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River dynamics

Context

The Reunion island rivers are characterised by small scale watersheds (less than 145 km²), basic linear sections of the order of 35 km and maximum heights around 2000 m. Under tropical climate, during the cyclone season and very heavy rainfall, the flows of these rivers can turn from a few m3 by second during low-water level periods up to several thousands when tropical depressions or majors cyclones occur.

The impact of the Reunion torrential dynamics is quite significant and misunderstood with regard to:

- The erosion in the heights of the island, more particularly in the cirques;

- Natural hazards (river blockages and breaks up, water level rises, floods, collapse and erosion of sheer banks);

- Transport and sedimentation of materials coming from the island central areas (development of alluvial fans, changes at the shoreline level...).

All these processes have major consequences on the safety of people and possessions, the evolution of natural environment and country planning. On the other hand, the development of urban areas and the needs for travelling within the island lead to settle on alluvial fans or build large civil engineering structures to cross rivers.

It is therefore impossible to forget the different roles played by the Reunion watercourses and designing these structures requires to get a deep knowledge of the phenomena at stake.

Activities currently carried out by scientific teams

The DynTorre research project falls within the framework of agreements signed in 2000 between BRGM and both regional and department authorities of the Reunion island. It aims at a better comprehension the dynamics of Reunion rivers.

In the end, this improved knowledge of geodynamic processes will enable the development of new monitoring approaches and computing models tailored to the Reunion context. These will encourage to better take into account natural risks, get knowledge related to the alluviation phenomenon (which could apply to the still complex extraction of materials in gullies) and find solutions for a less vulnerable and therefore sustainable development of the considered watersheds.

Relevance of a reference remote sensing database

The development of the Kalideos Isle-Réunion database will contribute to:

- Map the current status of river beds;

- Recount their changes during the last decades, through the comparison of Spot 5 images with the airborne photos from IGN campaigns carried out from the sixties up to the late nineties, while resetting them back in the context of cyclone phenomena.

Contact point

The coordination of this activity is ensured by Richard POUGET, responsible for the DynTorre project at the Reunion local branch of the Bureau de Recherches Géologiques et Minières (BRGM).