Landslides and debris flows: recognising warning signs

A slope is a balance. Gravity pulls soil and rock downwards; friction, roots and the cohesion of the material hold against it. As long as the holding forces are greater, the slope stays put. Water shifts this balance in two ways at once: it makes the soil heavier, and it reduces the friction between grains and layers. That is why heavy or prolonged rain and snowmelt trigger most slides; earthquakes can trigger them too.

Slides can be slow or fast. Some slopes creep downhill only a little over weeks and months; others give way in seconds. Debris flows are particularly fast: they form when water carries away large amounts of mud, rubble and wood. This mass follows stream beds and gullies and can push very far into the valley.

Before a slope fails, it often already moves a little. This slow movement bends buildings, fences and trees, opens cracks and changes where water flows. That is exactly where the warning signs you can recognise come from – though not always, and not always clearly. Fast debris flows often leave hardly any warning time.

The first step is to know your own risk. Many regions, especially in the Alpine countries, have hazard maps or hazard-zone plans for slides and mountain streams; your municipality can tell you whether your property is affected. Also watch for clues yourself: earlier slides nearby, water seeping out of the slope, or a steep embankment directly behind the house.

You should know the warning signs: new cracks in walls, plaster, ground or road; doors and windows that suddenly stick; tilting trees, poles or fences; water newly seeping from the slope; streams that suddenly turn muddy or dry up; unusual rumbling or cracking. A single sign does not have to mean danger, but do not simply dismiss it as normal building damage – report it to the municipality.

During heavy rain, be especially alert, at night too. Follow the radio and warnings, avoid stream beds and valley floors, and leave at once when the authorities call for evacuation. Plan your evacuation route so that it does not lead through stream beds or along the foot of slopes.

If you hear rumbling, trees cracking or notice visible movement, do not wait. Get sideways out of the slide path at once, onto firm, higher ground. Do not run down the slope ahead of the mass – it can be faster than you. If escaping from the house is no longer possible, go to the upper part facing away from the slope. Then call the local emergency number (112 in the EU).

During heavy rain, stream beds, culverts, bridges over mountain streams and valley floors are dangerous places, because debris flows follow exactly these channels. Do not stay there, not even to look. When hiking after heavy rain, avoid paths on steep slopes and in gullies, watch for fresh rockfall and abandon a tour if in doubt.

After a landslide, further slides can follow. Do not walk onto the slide area to look at the damage, and avoid the area until experts clear it. Report damaged power, gas or water lines immediately. Stabilising slopes is a job for professionals, not for private individuals.

Whether a particular slope will give way on a particular day cannot be predicted precisely. Experts can, however, identify areas at risk and monitor some slopes with instruments. For individuals this means: take hazard maps seriously, follow warnings and report your own observations – also to protect neighbours who may live further down.

Landslides do not only occur in high mountains. In uplands, on riverbanks, on road and railway embankments and on hillside residential areas the ground can also start to slide, especially after long periods of rain. If you live on a slope, you should know the warning signs, even if nothing has ever happened.

A common mistake is only thinking about escape once movement is visible. It is better to know the escape route beforehand and, during heavy rain, to pay deliberate attention to how streams and slopes behave. If you are prepared, you do not lose a second in a real event.