Reference, completed project

Solutions for managing the Foumbouni landslide in Mayotte

The Foumbouni landslide, in the municipality of Mtsamboro in Mayotte, is a slow, large-scale landslide affecting a 7-hectare area, on which around a hundred homes and one of the island's main roads are located. Since 2013, BRGM has undertaken work to improve our understanding of the phenomenon and identify appropriate management solutions.

21 November 2024

Damage to buildings (Mtsamboro, 2015)

The need

Following the heavy rainfall caused by Cyclone Hellen (March 2014), a phase of accelerated landslide movement exacerbated existing damage and led to the appearance of new damage to infrastructures in the Foumbouni district in the municipality of Mtsamboro in Mayotte. The municipal council and the French Government then asked BRGM to find out more about the structure, dynamics and hydrogeology of the site and to propose appropriate, sustainable solutions to minimise the risks associated with landslide movements.

The results

Since 2014, the movements and speeds of the landslide have been monitored through geodetic measurements. Movements of up to 40 centimetres were recorded in the northerly direction between January and July 2014, covering the period of heavy rainfall caused by Cyclone Hellen (239 mm on 29 March 2014). In the aftermath of this event, a damage-assessment survey recorded the damage to a large number of structures and homes, some of which were even totally ruined. Since then, no acceleration phase of equivalent or greater intensity has been recorded. Although the movements are slow to very slow unless there are exceptional events (annual movements measured in millimetres to centimetres since 2015), they still indicate landslide activity. Consequently; there is an ongoing risk that buildings will be gradually ruined. At the same time, the risk that the landslide will suddenly speed up cannot been ruled out.

The various investigations carried out (geomorphology, geophysics and core drilling, notably) have made it possible to define the contours and structure of the landslide, but have not identified a rupture surface. Piezometric monitoring, which started in 2015, has established a link between the rapid recharge of the aquifer and a slight acceleration of the landslide.

In 2021, a permanent Global Navigation Satellite System (GNSS) station was installed in the landslide area. The data acquired by this station, combined with data from piezometric and rainfall monitoring, has made it possible to establish movement-activation thresholds and thus define thresholds for reinforced monitoring, when conditions could lead to movements measured in tens of centimetres, or even more. Further data acquisitions will be used to validate these thresholds.

BRGM is a long-standing partner and our first point of contact when it comes to issues concerning the geological context of Mayotte. Their expertise enabled us to characterise the landslide risk in Foumbouni, which is particularly significant. Today, all construction is forbidden in the area, which is monitored on an ongoing basis, with the help of the local BRGM team. This partnership is extremely beneficial and meets our need in terms of gaining a better understanding of our subsurface.

Philippe Hirel, Deputy Head of the Natural Risks Unit, DEALM of Mayotte

Map showing the extent of the Foumbouni landslide are, with movement speeds

Using the results

These results enabled BRGM to propose appropriate, long-term technical solutions aimed at minimising movements. These solutions include measures to address both surface water management and groundwater drainage. A detailed study of the hydrogeological behaviour of the landslide should provide the essential data needed to assess the feasibility and relevance of implementing such solutions, in line with local constraints.

In the meantime, working in conjunction with the local authority and government departments, the knowledge acquired over the last ten years has been used to establish temporary evacuation measures in the event of a serious, imminent threat, and rehousing solutions for residents whose dwellings are most severely damaged. Studies are currently being conducted to define a specific warning system for landslides, with encouraging initial results. Continued monitoring of the landslide's behaviour in relation to meteorological impacts remains essential in order to define an appropriate system to make the village safe, in conjunction with the local authorities.