest rivers in Europe. And it is not always a long quiet river, as evidenced by orange vigilance to the risks of floods in Alsace.

This vigilance continues this Thursday, July 15, 2021, after a start on July 14. The fault of particularly heavy precipitation.

However, this is not an exceptional flood. Neither was the decision to fill the polder located in Erstein (Bas-Rhin, see on the map below) with water.

France 3 Alsace interviewed Vincent Steimer, director of the territorial units of the Rhine at the Voies navigables de France (VNF).

What is the frequency of these floods?

“It is a phenomenon which generally occurs rather in spring when the snow melts in the Alps: the Rhine is fed by the Alpine massif, in particular Switzerland. When the snow melts, when there is a sudden warming in the Alps. spring, that causes this phenomenon. This snowmelt sometimes combines with significant precipitation, which can further amplify the phenomenon. “

What can we say about the current floods?

“There, it is a little more unusual. We are in a strong episode of precipitation known these last days. Especially in the catchment area of ​​the Rhine, which includes Switzerland, Alsace, Germany … The precipitations are which fell heavily and swelled all the tributaries of the Rhine. Which brings us to this situation. “

This flow is around four times the average flow of the river.

Vincent Steimer, director of the Rhine territorial units at the VNF

When did it start?

“We had a first flood peak last week, with a peak at 3,000 cubic meters per second. This is three times more than the average flow of the river – which is 1,000. And here we have two new peaks of flood: one yesterday, which reached 3,800 cubic meters per second. And the news which arrives to us tomorrow, and which will reach approximately the same level. This flow is close to four times the average flow of the river. ”

How do you take your measurements?

“The Rhine is equipped with measuring stations, distributed under the bridges, on the edges … from Switzerland to Lake Constance. We record these flows, we keep traces, and we correlate all that. allows to know at any time the flows of the Rhine. Then, there are models to make forecast. All the data is shared with several interlocutors: EDF, VNF, German and Swiss services … We compare our forecasts , which makes it possible to know the situation and control the flow of the river. “

The Rhine overflows at Kehl, the German border town opposite Strasbourg.

© Catherine Munsch, France Televisions

How are you handling the situation?

“The Rhine is developed, channeled over a large part of our sector, therefore between Basel – in Switzerland – and Lauterbourg. Over these 180 kilometers, almost the entire Rhine is channeled: it is contained between two networks of dykes which allow transit these floods without the Rhine overflowing. “

“And we have the last 18 kilometers, downstream from the last dam on the Rhine – which is in Iffezheim, near the village of Roppenheim marks. Until Lauterbourg, it is called the Rhine in free current. On this part, the Rhine sees its level rise when it is in flood, and it can get out of its bed, overflow and flood the neighboring lands. “

What are the protective devices there?

“We have a curtain of high-water dikes in this area. They are built inland to contain overflows and prevent flooding in neighboring villages. The area is subject to reinforced surveillance. the Rhine is contained in dikes all year round, they are not in water, they are dry dikes. When the Rhine comes to load them, we monitor them continuously by setting up a dedicated team. “

The situation is under control.

Vincent Steimer, director of the Rhine territorial units at the VNF

How then to qualify the current situation?

“The situation is under control. On the canalized part, the structures are designed to pass this type of flood without too many problems. A certain number of measures are implemented, which aim to store water to limit the consequences. floods downstream. We do retention in the polders, like Erstein’s. “

What can we say about this Erstein polder?

“The water storage will begin this evening. We are acting on behalf of the State. And we, Voies Navigables de France, are the operator. We are in the process of maneuvering. But the impoundment is decided by Prefecture. “

What other works do you have to fight against floods?

“We have structures built on the other side of the river, in Germany. It is within the framework of an international consultation: each actor does not act in his corner. All the measures are triggered according to a pre-established plan. “

Up to what level can these works be used?

“The structures are designed to pass millennial floods, that is to say floods which have a one in a thousand chance of occurring. The current episode is a ten-year flood, therefore which has a one in ten chance of occurring. produce every year. It’s a relatively classic flood; quite large, but classic. “

Should we be worried about these short-lived ten-year floods?

“No, we have years without a flood, and others where we can have two or three episodes. For example, this year, we had two episodes. The flood peak in January, linked to a melting episode snow with precipitation. And in this month of July, it’s just the rains. In 2018, we had a flood, but not in 2019, and even less in 2020, which was rather a year of drought. is therefore quite random. “

We smooth the phenomenon over time, and it generates fewer consequences.

Vincent Steimer, director of the Rhine territorial units at the VNF

To come back to the works, what is their precise purpose?

“They have to slow down the maximum crest. When we come to retain the water at the right time, it allows for a flood that is less in magnitude than what it could have if we did nothing. has passed, we release this water that we have stored. And we smooth the phenomenon over time. It therefore generates fewer consequences. “

What consequences are we talking about?

“There were no real environmental consequences. For those on human activity, navigation has stopped. A lot of goods transit the Rhine, 20 to 30 million tonnes per year. So it is disturbing. supplying companies, particularly in Switzerland, which is supplied by the Rhine. But carriers are organizing themselves to put these goods on the train, or on trucks while the situation is restored. “

“In terms of actual floods, as the flood is contained by the embankment, there are no real consequences. We can have upwelling water tables causing flooding of cellars in homes which are close to the Rhine. . But these are not the floods that you can see on undeveloped waterways. “

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