Reference, completed project

Landslides: socio-economic vulnerability of the Cirque de Salazie on Réunion Island

Due to its steep terrain and climate, Réunion is highly vulnerable to gravity-related hazards. The Cirque de Salazie is the area of the island which is most affected by these phenomena. However, it is also an area involving a large number of human and economic assets, as well as strategic infrastructures that are vulnerable to such hazards, which therefore need to be taken into account when drawing up a regional risk prevention strategy.

17 November 2023

Village of Hell-Bourg, close to where the large-scale landslide at Fond de Rond-Point (Salazie, La Réunion) started

The need

The high-ground instability hazard affects around 80% of the Salazie area. The Cirque's human, socio-economic and building assets are therefore particularly exposed to these phenomena. In order to guide risk management policies in a coherent, hierarchical manner and prioritise the safety measures to be taken, BRGM undertook a regional risk mapping study. The aim of the study was to assess the hazard for phenomena likely to occur on a generational scale (30 years) and to characterise the issues at stake in detail, by incorporating a socio-economic dimension.

House at risk in the Salazie cirque

The results

Seven types of issue were taken into account for this study: human lives, the social vulnerability of the population, economic activity, buildings, vital networks, critical infrastructures and cultural heritage (monuments). The perception of risk and the relative importance of the various types of issue were defined using a hierarchical multi-criteria analysis based on consultation with a number of local stakeholders (elected representatives, technicians, experts, etc.).

Overall, the "human lives" issue stands out as having the most weight. However, perceptions of this category vary widely, particularly between elected representatives and technicians. Elected representatives tend to prioritise the functioning of the region, while technicians/experts prioritise the protection of the population. The mean weight assigned to each category of issues was used to produce the risk map. This corresponds to the cross-referencing of the risk of ground movement and the map of prioritised issues. This risk map identifies the sectors with the highest concentration of issues that are subject to a high risk of ground movement. The most exposed area is the Salazie-village/Camp Lilas sector. A risk map has been produced for each type of hazard (boulder falls, landslides, large-scale slow earthflows), providing guidance on the most appropriate safety measures for each phenomenon. The risk map thus produced makes it possible to identify the sectors that are most exposed to gravity phenomena and to prioritise the safety measures to be taken. It also allows us to identify areas that are less exposed to risk, where future urban development could be envisaged.

Finally, a number of safety measures that could be implemented in the sectors identified as most at risk were compared using a preliminary, semi-fictitious, cost-benefit analysis (CBA). The aim of this final stage, which was still exploratory, was to start investigating the use of cost-benefit analysis in the field of gravity risks by means of concrete cases.

Road cut off by a landslide in the Salazie cirque

Using the results

As part of the development of a regional strategy for the prevention of gravity-related risks, this study provided objective ranked data on the sectors most at risk and for which safety measures should be given priority throughout the Cirque de Salazie. The methodology used for the Cirque de Salazie can be applied to other areas subject to natural hazards. Exploratory CBA, on the other hand, will most likely be developed further, even if the discussions that have begun have led to the development of a CBA doctrine applied to gravity risks.