Explain why heavy rains can cause landslides.

Formation of landslides

Landslides occur when a mass of soil, rocks, or mud suddenly rushes down a slope. It starts with a loss of balance of the materials between the force of gravity, which pulls everything downward, and the resistance of the ground, usually provided by elements like tree roots or the very structure of the terrain. When this resistance gives way or significantly decreases, the earth slips. Often, it begins with a crack or a small detachment at the top of a slope, then it escalates rapidly into a large movement of soil. The materials that slide can be dry or wet, but when they are saturated with water, it exacerbates the phenomenon.

Effect of heavy rainfall on soil stability

When it rains heavily, the ground quickly becomes soaked and loses its stability. Water seeps between the soil particles, and under its weight, it causes the separation of soil grains: this reduces friction, thus decreasing the ground's ability to stay in place. It also creates an increase in pressure in the soil's pores, called pore pressure, which makes the soil extremely unstable. As a result, soil that was previously well-compacted can become slippery and start to shift, especially on steep slopes. The heavier the rain, the faster the soil loses its resistance, quickly increasing the risk of landslides.

Water saturation and infiltration process

When heavy rain falls on soil, water does not always immediately run off elsewhere: it penetrates deeply. This is called infiltration. At first, the soil calmly absorbs all this water, as its pores are still filled with air, much like a dry sponge that easily soaks up liquid. But if the precipitation continues, these spaces fill up completely with water, leading to the famous saturation. From that point on, it’s as if the soaked sponge can no longer hold any more: water begins to accumulate and exerts pressure that gradually pushes the soil particles apart. This phenomenon significantly weakens the natural adhesion between the soil elements, greatly reducing their resistance. Once saturated, the soil becomes particularly heavy and unstable, ready to slide.

Triggering geological mechanisms

When a slope is made up of fragile or loose layers such as clay, silt, or sand, it makes things significantly easier for landslides. These layers can quickly become saturated with rainwater, becoming heavy and slippery like wet soap. Meanwhile, cracked or fractured rocks allow water to penetrate easily deep within, increasing internal pressure in the materials. The result: less cohesion, greater risk of everything sliding down. Additionally, there may be a slope already weakened by recent snowmelt or light but regular tremors that further compromise the stability. In short, when all these little geological details combine their efforts, it’s a sure thing: the slope eventually gives way.

Environmental and social impacts of landslides

Landslides can quickly cause significant damage by permanently altering natural landscapes. They can lead to the destruction of ecosystems, uprooting trees, plants, and animal habitats in the process. An affected area may thus become poorly suited for wildlife for a long time. From a human perspective, it is equally problematic: sometimes it only takes a few seconds to lose entire homes or essential infrastructures such as roads or drinking water networks. Such disasters hinder access to emergency services and complicate the daily lives of affected populations for a long time. There are also psychological issues, as residents who are victims of such events often face deep stress and lasting uncertainty. Finally, on the economic side, repairing the damage is costly, not to mention the indirect effects on tourism or local businesses.