The PACA region has 1,035 km of coastline. Four hundred and twenty kilometres of coastline have cliffs, 40% of which have been built on.
Instabilities in coastal cliffs are largely due to gravity effects (landslides, boulder falls). These events can affect assets (housing, roads, coastal paths, beaches frequented for outings or any other leisure activities) that are at the top or foot of the cliffs.
Natural phenomena such as landslides are unpredictable and thus a major hazard for local residents and coastal users alike. Landslides have caused fatal accidents in recent years, mainly in non-urban areas used for recreation (swimming, walking).
A guidebook for local authorities
Produced by the PACA Observatoire Régional des Risques Majeurs (ORRM) in partnership with the DREAL and BRGM, this guidebook is designed to help local authorities manage the risk associated with rocky coastlines, as part of the Provence-Alpes-Côte d'Azur Regional Strategy for natural hydraulic and mining hazards. It also provides information for analysing the gravity-induced mechanisms behind the gradual disintegration of these rocky cliffs, in order to meet the specific objectives of the Climate and Resilience Act.
The guidebook complements an initial Guide "Gestion du littoral rocheux de la Côte Bleue", for managing the rocky coastline of the Côte Bleue (north-west of Marseille), published in 2022.
Signs to look out for to identify incipient instability on a cliff.
© BRGM - De Michele
Extract from the guidebook
Main warning signs of future cliff instability
At the top of the cliff
- Cracks on the crest or set back a few metres from the edge
- Cracks in patio tiles
- Cracks in swimming pool tiles
- Subsidence of the land further down with a slight depression
- Water flowing below the cliff
- Swimming-pool leaks or over-watering, creating a pool of stagnant water that can seep into the ground
- Action of tree roots in cracks and the effect of tall trees swaying in the wind
- Overhanging rock strata
On the cliff face
- Water flowing out of stratigraphic joints and/or cracks
- Overhanging rock that is parallel with other strata
- Cracks which can split slabs of rock that are liable to topple over
- Vegetation with extensive root systems that widen cracks
- Evidence of previous events, such as visible marks, scars and indentations, with a patina of a different hue
At the foot of the cliff
- Traces of water circulation (resurgence)
- Traces of recent erosion (gullies) in the marl-clay-sand strata at the foot of the cliff
- Piles of boulders, stones and rock masses indicating a recent landslide
- Weathering of the rock, which has become powdery, a sign of increased fragility
- Slabs of rock at the foot of the cliff, indicating a recent landslide
