Almost all Swiss communes now have hazard maps. The task now is to use this scientific information to plan the measures necessary to protect the population and important material assets and to adapt spatial development. A walk through the commune of Ollon in the western Swiss Alps reveals what is involved here.
Text: Cornélia Mühlberger de Preux
Anyone who walks through the Ollon would not suspect in a million years that the ground here is moving. This hazard cannot be detected with the naked eye. The presence of a few protective structures is the only thing that might arouse suspicion.
Ollon extends from the Rhône plain to the 2112-metre-high Chamossaire peak. The commune consists of 23 villages and hamlets. Old chalets stand serenely beside the newer holiday homes.
A broad spectrum of natural hazards
The area is beautiful and the view magnificent. However, caution is advised as the region, or at least a part of it, is at risk from the entire spectrum of natural hazards – landslides, rockfall, sagging, floods and avalanches. “We even have areas in which three hazard types overlap,” reports Pierre-Alain Martenet from the communal building and planning authority.
As our expert guide explains, a major landslide occurred near Les Tailles close to La Saussaz in the 1970s. Between 2004 and 2007, the commune had hazard maps compiled – long before the canton of Vaud undertook the systematic mapping of the canton in 2008/09. Several protective measures were implemented as a result: sliding slopes were stabilised and areas at particular risk were designated as prohibited for development. Today, no new buildings can be constructed in the area of La Saussaz – despite the fact that the terrain has been secured and is under constant monitoring.
Yellow, blue red
Before we climb the hills, Pierre-Alain Martenet opens the hazard maps for two parts of the commune which are particularly exposed to risk: La Saussaz and Arveyes. Yellow, blue and red areas feature prominently on the maps. In the red zones, the risk is classified as “high” which is why the construction of new buildings is prohibited there. People can continue to live in existing buildings as long as an evacuation plan has been defined.
In the blue zones (“medium risk”), special measures must be taken during the construction of buildings. For example, basements must be constructed in solid blocks and reinforced concrete and adjacent sites must be systematically drained. Simple measures, which property owners can carry out themselves to limit any possible damage, usually suffice in the yellow zones (“low risk”).
In addition to the hazard level, the maps also provide information about the extent, intensity and probability of occurrence of the individual hazard types. “The hazard maps are an indispensable tool both for protecting the population and infrastructure and for limiting the damage caused by natural hazards,” explains Bernard Loup from the Landslides, Avalanches and Protection Forest Section of the FOEN. The risk does not depend solely on the hazard itself and is mainly determined by the use made of the vulnerable areas. The more densely developed, inhabited and used these areas are, the greater the damage potential and, hence also, the risk. For this reason it is important to control developments at an early stage in the spatial planning.
“The compilation of the hazard maps requires a lot of time and the involvement of numerous partners,” says Pierre-Alain Martenet. To cover the entire canton of Vaud, around 12,000 maps were compiled, of which 20 cover the commune of Ollon. In addition to the communes, 32 consultancies specialising in geology, water and snow were also involved in this comprehensive project.
Spotlight on Arveyes
We reach Arveyes. At the lower end of the hamlet, there are around a dozen buildings, including a farm. The ground here is unstable too. There are numerous deep permanent landslides, and springs at the foot of the slope point to the presence of groundwater. In the hamlet itself and along the road, water is pumped out of the ground all year round using several pumps, which penetrate to depths of 30 to 60 metres. It has been possible to limit the extent of the ground motions with the help of this system, which was installed in the 1980s.
The Arveyes area is currently classified as a ‘planning zone’. All settlement development has been stopped for the time being until further clarification can be obtained. This should make it possible to pinpoint the risk as accurately as possible and estimate potential risk developments. Depending on the outcome of this process, it will be decided whether sites here can be built on or not. Moreover, the future management of this zone must be defined in a model regulation.
The results will be available between 2016 and 2018 at the earliest. “The assessment of the landslides takes a lot of time,” explains Pierre-Alain Martenet, “as they move at a rate of centimetres.” There is a lot at stake: the affected sites are very much in demand – firstly because they are very accessible and, secondly, because they provide an amazing view of the surrounding landscape. “This can give rise to conflicts. But the law must be applied: all known risks must be avoided,” says the expert.
Limited development potential in La Saussaz
As opposed to this, in La Saussaz, which we have now reached, changes in land use are a thing of the past. Certain plots have been designated as prohibited for development and cannot be built on today. Buildings located in the red zone cannot be extended and houses that are demolished cannot be rebuilt.
In Les Tailles, in contrast, it was possible to stem the hazard risk with the help of a large retention basin. Since 2011, the basin has been collecting material conveyed by the stream of the same name. The downstream area has been reclassified from a red to blue zone. It was possible to secure the existing buildings and sites that remain empty can now be built on again.
“The risks in Ollon could be reduced considerably with the help of technical measures. However a residual risk remains,” stresses Bernard Loup from the FOEN. The most efficient measure for the prevention of risks consists in avoiding construction in endangered areas. If buildings are constructed there despite the risk, possible damage can be limited through the use of resilient structures. “The safety of the population can also be improved through the development of an emergency plan,” he adds.
Leaving nothing to chance
The definitive hazard maps for the entire communal area of Ollon are now complete or will be soon. What needs to be done now is to inform the population about the situation and integrate the information about the identified natural hazards into the communal structure and land-use plans.
According to the Federal Act on Hydraulic Engineering and the Federal Act on Forest, the cantons are obliged to compile hazard maps and to take the information they contain into account in their structure and land-use plans. “Hazard maps are indispensable tools for controlling the development of risks in hazard areas,” confirms Roberto Loat from the FOEN’s Risk Management Section. They enable the authorities to limit the construction of new buildings in such areas or at least ensure that any building activity and land use there is hazard-appropriate. And they draw the attention of property owners in hazard zones to the fact that they would do well to increase the safety of their buildings by implementing protective measures.
According to the FOEN expert, building regulations will be verified for all hazard levels, including the lowest ones. An analysis of the storms of recent years showed that major damage arose in the yellow and white zones (“residual risk”) in which no regulations currently apply. For this reason, it makes sense to define requirements for these zones too. Spatial planning that is based on risks and not just hazards must ensure risk-appropriate use on all hazard levels.
Mapping all of Switzerland
Meanwhile, apart from very few areas, almost the entire inhabited area Switzerland has been mapped. Two thirds of communes have already integrated their hazard maps into their communal land-use plans. Compared with other countries, Switzerland is very advanced in this regard and the expertise we have here has triggered a lot of interest abroad.
“However, the work here is not finished and it will never be,” admits Roberto Loat. The hazard and risk documents must be updated periodically and new phenomena, for example surface runoff, which is responsible for around half of all damage, must be mapped. “Only when we have complete and up-to-date information can we take the correct measures to improve the safety of people and important assets.”
More space for watercourses
Up to a few years ago, the river Aire near Geneva flowed through a straight concrete channel. Following periods of heavy rain it repeatedly breached its banks and posed a flood risk to some of the city’s neighbourhoods.
A flood protection project, which is being combined with the ecological upgrading of the watercourse, was initiated in 2002. A long stretch of the stream bed was widened. The discharge slowed down as a result and the flood peaks in the lower reaches are dissipated.
Since 2011, the Waters Protection Act prescribes a minimum space for streams and rivers. The buffer strips along banks that already exist today must be extended – particularly along major watercourses. Around 20,000 hectares of land is required for this throughout Switzerland, mainly in agricultural areas. The land will not be lost to agriculture as extensive grassland use is still possible.
However, the areas that will be needed in the 80 years to come for the revitalization of cramped streams and rivers will no longer be available for use as arable land. The area involved is estimated at 2,000 hectares.
Switzerland’s total agricultural area – excluding Alpine pastures – is 1.02 million hectares.
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Last modification 20.05.2015