Today, the Federal Office of the Environment, FOEN, published its study “Extreme Flooding of the River Aare”. Their findings also include a re-evaluation of the flood risk to nuclear installations located on the Aare. ENSI will now require those nuclear installations concerned to update their safety cases.
Since 2013 ENSI has been cooperating with the Federal offices for the Environment FOEN, Energy SFOE, Civil Protection FOCP and for Meteorology and Climatology MeteoSwiss on the project “Extreme Flooding of the Aare and Rhine (EXAR)”. Under the guidance of the FOEN the main study was initiated in 2016 in the aftermath of the Fukushima reactor accident to improve the assessment of dangers resulting from extreme flooding of the Aare.
The new hazard analyses based on the latest state of science and technology have now been published. An initial assessment by ENSI indicates that: the flood levels reported in the study for the 10,000-year event would be safely managed by the existing safety margins of the nuclear installations along the Aare. However, detailed clarification is required, especially for river bank erosion.
Based on the “Extreme Flooding of the River Aare” study, ENSI is requiring the nuclear installations located on the Aare (nuclear power plants Beznau, Gösgen and Mühleberg as well as PSI and ZWILAG) to update their safety cases in respect of flooding.
The press release wording:
Bern, 22/02/2021 – The study “Extreme Flooding of the River Aare” provides the basis for assessing the risk of flooding for extremely infrequent events. Detailed flood hazard analyses have been conducted for five sites, including the Mühleberg, Gösgen and Beznau nuclear power plants. This provides the authorities and installation operators with a sound foundation for improving the protection of critical infrastructure against extreme flooding.
In March 2011, the Fukushima nuclear disaster was triggered by a tsunami. As a consequence, a number of federal agencies decided to develop a coherent basis for assessing the hazard posed by extreme flooding of the River Aare. Participants included the Federal Offices for the Environment FOEN, for Energy SFOE, for Meteorology and Climatology MeteoSwiss, for Civil Protection FOCP and the Federal Nuclear Safety Inspectorate ENSI. The results were presented to the media at a background discussion on 22 February 2021. Thanks to the study “Extreme Flooding of the River Aare”, coordinated by the Federal Institute for Forest, Snow and Landscape Research (WSL), data on hazards up to and including 100,000-year flood events are now available for the catchment area.
Precipitation and flow rate time series over almost 300,000 years
For 50 sites, it has been possible to simulate precipitation and flow rate time series on an hourly basis for a period of almost 300,000 years. In shows that, in the event of extremely rare precipitation events (recurrence period of 100,000 years), peak flows of more than 7,000 m3/s are possible upstream of the Aare’s confluence with the Rhine, or twelve times the mean flow.
The study took into account various factors that could influence the flow of the Aare: landslides, blockages at bridges due to drift wood (log jams), riverbank erosion, dam failures or human error in the operation of the weir systems. In addition, expert assessments, calculations and simulations of natural processes and civil engineering structure failures were drawn upon. These are, however, subject to uncertainties, the extent of which has been quantified as far as possible.
As part of the study, a local risk assessment was carried out for the nuclear power plants at Mühleberg, Gösgen and Beznau, for the site of the Paul Scherrer Institute (PSI) with the interim storage facility for radioactive waste (ZWILAG) and for the city of Olten as an important transport hub (see box).
Incorporation of new findings into the risk assessment for installations and infrastructure
The catchment area of the Aare covers some 43 percent of the country’s total area and includes some of the most densely populated regions in Switzerland. In addition to the five sites studied, there are 19 dams together with national transport infrastructure along the course of the river and its tributaries. The new findings on the risk of extreme flooding provide an important basis for their risk assessment.
The FOEN has presented the results of the study to the cantons and national infrastructure operators. The competent authorities will include the new findings in the risk assessment and in the implementation of flood protection measures. According to ENSI, the water levels relevant for the safety assessments of the nuclear power plants are in a similar range to those used in previous analyses. Nevertheless, ENSI will ask the operators of the nuclear plants to revise their safety assessments on the basis of this study, as provided for in the regulations. The SFOE, for its part, will ask the operators of the dams subject to federal supervision to review the existing evidence of flood safety in the light of the new findings.
Under the guidance of the FOEN and SFOE, the methodology developed for the catchment area of the Aare is now being applied to the catchment areas of other large watercourses. The results will then be made available to the competent authorities and facility operators with the aim of reviewing and, if necessary, improving the flood protection of critical infrastructure when extreme events occur.
Extreme flooding: Hazard analysis for five sites
The following flood heights of the sites of the nuclear power plants refer to the sites of emergency buildings located near the reactor building.
Mühleberg nuclear power station (final shut down in 2019): In the event of a 1,000-year flood, the site would remain dry. In the event of a flood recurrence period of 10,000 years, the plant site would be under 18 centimetres of water. On the other hand, a 100,000-year flood would flood the site to a depth of almost one metre.
Gösgen Nuclear Power Plant: For the 1,000 and 10,000-year floods, the blocking of the pedestrian walkway in the immediate vicinity, that is, its partial jamming with drift wood, would be relevant. The flood heights at the emergency building would be 5 and 65 centimetres. If a 100,000-year flood event occurred, the water depth would reach approximately 1.15 metres.
Beznau Nuclear Power Plant: Here too, a decisive factor contributing to flooding of the power plant site would be log jams caused by drift wood. In the event of a 1,000-year flood, the site would remain dry. A 10,000-year flood event would flood the emergency building to a depth of 38 centimetres and a 100,000-year flood event would flood the building to a depth of about 1.1 metres.
PSI site, Villigen: All rainfall-triggered flood events could be drained away by the River Aare without site flooding occurring. Flooding would only occur if the 200 metre long PSI steel-reinforced concrete bridge were to be blocked by drift wood during a 100,000-year flood event. In such a case, it would be expected that the site would flood to a depth of 20 to 25 centimetres.
Area around Olten train station: The site would only be flooded if a 100,000-year flood event coincided with bridges being blocked by a log jam. The flood would begin 1.8 kilometres above the Trimbacher Bridge. The industrial site would then be partially submerged under 2 metres of water. The area around the station would also be flooded up to a depth of 3.1 metres.