KARLSRUHE, Germany — It is one of the tightest and fastest bends in the Rhine River, a nearly 90-degree curve where a small rocky island dotted by trees further narrows the shipping channel.
Engineers, surveyors and craftspeople have spent months surveying the area, measuring the flow and speed of the water, which is so clear that the contours of the riverbed and its sediment are visible.
But the team does not need boats to do their work. Instead, a short walk through a warehouse gets them to all spots on the river, laid out before them on a 4,500-square-foot scale model resembling a hyper-realistic miniature railroad set, but for boating enthusiasts.
This million-euro model is no child’s play. It is a part of one of the Europe’s most ambitious and expensive experiments in climate adaptation, a 180 million euro effort to save the Rhine’s historical role as a vital transport link for Germany’s economy from the adverse effects of climate change.
How nations can mitigate the escalating impact from a changing climate features prominently in discussions in Sharm el-Sheikh, Egypt, where world leaders are meeting at this year’s U.N. climate conference, COP 27.
For Germany, that debate has already begun in earnest as higher temperatures and longer droughts have taken an increasing toll on its economy by making already difficult natural bottlenecks on the Rhine practically unnavigable more often and for longer stretches of time.
The challenge of the riverbed “optimization” project, as it is known, is to deepen the river at those critical points — without causing unwanted side effects.
“Across all modes of transportation, there is no other measure that makes as much sense — both in terms of return on investment and for the economy as a whole — as optimizing the riverbed,” said Oliver Luksic, a state secretary in the federal transportation ministry.
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Floods and droughts have long been part of the Rhine’s history, demonstrated by the “hunger stones” — engravings from the Middle Ages far below the water’s surface that marked historically low levels. They have emerged along with other artifacts recently, serving as something of an alarm.
“Low water is not a new phenomenon, but it will happen more often and last longer,” Mr. Luksic said.
These recent low-water events have had a distressing impact on Germany’s economy. Up to 80 percent of Germany’s ship-bound cargo — everything from coal, wheat and cars to containers from China — are transported on the Rhine on their way to not only the south of the country, but also to parts of France, Switzerland and, over the Main-Danube Canal, Eastern Europe.
In 2018, when water ran low for weeks, it cost the German economy an estimated 0.3 percent, or about 12 billion euros, according to Carsten Brzeski, a chief economist at ING Germany. This summer’s drought cost the country approximately 8 billion euros because the low water blocked shipments of coal needed to generate electricity.
As it has become painfully evident how important the functioning river is to the economy, the deepening of the Rhine, long treated as a purely technical problem, has become a serious political issue for Germany.
Though investments are likely pay off in the medium term, the optimization of the 30 mile stretch does not solve the underlying problem of the low water in the Rhine, the work only takes care of the most obvious bottleneck.
In the western part of Germany, what is known as the “middle” Rhine flows past villages, castles and mountainside vineyards. The area is so culturally rich that UNESCO declared it a World Heritage Site in 2003. Tourists, both domestic and international, flock to the area. On the water, commercial freighters compete for space with sleek river cruise ships and colorful day-tour boats.
But the bucolic scenery above the water line betrays how treacherous the river can be as it descends fast and shallow through the hills. Climate change is exacerbating the problem of this bottleneck, which is most treacherous in a 30-mile stretch near the village of Kaub.
The optimization project is focused on making six distinct spots in this critical passage just as deep as river sections above and below the stretch.
In the actual village of Kaub, a yard-high digital display on the side of an ancient tower on the banks of the Rhine marks the single most important number in German logistics — a real-time readout of the river’s depth at the tightest spot.
When the number dips below 78 centimeters — meaning the natural river water is only two and a half feet deep and the artificial shipping channel is less than six feet deep — transportation is essentially blocked, and only small ships with very limited cargo can pass.
But deepening the river without causing adverse side effects is not easy, said Andreas Schmidt, who runs a department of 125 people at the Federal Waterways Engineering and Research Institute and is overseeing the project.
“We can model highly complex three-dimensional flows numerically, but once you add the movement of sediments, you quickly reach the limits,” he said.
Using their replica of the river and a thoroughly tested scale substitute for the actual sediment and gravel at the bottom of the Rhine, Mr. Schmidt and his team go about testing ways to deepen the channel.
Just cutting into the rock or digging into the sediment without any countermeasures will not necessarily deepen the river, since the water will simply fill out the new channel and run lower.
So the team is experimenting with the idea of sinking three giant concrete sills to the river floor, which will raise the water. The risk is that the modified river bottom will simply cause a buildup of sediment downstream and block ship traffic anew. Even if it works, the fix would solve a problem at just one of six spots on the stretch of river that need to be climate-proofed. Nothing is certain.
The Rhine has been artificially straightened, deepened and otherwise manipulated for hundreds of years as the many artificial structures visible in the river attest. But since the last major work was competed in 1995, the number of stakeholders has increased, said Sabine Kramer, the project manager.
Not only are environmentalists, business owners and civic leaders consulted on the changes, but, given the region’s special status, UNESCO must also be consulted before work starts. Besides the technical challenges, it is one of the reasons planning the project, which is scheduled to be completed by 2035, has been so onerous and slow.
Mr. Schwanen, who represents inland navigation, says that given the importance of the stretch of river, the preparation work is taking too long. He believes that recent blockages caused by low water could have been prevented if the project had been given higher priority earlier.
“We’ve known for 20 years that low water events lead to blockages at this one spot,” he said. “It’s time we get this done.”