Water level and depth Why can ships still sail on the Rhine despite the low water level?
Bonn · In Bonn, the water level is still 85 centimetres, elsewhere it has already reached zero - and yet ships are still sailing on the Rhine. We explain how the water level is calculated and what it says about the water depth.
The water levels along the Rhine have been falling for days. At the Kaub gauge in Rhineland-Palatinate, 33 centimetres were measured on Tuesday morning. In Bonn it is 85 centimetres, in Cologne 74 centimetres. And in Emmerich near the Dutch border, a historic low was reached on Tuesday: zero centimetres.
But how do all these different values actually come about? And why, despite the low water levels, can ships still sail on the Rhine without running aground?
Florian Krekel is press officer at the Rhine Waterways and Shipping Authority and can answer these questions. "The water level is a fairly arbitrary value," he explains. "Water levels can't be compared with each other just like that, and without further reference values they also don't give any information about how low the Rhine is at that point.“
Low water in the Rhine: gauge provides no information about water depth
This is because the so-called gauge staff, at the lower end of which the water level is measured, does not hang at the lowest point of the Rhine, but on the left or right bank. So at a water level of 0 centimetres, the river bed is not dry. Only the staff gauge is no longer immersed in the water.
Some of the staff gauges were set well over a hundred years ago, depending on local conditions. This is the reason for the different water measurement levels at different points. Often, the gauge was hung in such a way that even in extreme drought no minus values would occur. Now, however, precisely this case could occur: Florian Krekel expects the water level in Emmerich to take on negative values in the coming days. This is due not only to the prolonged dry phase, but also to natural and structural changes in the waterways over the past centuries.
Without further information, little can be done with the water level, Krekel emphasises. Because: "The water level is not the water depth." To determine the water depth in the so-called navigation channel, two other values are needed in addition to the water level: the so-called guaranteed water depth, which is 250 centimetres for the section between Koblenz and Krefeld, and the equivalent water level. The latter was introduced by the Water and Shipping Directorate as a comparative value and is redefined every ten years with a complicated and elaborate calculation. Among other things, sedimentation, erosion and low water levels in a given period are taken into account for the calculation.
Navigation on the Rhine is not stopped by authorities even at low water levels
In Bonn, the equivalent water level is 141 centimetres. So if the water level in Bonn were to drop to 0, the water depth in the navigation channel would still be 109 centimetres (water level - equivalent water level + guaranteed water depth).
And up to what level can ships still sail on the Rhine? "There is no value at which navigation is stopped by the authorities," Krekel explains. "To exaggerate: you can still sail for quite a long time with an inflatable boat." The decision as to whether a ship can still pass the Rhine is the responsibility of the skipper. It depends not only on the draught of a ship, but is also influenced by economic factors: because if a ship can no longer be fully loaded, a trip is often not worthwhile.
Original text: Sandra Liermann
Translation: Mareike Graepel
