The Federal Office for the Environment FOEN, the Swiss Insurance Association SIA and the Cantonal Buildings Insurance Association VKG together presented the new surface runoff risk map in Bern on 3 July 2018. Floods do not occur only as a result of streams, rivers or lakes bursting their banks. Rain which cannot seep into the soil and runs over open ground can lead to flooding. This surface water causes up to 50% of the damage from floods and its occurrence has increased over recent years. There have been various examples in recent months, such as in Lausanne in June of this year and in Zofingen in July 2017 (see box 2). As the climate warms, more frequent and intense precipitation occurs and therefore more surface runoff can be expected. The new map helps Switzerland to adapt to climate change and these phenomena.

Surface runoff can cause serious damage

The nationwide map shows which regions are at risk and how deep they may be under water (see box 1). Around two thirds of buildings in Switzerland are potentially affected by surface runoff. The map helps architects, developers, planners, authorities and emergency response services with the planning of protection measures. Relatively simple measures on buildings can hold back the water and prevent damage: for example, by raising the level of light wells or installing barriers or small raised sills at underground car park entrances. The surface runoff risk map also covers unpopulated areas and can be useful to farmers for soil protection measures.

The canton of Lucerne has been using the map for two years and reports positive results. The additional costs of integrating surface runoff protection at the planning stage of a construction project are low or non-existent.

The surface runoff risk map of Switzerland is now available free of cost online to all cantons, insurance companies, developers and other interested groups at www.map.geo.admin.ch. It has no legal force but is informative in nature and supplements the existing cantonal risk maps. Switzerland is one of the first countries to have produced a map on surface runoff and is playing a leading role in this field internationally.

Box 1: Innovative computer model for the whole of Switzerland
A computer model for the whole of Switzerland was developed for the map. In addition to precipitation levels, soil cover and properties, soil storage capacity and a high resolution digital terrain model were important for the modelling. The resulting map shows where surface water runs off, which areas are affected and how high the water might rise.

Box 2: The floods at Zofingen in July 2017
On 8 July 2017 the region of Zofingen in the canton of Aargau was hit by a heavy storm such as only occurs every 100 years. The River Wigger did not burst its banks, but floods, landslides and power cuts did occur. Businesses, basements, underground car parks, underpasses and the station car park were under water due to the surface runoff. Drinking water supplies were partly disrupted. Fire service and civil defence personnel were deployed for several days. Fortunately, no-one was injured, but property damage was extensive, totalling more than CHF 90 million.

Gap in flood protection closed: New map on surface runoff