The renovation of the Reuss dam is urgent

The Reuss dam had to withstand huge masses of water last week, and various weaknesses were revealed. These are now to be eliminated.

Can the Reuss Dam withstand further flooding? This is a question that caused particular concern to the residents of the Reuss plain (Reussebene) last week. The earth wall that keeps the river at bay near Hünenberg threatened to break due to the heavy rainfall. As a leak also appeared, it even had to be stabilised.

The situation is no longer precarious in the meantime, as the water masses are slowly becoming less. But questions arise. Why weren't leaks on the Reuss dam patched up sooner? And a renovation project for the Reusshalde to the Sins bridge section of the dam has been in place since 2015. Why hasn’t this been implemented yet?

New federal requirements
According to construction director Florian Weber, everything came to a standstill because the project had to be coordinated with the Flood Protection Concept on the Aargau side of the Reuss. "As part of the consultation for our project in 2017, the federal government made it a condition that there should be an in-depth coordination between the flood protection concepts on both sides of the Reuss," explains Weber. The development of the flood protection concept by the Canton of Aargau and its integration into the "Upper Reuss Valley" flood protection concept took three years. Subsequently, the canton of Zug had to revise the project on the basis of this concept. The construction project is now available. "To this end, a further consultation was carried out with the Canton of Aargau and the municipality of Hünenberg. We are currently evaluating the statements," says the construction director.

It is not yet clear when the renovation of the Reuss dam can begin. Florian Weber: "If no further revision is necessary as a result of the positions of the Canton of Aargau and the municipality of Hünenberg, we can submit the project to the Confederation."

The Federal Office for the Environment (FOEN/Bafu: Bundesamt für Umwelt) will check the correctness of the legal requirements and, if there are no more reservations, will also announce the federal subsidies. "We can only move forward to the public version after this decision. During this period, we will also apply to the cantonal council for the necessary loan via the government council," explains the construction director. "It’s clear that we are aiming for an implementation as quickly as possible."

The gross costs for the project amount to around CHF 9 million. Federal subsidies are expected, resulting in a net cost of around CHF 7 million for the Canton of Zug. The canton of Aargau does not have to participate in the current project. This also applies in the opposite case, if measures are implemented on the Aargau side.

Strengthening, increase in height and widening
The project envisages that the existing dam will be reinforced. This means that the dam will be raised and widened, the dam crown should be able to be driven on, and there should be a free area at the foot of the dam. An ecological upgrade will also take place, in particular with the widening of the Beugerank area, a new dam connection in the Reusshalde area and the optimisation of recreational use. "The project is designed to manage any runoff into the Reuss, which occurs on average once in 50 years, that is safe and will cause no damage, either both for the construction itself or for the environment," says Florian Weber.

If there is a major incident – a so-called ‘overload case’ –damage to the construction and the surrounding area would have to be accepted. "Depending on the extent of the overload, this can go as far as a dam burst and large-scale flooding of the Reuss plain." In the event of an overload incident, contingency planning comes into play. This already exists and has "also proven itself in the flood event just past".

Photo 1: The Reuss dam had to be stabilised and raised last week.
Photo 2: The water level of the Reuss was extremely high last week
Photos: Matthias Jurt (Hünenberg, 14 July 2021)

Possibility for intervention was limited
During the recent high water incident, there was the accusation that it had been negligent not to press ahead with the renovation of the Reuss dam. "The renovation is urgent," says Weber. "Nevertheless, the delays cannot be described as negligent." The dam has withstood a 30-year event virtually along its entire length, albeit with unsatisfactory security and limited possibilities for intervention.

"That's why the emergency planning came into play earlier than would have been the case with a renovated construction."

Even if there is an urgent need for restructuring, the necessary processes could not be circumvented. "The project must be developed and approved following the necessary steps. In addition, the political process is also mandatory for the granting of credit by the cantonal council."

Leaks are possible everywhere
Couldn't we have known that there were potential leaks in the dam, like the one that needed to be stabilised? "Leaks in the dam can only be detected if the water level of the Reuss is high to very high. The water pressure on the dam then becomes higher and higher, and river water infiltrates into the old dam and emerges on the land side," says Weber. If this flow rate becomes higher, more and more material is washed out of the dam, which leads to cavities. The result is that the dam will become even more unstable and could collapse. The dam crown then sinks, and, due to an overflow of the Reuss water, is washed away relatively quickly. "It is basically the case that leaks could occur anywhere in the old dam between Reusshalde and Zollhuus," explains the construction director.

Level of Lake Zug only reducing slowly
The level of Lake Zug has only reduced slightly in recent days – in contrast, for example, to Lake Lucerne. It reached a water level of 424.3 meters between Saturday, July 17, and Tuesday, July 20, and then dropped to 424.24 meters by Friday, July 23 (last reading, 1 p.m.). It’s now assumed that less water will have to be released in Cham to relieve the Reuss. "This assumption is wrong," says construction director Florian Weber. The regulation of Lake Zug has nothing to do with the Reuss outflow. "In addition, the Reuss is currently not endangered at all. All four weirs on Lake Zug are fully open." There simply cannot be more water flowing out of Lake Zug than is currently the case. "The reduction rate depends on the ratio of the inflows and outflows, as well as the size of the lake. These influencing factors are different for each lake, and the differences in the reduction rates present themselves accordingly," explains Weber.