The landslip at Salazie on La Réunion. © BRGM - René Carayol

A global model to predict ground movements

In La Réunion, which experiences very large scale ground movements, the BRGM has developed a modelling tool which is capable of predicting the severity of ground movements from rainfall data.

The Salazie, Cilaos and Mafate cirques on La Réunion were formed by erosion in the wake of gigantic ground movements, and the land has continued to slip ever since.

Bertrand Aunay, the BRGM's project manager for Hydrogeology and Natural Risks explains: "The two landslips at Salazie, in inhabited terrain, were exceptional in the scale and the factors that triggered them".

Downstream from Hell-Bourg, no less than 300 to 350 million m3 of soil have been shifting in the last ten years.

The MvTerre project produced a model of the ground movements at Ilet-à-Vidot (Salazie cirque, La Réunion). © René Carayol

"The disruption is considerable", says Bertrand Aunay, "especially for buildings and road infrastructure, with major consequences for the economy. There is no major risk of runaway ground movements, except very locally, but the situation has claimed a dozen victims in the last forty years".

Detecting, monitoring and modelling ground movements

Following the MvTerre research project (2002-2008), which produced geological maps of the worst affected sectors, MvTerre-2 (2010-2014) focused on detecting, monitoring and modelling these large-scale ground movements.

The project, conducted by the BRGM in partnership with the EU and the La Réunion DEAL and Regional government, focused in particular on the Salazie cirque, installing different types of equipment to monitor land movements (10 permanent GPS points and 150 geodetic markers) simultaneously with water circulation (monitoring of 3 piezometers and 7 springs or rivers). The project also used geophysical investigation techniques and remote sensing.

"One of the main lessons from these studies", says Bertrand Aunay, "was the crucial role of hydrogeology. Although erosion is a factor, groundwater movements are the driving force of these landslides".

The studies are ongoing and have already produced a global modelling tool with a transfer function that uses input data on rainfall and groundwater levels to produce, as output, a precise description of land movements and hence predictions of these movements according to rainfall conditions.

The next step will be to assess local runaway land movements. Several awareness-raising tools have already been developed in parallel, including a film for the general public and educational materials for schools.

BRGM - 3 avenue Claude-Guillemin - BP 36009 45060 Orléans Cedex 2 - France Tel.: +33 (0)2 38 64 34 34