Stage 10 | Morzine les Portes du Soleil - Megève / Landslides and the Tour

On the steep hillsides of mountainous regions, large packages of rock and loose debris can slide down in slow or catastrophic events: so-called landslides. The remains of landslides are visible in many places along the Tour de France route, especially on the sides of the large cols during mountain stages. Landslides are common in all mountainous regions on Earth. It comprises the movement of rocks and soil along a slope, and may include falling, sliding, or flowing (see Figure). Especially the last form, of flowing, will be remembered by many a Tour de France viewer after stage 19 in the Tour of 2019. That stage was canceled just below the top of the Col de l’Iseran after a mud flow blocked the road in front of race leader Egan Bernal.

The mechanisms that lead to landslides

A landslide forms when the forces, especially gravity, that pull slopes down exceed the strength of rock and soil on a slope. Landslides may occur on slopes that were already on the edge of collapsing after the subsurface is weakened due to rain, melting snow, changes in water level, erosion due to streams, changes in groundwater supply, earthquakes, volcanic activity, human activities, or a combination of these. Landslides are very hard to predict and we therefore cannot prevent all landslides. Slopes with a high landslide risk can be stabilized through a number of methods, amongst others lowering of the slopes, increasing drainage to avoid soil saturation and loss of strength, and increasing the slope strength by for example a combination of concrete and earth anchors (as sometimes observed along road cuts).

The deadly landslide in Passy

Today, the Tour family will pass one of the most devastating landslides in French history, in the village of Passy, close to Mont Blanc and at the start of the final climb of today. During the night of 15 to 16 april 1970, Passy was struck by a catastrophic landslide (Figure). This slide was likely caused by the melting of the thickest snow cover in years. The melt water then infiltrated the soil leading to a decrease in soil strength. A slide plane formed at the transition between soil and bedrock, and the upper part of the hillside collapsed and slid down. The Passy landslide claimed 71 lives, mostly children, when it destroyed part of the Roc des Fiz sanatorium. Ironically, this sanatorium was intended for children between 4 and 16 years to recover from tuberculosis in the healthy mountain air. In 2019, a memorial was raised at the location of the former sanatorium (Figure).

I am a geomorphologist specialized in high-mountain processes on Earth and Mars. My main goal is to minimize landslide and debris-flow hazards, by understanding and recognizing their triggers, flow dynamics, erosion processes, and deposits. This quest has taken me from mountain ranges all over the globe all the way to the mountains of Mars.

Tjalling de Haas