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Water & ice

Whether fluid or frozen: water can cause a variety of natural hazards

Floods in Thun, 2005
Floods in Thun, 2005


Flood Hazards

During a flood, the water level or volume of water exceeds a certain threshold. Typical causes are intensive or prolonged periods of rain and/or snowmelt. The water volume can increase rapidly and contain sand or gravel. In steep gullies, such as in mountain streams, intensive bedload rearrangement and deposition of sand and gravel dominates. It results in debris flow deposition, bank erosion and overbank sedimentation.

Debris Flows

A debris flow is a downhill flowing stream of mud and coarser sediment. Debris flows typically occur in mountain stream areas with a stream gradient of more than 15 percent. Typical of debris flows: the high density, the often high flow velocity and the enormous transport capacity. A debris flow can carry whole trees and rock blocks of several cubic meters and cause extensive damage.

Flooding, bank and bed erosion

Shallow waters in the valley endanger by means of flooding. River bank and bed erosion can occur. They lead to undermining or silting. Flooding becomes dangerous when large water depths or strong currents occur in the flood area. At the same time, a lot of sediment can be deposited. Strongly flowing water can pick up and transport solids from the bank or channel bottom. The river course is displaced. River bank landslides can endanger structures and facilities along the water. The entrained debris often damages farmland and buildings.

Melting Permafrost

The term permafrost (permanent ground frost) denotes subsurface material such as rock or scree, whose temperature lies below freezing all year-round. According to model calculations, the extent of the permafrost in Switzerland comprises approximately 5 percent of the total land area.
Permafrost ground lies beneath an oft several meters thick cover layer which thaws at high temperatures. Permafrost is therefore neither visible nor measurable on the surface. The permafrost layer can itself be up to several hundred meters thick. Permafrost occurs both in rock and loose material (e.g., gravel, scree). Loose material which is supersaturated with ice often creeps slowly downhill. This can be problematic for structures located on permafrost ground e.g., cable car stations. Climate change can influence the frequency and intensity of landslides and other dangerous mass ground movements.

Unstable glaciers

A glacier is a massive ice stream which flows downhill in slow motion and at the same time is eroded in its bottom portion by melting. Various hazards can emanate from glaciers: ice falls from hanging glaciers, advances from glacier tongues and floods from ice-blocked lakes or due to glacial retreat of newly formed lakes.

Risk management is necessary

Through Integrated Risk Management (IRM), possible damage should be selectively prevented or limited. This requires that hazard fundamentals must be recorded and processed - for example through event registers and hazard maps.
Protective measures: See page Rockfall-Landslides-Debris Flows

Who is who

Many organizations in Switzerland (e.g., federal offices, cantons, universities, museums, private bureaus) deal with natural hazards which are the result of water and ice, such as floods, melting permafrost or glacier instability.


The Federal Office for the Environment (FOEN), the Swiss Geological Survey as well as the Geodata portals of the cantons and the federal government provide visual access to the natural hazards data. offers an ideal entrance to the search for such data. Numerous reports and illustrations are obtainable via FOEN.

Geologische Gefahren


Hazard Prevention Division (FOEN)
Natural Hazards, PLANAT
Natural Hazards: Glaciers

Aquaprotect flooding areas: 100-year recurrence period

Potential permafrost distribution