The flood alert map shows the current flood danger level (1 to 5) for all and for the regions in relation to small and medium-sized rivers.

The rivers of national interest are divided into individual stretches of river, each of which has a defined discharge gauging station. The forecast is based on a hydrological model and various weather models. Depending on how the forecasters assess and interpret the models, individual stretches of river and lakes are colour-coded by danger level. If necessary, the FOEN forecasters may request more detailed information on weather developments and the status of snow cover from MeteoSwiss and/or the WSL Institute for Snow and Avalanche Research (SLF).

The FOEN also issues alerts and information when a strong increase in discharge is expected locally on small and medium-sized rivers. In such cases – especially during thunderstorms with heavy rainfall – it is not possible to predict precisely which water bodies will be affected. Therefore, the regions likely to be affected are marked on the flood alert map. For small and medium-sized rivers, there are only two possible danger levels in advance of an event: low/no danger (green) and flood danger (yellow, no indication of severity). Red is also used to indicate a very large flood in progress; this is based on current readings and is not issued in advance.

Rivers and lakes of Swiss national interest are:

• Rivers and lakes along which high potential for damage exists.
• Main rivers which flow through several cantons or international rivers and lakes bordered by several cantons or forming a frontier with neighbouring countries.
• Rivers which have had their course altered in major historical correction projects with significant involvement by the Confederation, the long-term effects of which are to be evaluated by regular surveys.
• Rivers which are having their course altered under major current flood protection projects with significant involvement by the Confederation, the effects of which are to be assessed by regular surveys.
• Rivers where there are extensive floodplains of national importance.
• Tributaries which have a water and bedload carrying capacity or input (flow rate, bedload supply) which is of crucial importance for the main river system.

The 5 flood danger levels

Hydrological forecasts: Cooperation with expert national and international services