Assessment of the hazard of falling boulders on the eastern flank of the Calanque de Port-Miou in Cassis (Bouches-du-Rhône)

BRGM assessed the rock hazard on the eastern flank of the Calanque de Port-Miou with the new MEZAP method (used for zoning rockfall hazards as part of natural risk prevention plans), which had been adapted to the local context in terms of timing and intensity to take account of specific vulnerabilities in the area.
25 September 2020
Calanque de Port-Miou in Cassis

Calanque de Port-Miou in Cassis (Bouches-du-Rhône, 2019).

© BRGM - E. Equilbey

The need

Following a request from the Sea, Water & Environment Department of the Bouches-du-Rhône departmental authority for inland and marine areas (DDTM), BRGM conducted an assessment of the rock hazard for the entire site of the eastern flank of the Calanque de Port-Miou in Cassis. The proposed approach was adapted to take account of the decision to renew the authorisation to temporarily occupy publicly-owned coastal land (for a 15-year period: boats at anchor and pedestrian traffic on the jettees), in a zone exposed to rock falls.

The results

The intensity categories of the MEZAP method (working group meeting under the authority of the Ministry of Ecology to determine a methodology for zoning rockfall hazards, publication pending), which are geared to the moderate- to high-energy damage to a conventional dwelling house, were adapted to take into account the much lower energy thresholds for damage to pleasure boats. Risk intensity categories specific to this type of issue were therefore defined, then transposed from energy to volume, as with the MEZAP.

The risk map drawn up with regard to pedestrian traffic at the bottom of the Calanque (cove) confirmed the relevance of the order prohibiting access to the public: only 120 linear metres of jettee (the midway point) are not concerned by high-risk zoning.

With regard to organised boat mooring areasfour high-risk zones have currently been identified:

  • a first zone just below the most active cliff area known (particularly high, at more than 25 m);
  • after an interruption 60 m long, a second risk zone further south on a line of cliffs that are again high (20-25 m elevation);
  • then two separate zones at the northern and southern ends of the Calanque with lower cliffs (10-12 m high). 

One of the few fixed buildings (Port-Miou sailing club) is also located in a high-risk zone.

Using the results

This first site-wide rock risk diagnosis was based primarily on remote visual observation. In addition, certain factors that are difficult to quantify (such as vegetation) were not taken into account. The method used remains cautious and safe (geared mainly to potential damage), while the levels obtained still need to be put into perspective in view of the site's low rock activity. No immediate or imminent visible risk was identified. For high-risk areas that could remain for the redevelopment project currently being sketched out, a more detailed specific rock diagnosis is recommended (personnel on rope harnesses, along with trajectography profiles) in order to confirm or rule out the relevance of the risk over the next two decades. In the event of a persistent risk, appropriate development solutions may be proposed, including:

  • A freeze on moorings in highly exposed locations (eliminating the issue);
  • Moving the mooring line further downstream (moving the issue);
  • Implementing suitable counter-measures on the rock (eliminating the hazard or risk). This could be done using methods (removal, anchoring) having little or no visual impact on this part of the Calanques National Park.

The partners

  • Bouches-du-Rhône departmental authority for inland and marine areas (DDTM13)
Map of the rock risk

Map of the rock risk for pedestrian use and boat mooring (Calanque de Port-Miou in Cassis, 2019).

© BRGM - E. Equilbey