Integrated risk management in action: Zurich Arms itself against an Extreme Flood Event

After the floods of 2005 one thing was clear: the river Sihl poses a serious risk to Zurich in the event of a flood. In the case of a 100-year flood, large parts of the city and, including the main station, Switzerland’s most important rail transport hub, would be affected. Based on a systematic approach, the canton of Zurich is working with all stakeholders to identify all of the options available to reduce the risk.

Confluence of the rivers Sihl and Limmat
Confluence of the rivers Sihl (murky water) and Limmat at Zurich’s main station in August 2005. Only a last-minute weather reprieve prevented the Sihl from breaching its banks.
© AWEL, the canton of Zurich Office for Waste, Water Energy and Air

Text: Lukas Denzler

Our ancestors knew exactly where it was practical to form settlements. River banks and lake shores were among their preferred locations. The hydropower could be used to operate mills, the waterways provided transport routes and fish supplemented their diets. On the other hand, however, rivers and lakes can breach their banks and shores, and simply do this from time to time.

This also applies to the river Sihl, which enters the river Limmat shortly after Zurich main station, beneath which it flows. Over the course of the flood-rich 19th century, it caused extensive floods in 1846 and 1874. However, Zurich was a very different city at the time.
The last major flood of the river Sihl occurred in 1910. The flood water reached the western boundary of the city near Schlieren. “After this event, the entrances of some new houses on Löwenstrasse near the main station were built a few decimetres higher and fitted with steps,” explains Matthias Oplatka, Project Manager at the canton of Zurich Office for Waste, Water, Energy and Air (Amt für Abfall, Wasser, Energie und Luft, AWEL). This is an extremely effective measure against the risk of flooding. However, this insight was soon forgotten and, today, the entrances to most buildings are at ground level.

In 1937, Lake Sihl was dammed for the Etzelwerk hydropower plant. The power plant provides traction power for the Swiss Federal Railways (SBB) and private railway companies. During periods of intensive precipitation Lake Sihl retains water and the people of Zurich believed for decades that the risk of flooding by the Sihl was under control.

No floods thanks to luck with the weather

However, in August 2005, the situation was extremely critical. “If Lake Sihl had risen just four centimetres more, so much water would have had to have been released for dam safety reasons that there would have been flooding in Zurich,” explains Matthias Oplatka as the river Sihl was already filled to capacity at the time. Thanks to the favourable weather conditions, the city was spared. At the same time, heavy storms were raging in the canton of Bern and in central Switzerland. If the centre of the precipitation had been somewhat further east in the catchment area of the Sihl and its tributaries, the Alp and Biber, large parts of Zurich city centre and the main station would have been flooded.

In 2005, the discharge of the Sihl in Zurich was 280 cubic metres per second (m3/s) – a value that was only exceeded over the 90-year-series of Sihl discharge measurements in 1934 when 340 m3/s was recorded. In 1910, it was as high as 450 m3/s. However, at that time the riverbed of the Sihl beneath the main station was slightly lower and there was neither a station shopping centre nor underground stations below the level of the river.

Zurich – a cluster risk

Zurich has a lot of buildings and infrastructure in a small area and its main station is Switzerland’s most important railway hub. The city on the Limmat is a concentration risk in itself. A risk assessment carried out jointly by the city, canton, the cantonal buildings insurance company and Swiss Federal Railways (SBB) in 2010 estimates the damage potential from an extreme flood event with a Sihl discharge of 550 m3/s as 5.3 billion euro. Up to 3,600 buildings would be affected and four to five square kilometres of urban area would be flooded. According to experts, in extreme cases, peak discharges of between 550 and 650 m3/s would be possible.

To this would be added economic costs arising from breakdowns and interruptions to the operation of energy, telecommunications and transport infrastructure. The intensive use of basements is very common in Zurich. From a Sihl discharge of around 300 m3/s, which can arise statistically around every 30 years, the safety standards recommended by the National Platform for Natural Hazards are no longer complied with at different locations along the Sihl. Moreover, if Zurich’s main station were to be closed for an extended period, this would have huge impacts far beyond the city of Zurich itself.

“Flood protection in Zurich has a very high priority for us,” stresses Manuel Epprecht from the Flood Protection Section at the FOEN, which represents the federal authorities in the Sihl, Lake Zurich, Limmat Flood Prevention Steering Committee. The specific challenge here arises from the urban conditions. Solutions must be found for this densely populated area and implemented.

Following the floods of 2005, in which the canton of Zurich escaped lightly with damage totalling 14.5 million euro, the canton took immediate measures. As part of the construction of a new railway line, the river bed of the Sihl was lowered somewhat beneath the main station. In addition, together with partner consultants, the AWEL developed a forecast model for the Sihl discharge volumes. If a critical situation is likely to arise, it is possible to reduce the water level in Lake Sihl to create retention capacity for the expected intensive precipitation. This measure was ordered by the canton based on forecasts in May 2013 and late July 2014.

Solutions for greater protection

Further measures are planned in the medium term: from 2017 a trash rack in the river Sihl upstream of Zurich should retain driftwood in the event of a flood.

Using a 30-metre-long model of the main station on a scale of 1:30, tests are being carried out at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at the University of Science and Technology (ETH) in to establish how much water the five Sihl culverts below the platforms can actually hold. The aim is to optimise the structures of the main station and riverbed. Finally, it is planned to adapt the weir regulation of Lake Sihl in compliance with the requirements for dam safety and to exploit the available scope in terms of optimal flood protection.

However, all of these efforts may not reduce the risk in Zurich to an acceptable level. For this reason, further-reaching measures are being examined. At the beginning of the process in 2011, several interdisciplinary teams were given the task of presenting solutions – including unconventional ones.

The proposals were based on the three main ideas: ‘retaining water’, ‘diverting water’ and ‘channelling water’. The result was an overview comprising 35 solutions, which were reduced to five alternatives through an initial process. Two possible solutions ultimately emerged: the first consists in the construction of a relief channel around 20 river kilometres upstream of Zurich which would divert some of the water from the Sihl in the event of a flood; the second would involve the expansion of the pumped storage system at Etzelwerk power station.

Both variants should protect Zurich in the case of an extreme flood event. At investment costs of between 65 and 125 million euro, they are also economically viable. According to Matthias Oplatka, a decision regarding the next steps to be taken may be expected in mid-2015.

Risk analysis reveals weaknesses

The Building Department of the canton of Zurich compiled the flood hazard map for the city in 2009. Based on this, the city was legally obliged to take suitable measures in the areas of spatial planning, water body maintenance, structural flood protection and emergency planning. An implementation strategy also had to be developed within a period of two years.

“Before we were able to start the work, we had to clarify the relevant responsibilities,” says Bernhard Kuhn, who was responsible for the coordination of projects in the area of natural hazards until autumn 2014. For the implementation of the hazard maps, the city of Zurich formed a project group, which included representatives from twelve service divisions from five departments, the AWEL and the cantonal buildings insurance company (GVZ). Bernhard Kuhn identifies the urban risk analysis as an important milestone in the project. A commune must know where the greatest damage potential exists in the case of a hazard event. The results of the analysis also prompted strong support from the city’s government.

The city parliament decided to add an article on natural hazards to the city’s building code in June 2014. This regulates the consideration of the hazard map in planning applications in greater detail. The city informed the approximately 10,000 owners of buildings at risk from floods twice with personally addressed letters. “Although the personal responsibility of building owners is recognised in principle, it is not easy to gain their support for preventive measures,” explains Bernhard Kuhn. Due to their rare occurrence, floods do not feature very prominently in the memory of Zurich’s urban population.

Advising property owners

The cantonal buildings insurance company is also in contact with the property owners. The company insures all properties in the canton against damage from fire and natural hazards. “We have intensified our advisory activities in recent years,” says Claudio Hauser from the GVZ. In businesses with high turnover, you can often achieve a lot through awareness-raising, particularly when an owner realises the extent of the financial consequences that can arise from an interruption to trading due to flood damage.

According to Claudio Hauser, it is important to consider flood protection measures at an early stage in the planning of new buildings and building renovation projects. In many cases, flood protection measures cannot be carried out in existing buildings at a reasonable cost. Mobile protection measures offer a possible solution in such cases, however.

Flood-proof City Parking

Zurich’s City Parking car park is a good example for the use of mobile flood protection. The car park is located near the main station and the river Sihl. The four story car park, which is entirely underground, was constructed between 2002 and 2004. According to Richard Heierli, former Zurich city engineer and President of the building commission of City Parkhaus AG, at the time, flooding was not an issue even in this exposed location. If the water level of the Sihl and Schanzengraben had been slightly higher in 2005, the water would have penetrated the car park through the ventilation flaps. As City Parking’s managing director Andreas Zürcher explains, based on the hazard map and other information, it became obvious that the risk had to be reduced – the interruption of operation for several months would cost millions of euro in losses.

Following consultation with the GVZ, the managers decided to adopt mobile flood protection measures which can be assembled by the company’s own staff within a period of two hours. The ventilation flaps can be sealed and the areas at risk from flooding at the exit, lift and staircase at the Löwenplatz exits can be protected. However, this is of no use if the procedures do not work in the actual event of an emergency. For this reason, the assembly of the system’s components is practised every two years. The cost of these measures was only 125,000 euro.

Strengthening the risk-based approach

Manuel Epprecht from the FOEN views the various preventive measures undertaken in Zurich as exemplary and is impressed by their range and professionalism. Under the auspices of the AWEL, the most important element of this integrated risk management approach was demonstrated in a report. On the request of the FOEN, the “improved channelling” variant is being kept open in this report should the other two variants – the relief channel to Lake Zurich or the combined solution with the development of pumped storage in the Etzelkraft power plant – prove infeasible for political reasons.

As the experience in Zurich shows, the risk-based approach to flood protection must be further strengthened. It is worth investing in risk protection in locations where extensive damage can be expected.

Serious potential consequences for Zurich main station

According to the SBB Media Department, Zurich’s railway junction could be brought to a partial or complete interruption by a flood. The impact on rail travel throughout Switzerland would be enormous. Around half of the journeys made by the approximately one million passengers who use the SBB every day take place in the Zurich region.

The SBB expects that from a discharge rate of between 360 and 400 m3/s, the Sihl would breach its banks on the southern edge of the city. Between 30 minutes and three hours later, the flood would reach Wiedikon railway station and shortly after the tracks in the main station. The underground systems would also be affected. The actual flooding of the underground stations and tunnels would unfold over a period of several hours to up to one day. The new Löwenstrasse station, the Museumstrasse S-Bahn suburban rail station and the railway tunnels to Oerlikon and Stadelhofen would also be affected.

The SBB does not expect any injuries to persons in the event of a flood as an evacuation system is in place throughout the entire main station. People in the halls and corridors and in the shopping areas can be kept informed about the nature of the event using pre-recorded texts in German, French, Italian and English. Evacuation of the main station would be ordered by the senior management of the city of Zurich and the SBB’s emergency or crisis team.

Further information

Last modification 20.05.2015

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