Carte géologique de Mayotte
The BRGM’s Mayotte agency is located in Mamoudzou. It employs seven people specialising in the fields of geological infrastructure, groundwater, geophysics and natural risks.
An island in the Comoros Archipelago
Mayotte became the 101st French département on 1 April 2011.
On this small island (374 km2), with its population of 270 000, rapid economic and demographic growth is putting increasing pressure on the environment and natural resources.
Defining priorities to manage the island sustainably is one of the main challenges for the years to come.
For more than twenty years, the BRGM has been gathering large quantities of data on the geology, hydrogeology and natural risks in this Indian Ocean archipelago, thereby helping to introduce a planning policy that has become essential given the island’s galloping population growth.
The island’s economic and planning goals are set out in a series of policy papers: a water management masterplan (SDAGE), a land use and sustainable development plan (PADD), the future regional land use masterplan (SAR) entitled “Mayotte 2025”, and a State-Region planning contract (CPER), to which the BRGM is contributing expertise in its core areas of competence.
Working with public and private partners
The BRGM has considerable experience of working with partners on projects to support public policy development, as well as on research and training at every level in regional policy-making and spatial planning. These partners include the Prefecture (SGAR, SIDPC), the Département Council, municipal authorities, the DEAL, DAAF, SIEAM, ARS and SIM, the Marine Nature Park and the Conservatoire du Littoral.
The BRGM also works with private companies of all sizes on methodology development and RDI (Research, Development and Innovation) to support their fields of activity.
What's new in Mayotte?
Volcanic eruption and earthquakes in Mayotte: activities to support crisis management and understand the phenomenon
Since 10 May 2018, several thousand earthquakes of small and medium magnitude have been recorded on the island of Mayotte, including the strongest earthquake recorded anywhere in the Comoros area. Onshore studies and offshore measurement campaigns conducted in 2019 have revealed a major underwater volcanic edifice 50 kilometres off the coast of Mayotte.
Since the start of this seismic crisis in Mayotte, the BRGM has been investigating the phenomenon and taking part in crisis management.
BRGM involvement and responses
As the only public geoscience organisation with an agency in Mayotte, the BRGM has monitored seismic activity continuously since the start of the phenomenon. This ensured that government agencies received essential information to support crisis management.
The scale of this ongoing seismo-volcanic event has spurred international interest, led by the BRGM, in understanding the phenomena under way and how they are evolving. Several major scientific projects have been launched, which deployed inland and offshore seismometers to reinforce the observation network.
Several measurement campaigns, both inland and oceanographic, have taken place since 2019 with the BRGM highly involved. In September 2019, the Mayotte Volcanological and Seismological Monitoring Network (REVOSIMA) has been set up to monitor seismic and volcanic activity affecting the island of Mayotte. Jointly operated by the IPGP with BRGM and its regional office in Mayotte, REVOSIMA is based on a close scientific and technical partnership with Ifremer, CNRS, BRGM, IPGS and RENASS, IRD, IGN, ENS, the University of Reunion Island, the University of Clermont Auvergne, CNES, Météo France, and SHOM.
Natural resources: groundwater protection and management
Mayotte’s heavily used surface water resources could be supplemented by groundwater resources, which are more evenly distributed across the island, closer to supply points and less vulnerable to pollution. The SDAGE (water management masterplan) for Mayotte addresses this issue. Diversifying the resources used (rivers, hill reservoirs and wells) can also help to secure water supplies.
Because of the rapidly increasing rate of water consumption since 2014 (+12% between 2015 and 2019, sources IEDOM), in combination with meteorological hazards, Mayotte regularly experiences periods of water supply crisis.
The Dapani F2 water abstraction borehole
BRGM involvement and responses
The BRGM was commissioned by the SIEAM (Mayotte’s intermunicipal body for water and sewerage management) to provide scientific and technical assistance and geological, geophysical and hydrogeological expertise to support groundwater prospecting and abstraction. The 6thcampaign for groundwater prospecting and abstraction in Mayotte has been scheduled, with 6 boreholes to be drilled.
It was decided by the crisis management authorities that all usable water resources on the island should be identified. The BRGM was asked to draw up a bibliographic inventory of all available and potentially usable water wells. The inventory identified three disused wells from the bibliography, whose technical and hydrogeological characteristics (depth and tubing diameter; usable flow) warranted consideration of their use in the short term.
- Hydrogeological studies of catchment areas around abstraction points
- Monitoring the inland water quality surveillance network for Mayotte and installation of a piezometric network in compliance with the EU Water Framework Directive (WFD)
Coastal risks: knowledge and protection of coastal environments
Mayotte’s rapid development is increasing pressures on the island environment, especially in coastal areas (erosion and coastal flooding, with a growing need to protect people and infrastructure). The aim is to increase knowledge on coastal environments to strengthen adaptation to coastal risks.
Au premier plan : plage de sable blanc partiellement recouverte de blocs de basalte noir. En arrière plan : lagon de Mayotte dans lequel se jette le dôme coulée de Saziley appelé la tête de crocodile (Mayotte, 2011).
© BRGM - Nicolas Ollivaud
BRGM involvement and responses
The BRGM is conducting a series of research projects on coastal risk prevention. One of these, CYCLOREF, is working on simulations of storm surges and their effects.
It is also pursuing its work to build up a statutory zoning plan and the relevant regulations, which will be put forward to the Mayotte département council as input for its coastal risk prevention plans (PPRL) for coastal municipalities. To support implementation of these PPRL, the BRGM is also mapping the ‘coastline retreat” hazard in Mayotte (RTC project).
- Morpho-dynamic study of Mayotte’s coastline.
- Project for a coastal observatory.
- Crisis management support.
Natural risks: morphological, geological and climatic hazards
Given Mayotte’s particular geological, morphological and climatic context, the various development projects for the island are highly exposed to natural hazards other than specifically coastal hazards, including landslips, cavity collapse, rockfalls, erosion, flooding and seismic activity.
Inondation avec glissement de terrain à Mayotte.
BRGM involvement and responses
The BRGM works closely with central and local government services, providing advice, expertise and post-event feedback on natural risks.
It also works to further knowledge, in particular through natural hazard mapping and development of the island's first natural risk prevention plans (PPRN).
The BRGM is contributing to the development of an “Erosion roadmap”, through LESELAM, a multiannual partnership-based project (2015-2017) on action against soil erosion and lagoon siltation in Mayotte. This project has three key objectives:
- to quantify erosion phenomena in several pilot catchment basins
- to set up a demonstrator of remedial practices
- awareness-raising and training for the different players affected by erosion.
The aim is to bring recommended erosion control measures into current practice, with a shift from the catchment basin scale to Mayotte as a whole in order to limit the negative impacts of soil erosion on the island as early as possible.
The BRGM also has several missions concerning seismic risks: regular information when earthquakes are sensed, characterisation of liquefaction hazards beneath schools and implementation of a schools awareness programme, “Sismo des écoles”.
BRGM also plays a role in raising the population's awareness of natural risks. For example, it has published two short guides on good construction practices in risk zones:
Geology: mapping applications and user support
To prevent and manage natural risks while meeting needs for resources and better land use management, it is essential to improve knowledge and to produce and disseminate widely usable data.
Heliborne geophysical surveys: measuring magnetism across Mayotte for the GéoMayotte project.
BRGM's involvement and responses
The BRGM has produced a geological map of the island and undertaken a heliborne geophysical survey (GéoMayotte) to collect the data needed to build up the regional data infrastructure that will help, in particular, to identify potential mineral resources and zones that are vulnerable to ground instability (3D TDEM model).
- A regional quarrying plan, for the DREAL;
- Prospecting for aggregate deposits;
- Input to and management of the BSS subsoils databank and the BD MVT databank on ground instability.