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Urban flooding slowly becomes a priority issue for Switzerland

Children have fun on a playground surrounded by the water of Lake Neuchatel in the port of Cudrefin following heavy rainfall in recent days on Friday July 16, 2021 in Cudrefin in the canton of Vaud.
Children have fun on a playground surrounded by the water of Lake Neuchatel in the port of Cudrefin following heavy rainfall in recent days on Friday July 16, 2021 in Cudrefin in the canton of Vaud. KEYSTONE

Climate change is pushing Switzerland to change the way it handles urban flooding. As flood risks grow, cities and towns are implementing innovative ways to mitigate the risks.

For many older people in Melchnau, a 1500-inhabitant village in canton Bern, the year 1986 will never be forgotten. They all went through what could be called “a collective trauma” to overcome the flood damage.

Christian Eicher, a retired civil engineer who spent much of his career working on flood control and urban drainage, told SWI that according to historic news records, on June 20 that year, the local area received an extremely heavy thunderstorm. “The streets of Melchnau flooded, as more than 50mm of rain fell per hour. Houses and meadows flooded and roads were ripped open. The major urban throughfare was swamped by up to one metre of water.”

“In 1986, the local government had little experience in preparing an urban flood-management plan and had no idea how much its implementation would cost. Despite this, an early Risk Map was commissioned which also included extreme-runoff flow estimates – but there was no concrete follow-up to reduce the flood flows or to protect against,” Eicher explained.

Only about twenty years later, in 2007 and again in 2010, heavy thunderstorms with extraordinary rainfall volumes, some combined with previous rainfall and consequently saturated soils, led to widespread flood damage and traffic disruptions. Eicher followed up on the two floods, documented them by taking photos and reported to the municipal authority with some preliminary suggestions for a possible course of actions to show how frequent and devastating the floods have been over the past three decades.

Melchnau 2019 / 1986
Melchnau 2019 / 1986 Mobiliar Lab for Natural Risks / BVE, Tiefbauamt Bern
Melchnau 2019 / 2007
Melchnau 2019 / 2007 Mobiliar Lab for Natural Risks / Christian Eicher
Melchnau 2019 / 2010
Melchnau 2019 / 2010 Mobiliar Lab for Natural Risks / Christian Eicher

Urban flooding is a global issue

Melchnau is not an isolated case in Switzerland. Over the last 40 years, four out of five Swiss municipalities have experienced flooding, about two-thirds (62%) of buildings are exposed to surface runoff, and one in seven people in Switzerland live in a building exposed to flooding, according to the Mobiliar Lab for Natural Risks at the University of BernExternal link.

Floods are considered by Switzerland as a “secondary” perilExternal link, defined as independent, high-frequency, small- to medium-sized events that are more difficult to monitor than hurricanes and earthquakes. But this doesn’t mean that they don’t cause substantial damage.

The Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) has been systematically recording and analysing flood damage since 1972. According to a report published in 2021External link, “floods are the natural hazard with the highest amount of damage in Switzerland”. A flood in August 2005 caused the most damage recorded in the WSL damage database – around CHF3 billion ($3.3 billion).

Climate change has pushed urban flooding to the forefront of local governments’ agendas and led to innovative solutions being implemented worldwide. A 2022 paperExternal link in the scientific journal Nature Communication says that 1.8 billion people, or 23% of the world’s population, are living in areas directly exposed to a 1-in-100-year flood risk. According to reports from the United Nations Office for Disaster Risk Reduction, floods caused global damageExternal link amounting to an estimated $651 billion between 2000 and 2019 alone; the number of major floodsExternal link has also more than doubled during this period, from 1,389 to 3,254. Notably, losses from these events are expected to increase by a factor of 20 by the end of the 21st centuryExternal link.

Current dominant strategies to combat urban flooding

Australia has long been known for its flood-control measures and an innovative water-management approach since the 1980s known as Water Sensitive Urban Design. This strategy reduces stormwater flows and increases soil moisture through decentralised infrastructures such as green roofs, permeable pavements, constructed wetlands, and rainwater tanks. Similar concepts elsewhere include the United Kingdom’s Sustainable Urban Drainage System, the United States’s Low Impact Development, and Singapore’s Active, Beautiful and Clean Waters Programme.

In Denmark, Copenhagen is taking a novel approach to preventing repeated flooding. The ongoing SkybrudsplanExternal link, or Cloudburst Management Plan, costs €1.8 billion (CHF1.76 billion) and aims to transform the city into a “sponge”: the idea is to redesign public spaces and infrastructure so they can absorb and retain vast amounts of rainwater, much like a sponge would, during periods of intense rain, and release it back into the water cycle.

In China, 70 rapidly growing citiesExternal link have also adopted the “sponge city” concept to manage water flow and reduce the risk of flooding.

Prioritising urban planning

Compared to these countries, Switzerland’s implementation of flood-management strategies has lagged behind. It was only in 2018 that the Federal Office for the Environment (FOEN), together with the insurance industry, began developing and publishing the first Swiss-wide surface-runoff hazard map.

According to the Swiss authorities, Switzerland’s annual financial investment in flood protection infrastructure remained between CHF50 million and CHF230 million between 1970 and 2008. Since 2008, the costs have significantly increased, fluctuating between CHF250 million and CHF400 million. However, this is nowhere near enough to offset the damage caused by a rare one-off flood. A 2020 publication by the Swiss Federal Office for Civil Protection (FOCP) shows that a very rare flood could cause damage in excess of CHF10 billion.

Switzerland's financial investment in flood protection infrastructure
Switzerland’s financial investment in flood protection infrastructure Kai Reusser /

Switzerland is a few years behind, but climate change is bringing its own set of challenges to Swiss urban planners and urban stormwater managers. “In Switzerland only in recent years it has been realised the significant impacts that pluvial flooding can have to infrastructure and socio-economic activities,” says João Leitão, the head of the Urban Floods and Hydroinformatics group at the Swiss Federal Institute of Aquatic Science and Technology (Eawag). Despite that in general the damages caused per pluvial flood event are smaller than those caused by fluvial flooding, in the long-term the cumulative impacts of pluvial floods can be comparable to those of fluvial floods due to their higher frequency.

The “growing pains” of Swiss cities and settlements also exacerbate the risk of urban flooding. Construction must keep pace with population growth and urbanisation. According to the World BankExternal link, Switzerland’s urban population nearly doubled between 1960 and 2022. Currently, almost three-quarters of the Swiss populationExternal link live in urban areas, and around 80% of the country’s economic activity is concentrated there.

Based on what he has observed in Melchnau, Eicher says that the increase in impervious surfaces reduces infiltration. “With concrete and asphalt covering areas that were once given over to grass and soil, the water from heavy rains has nowhere to go, resulting in flooding,” he says. “Intensified agriculture, increased mechanization and heavier machinery are compacting soils and reducing the important `sponge` effect of the so-called pervious surfaces. The result is more, earlier and faster runoff from surfaces that are supposed to retain most of the rainfall.”

The growing concentration of population and infrastructure in urban areas exposes the country to even greater financial losses from floods. According to Robin Poëll, a spokesperson for the Federal Office for the Environment, around 20% of the Swiss population currently lives in areas that are at risk of flooding due to their proximity to rivers and lakes. About 30 % of jobs and a quarter of Switzerland’s material assets (CHF840 billion) are located in these areas.

Given the probability of occurrence and damage potential, “flooding has become one of the dominant risks in Switzerland, and flood protection is a high priority on the Swiss agenda,” says Poëll.


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Complex distribution of responsibilities for flood-risk management

While the Swiss government is increasing investments in flood-protection infrastructure, Swiss researchers are recognising that traditional approaches to urban flood management alone may no longer be sufficient to ensure flood resilience. They’re now exploring novel strategies to combat urban flooding.

The Mobiliar Lab for Natural Risks launched a research initiative in 2018 entitled Flood-Risk Research Initiative – From Theory to Practice, which aims to develop a flood-dynamics tool and a flood-damage potential tool capable of warning not only of upcoming floods, but also of their effects. These include the impacts of rising and receding floodwater on populations, workplaces, buildings, and roads.

“For the first time, the potential damage from flooding can be visually identified throughout Switzerland – down to the neighbourhood level,” says Andreas Zischg, a professor at the University of Bern who participated in the project. “We can see when, where and how many people might need to be evacuated, and when and where roads might become impassable,” he says. “This information can be used by local civilian command teams, insurance companies, logistics companies and others to aid in risk communication, training and operational-level planning.”

In 2019, Leitão and his team developed a radical market-ready device called CENTAUR (Cost-Effective Neural Technique to Alleviate Urban Flood Risk) that can be easily installed in existing drainage systems to reduce local flood risks in urban areas.

It is designed to optimise the use of existing available in-pipe capacity by combining sophisticated computational techniques with flow-control devices to attenuate and store water during periods of high flood risks.

To date no Swiss city has adopted it, although it has been installed in Coimbra, Portugal’s fourth-largest city, and a few municipalities in the UK. Leitão says that the complex distribution of responsibilities for urban pluvial flood-risk management in Switzerland and the co-existence of different drainage system types in newer and older parts of cities, may explain the difficulty to explore novel pluvial flood management solutions.

As in other countries with federal political systems, such as Germany or Austria, Swiss flood-risk management is characterised by a complex distribution of responsibilities across different levels and sectors of government. In addition, the introduction of an innovative approach to water management and hydraulic engineering touches on areas such as urban planning, energy, nature conservation, and many others.

But things are moving: a few cities in Switzerland have already begun to introduce the sponge city concepts in new projects to mitigate flooding. Leitão cites the VSA (Swiss Water Association) sponge city project that promotes the use of blue and green infrastructure in Switzerland to reduce flooding and stormwater discharges as well as improving ecosystem services, such as reduce heat, increase biodiversity, etc.

When he stops by the river in Melchnau, Eicher can now see changes. In 2021, the municipality decided to invest more than CHF4 million in a comprehensive flood-protection project, including the construction of four retention facilities and the widening of the stream channel to reduce the risk of creeks overflowing their banks. “Better late than never,” he says.

Edited by Virginie Mangin/gw

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