Lake Zurich has been lowered every autumn since 1977. This year, however, is different. A look at the decisive weir.
An unusual measure: the Platzspitz weir is currently set to the peak level of Lake Zurich in summer.
Christoph Busenhart from the Zurich City Electricity Works (EWZ) is standing in front of two hand-drawn blueprints. Someone framed them and hung them on a wall of the operations building. There they seem to have fallen out of time. The mechanical components with their chains and rollers sticking out of the ground here and there also look museum-like.
But the impression is deceptive. The supposedly outdated inventory is part of a sophisticated machine that has been doing its job reliably for seventy years: the Platzspitzwehr in Zurich.
This has only two tasks, albeit important ones: On the one hand, it ensures that the lake level is as stable as possible while allowing certain fluctuations that simulate the natural course of the year. It thus prevents extreme low water and protects the population from flooding. On the other hand, it directs the lake water through the upstream channel into the turbines of the Letten power plant.
“Everything works entirely without a hydraulic drive,” explains Busenhart, clearly proud of the technology he commands as power plant manager. “We can adjust the height with just the water that we direct under the flaps.”
Christoph Busenhart from the EWZ.
However, the weir can hardly be set higher than now. The red reading on the scale on the wall is just below the maximum value of 406 meters – which is unusual for mid-December. There is a reason for this: the listed building has recently been the focus of a measure intended to help ensure Zurich’s energy security.
The lake as a battery
Russia’s invasion of Ukraine has sparked concerns about gas and electricity supplies across Europe. The situation could become critical, especially over the winter. In addition to voluntary cost-cutting measures, months ago people began looking for ways to increase domestic electricity production.
At the end of September, the Federal Council recommended in a letter to the cantons that they examine a temporary increase in the storage levels of run-of-river power plants. It turned out that not only the reservoirs in the mountains could make a contribution, but also the Letten power plant in Zurich.
The underlying idea is simple: should the electricity become scarce, energy could be fed into the grid from a kind of huge battery – a battery whose charge level should be kept at 100 percent as a precaution: we’re talking about Lake Zurich.
To this end, the decision was made to freeze the level at summer level instead of lowering the water level by a few centimeters in October, as has been the norm for decades.
What sounds like a small thing is not: every lost centimeter of lake level corresponds to around 900,000 cubic meters of water, which in case of doubt is not available to drive the turbines of the power plants on the Limmat. And once the level is low, it doesn’t rise again so quickly. At least not in winter, when the precipitation is sparse and remains as snow in the Linth catchment area in the Glarner Alps.
A long September 30th
Apart from estimating the specific benefit in terms of power generation, it was also necessary to check whether the measure was legal at all. The regulation of the sea is not a question of arbitrary decisions. It meticulously follows regulations passed in 1977.
In addition to a page with four sparsely formulated statutes, this essentially consists of a large sheet of graph paper on which a number of lines drawn with a ruler establish a binding relationship between discharge and lake level for each day of the year.
According to statute no. 4, deviations from the regulations are permitted in “emergency situations”. It was probably more about flooding than energy problems. But none of the signatories opposed it, neither the four cantons bordering the See and Limmat nor the federal government. The experiment could be continued.
As far as Lake Zurich is concerned, it has been September 30th for a total of three to four months since October – the peak of the summer regime. The computer control of the weir was fixed on this day instead of following the changes applicable for the winter.
If there is no threat of flooding, the runoff is also limited to a maximum of 120 cubic meters per second and the rainwater is retained for later use. The 120 cubic meters is the maximum volume that the two turbines of the Letten power plant can convert into energy.
In the meantime, the EWZ has calculated the yield of these measures: The ten power plants on the Limmat will be able to generate an additional three to five gigawatt hours of electricity during the critical winter months. Sixteen thousand people in four thousand households can benefit from it for three months.
measure with an expiration date
Christoph Busenhart is satisfied with the result: “It’s fantastic that we can make an important contribution.”
Marco Walser, Head of the Geoinformation and Hydrometry Section of the Office for Waste, Water, Energy and Air (Awel) of the Canton of Zurich, also rates the experiment positively. The opportunities clearly outweighed the risks and justified the deviation from the regulations. “However, we would like to emphasize that this is a temporary measure. Soon we will return to the old system.”
Marco Walser from Awel.
Because this definitely makes sense and is an expression of a long-lasting compromise between various interests: fishing, shipping, energy production, nature conservation and flood protection.
The lowering of the water level in autumn is primarily there to simulate natural fluctuations and to preserve the habitat of certain plants and animals. Even in the 19th century, the level fluctuated significantly more: by around two meters over the course of the year, with high levels in summer and low water levels in winter. This was ecologically valuable, but all the more difficult for the people who lived near the shore.
At the beginning of the 20th century, the outflow of the lake was improved by removing various mills and other installations from the Limmat and deepening the river bed. In addition, the city built a so-called Nadelwehr in 1876, which was replaced in 1951 by the roof weir that still exists today at the northern end of the Platzspitz Park.
Since then, the annual fluctuations have generally only been around 50 centimeters. These are intentional. Aside from the fact that they favor biodiversity on the flat shores of the Obersee, the more extensive level reductions in the months of December to March make room for meltwater from the mountains. The weir is an important part of flood management.
For this reason, this year’s exemption is also limited. By mid-February at the latest, water will begin to be drained in order to return to the old regime by March 15. Then, as usual, Lake Zurich should reach its lowest level.
According to Marco Walser from Awel, it is very unlikely that flooding could occur before then: “The risk of flooding is not significantly higher than in summer, since operations are still guaranteed in accordance with the regulations and winter floods tend to be less severe than summer floods. In addition, the greatest danger does not come from the lake itself, but from the Sihl, on whose alluvial fan large parts of the city were built. »
A new weir for Zurich
The Platzspitz weir is checked and serviced annually. The wooden planks under the gates are made of untreated oak. The moisture does not affect the wood, it is even good for it. “The biggest enemy of the weir is the zebra mussel,” says Busenhart. Because these sharp-edged molluscs cut through the rubber seals.
However, after more than 70 years of operation, the plant has reached the end of its life. In particular, no guarantees can be given for the steel used, explains Busenhart: “It dates from after World War II and does not meet today’s quality standards.”
A replacement building has already been arranged and, if everything goes according to plan, will be put into operation in 2027. The new weir will be more robust and more precisely controllable. Due to the requirements of monument protection, however, everything remains optically the same.