RIVIÈRE DES GALETS - 20 YEARS OF RISK MANAGEMENT
ONE OF THE MOST POWERFUL TORRENTIAL RIVERS IN THE WORLD ON REUNION ISLAND
Francis Fruchart1, Gilles Delphin2, Sylvain Reynaud3, Christian Séraphine4, Joël Cadudal5, Paul Bonnet6 and Hervé Lemahieu7
INTRODUCTION
Reunion Island close to Madagascar holds world records for pluviometry. For instance 1825mm a day (1960 Denise cyclone) or 4869mm within 4 days (2007 Gamède cyclone) have been recorded on the
‘Piton de la Fournaise’ Volcano. ‘La Rivière des Galets’ (Pebble River) is one of the most powerful of the island. Discharge can reach 2.500 m3/s and velocity 10m/s. Its Slope is 2%.
Close to the sea, the banks are equipped with imposing groynes. The downstream fan is equipped with huge dykes in order to protect available land. Plots of lands behind the dykes remain suitable for development because available industrial area are lacking on this mountainous island. The industrial area to be developed is also very attractive because close to the harbour of ‘Le Port’, the only one of the island.
These plots of land are subjected to dykes break hazard. Therefore mastering the hydraulic risk was mandatory. For achieving this goal, a real partnership has been set up for 20 years between the following institutions:
the local organization SIVU, responsible of the structures and in charge of industrial development, on behalf on the 2 riparian cities: Le Port and Saint Paul, owners of structures and land,
the Reunion Island DDE, state administration, in charge of monitoring and maintenance of structures,
the Engineering Department of CNR, National Company of the Rhone River, Lyon France, as consultant.
CONCEPT OF RISK MANAGEMENT
Mastering the risk has been pragmatically tackled developing several constituents during 20 years:
Risk of dyke break hazard assessment
Numerous studies have been carried out: dyke stability assessment, mobile bed scale model at CNR laboratory Lyon France, both in order to define strengthening of dykes; morphological study of the Rivière des Galets, which proved very dangerous incision trend of the river bed (till 8m incision).
1 Francis Fruchart, expert in hydraulics morphology, engineering dept, CNR National Company of the Rhone River Lyon France (e-mail: F.Fruchart@cnr.tm.fr)
2 Expert in civil works, CNR Lyon France
3 Engineer in hydraulics, CNR Lyon France
4 Head of works dept, Le Port Reunion Island
5 Head of hydraulic structure dpt DEAL (ex DDE) Saint Denis Reunion Island
6 Retired from ex DDE, responsible of Rivière des Galets structure construction
7 Région Reunion Planning dept Saint Denis
12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
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Vulnerability mitigation of the area behind the dykes
Type of constructions and activities to be authorized is controlled through severe urban planning rules, in order to avoid any extra-disaster or any casualties, in case of a dyke break. Accepted risk and responsibility have been the main thread along years.
Repairing works and preventive reinforcement works
Reinforcement of existing dykes and groynes, earth works behind the dykes (reverse slope) have been carried out in order to mitigate damages in case of dyke break. An emergency channel has been dug, boulders stocked for quick repairing in case of dyke breach. Damages after each cyclone are every time quickly repaired in order not to let a weak point in case of another cyclone.
Structure monitoring
A monitoring system has been set up for dykes and groynes. Regular inspection visits are organised, leading to structure diagnosis, specific expert missions when needed, reinforcement proposals, annual reports.
Training
Regular trainings have been organised for 20 years either in Reunion Island or in Lyon. Trainings covered theoretical and practical topics such as torrential hydraulics, morphology, dyke monitoring and maintenance, experience feedback, with examples from the Rhone River.
CONCLUSION
All these steps have been performed with confident collaboration between complementary stakeholders. This approach has led to a comprehensive and consistent mastering of hydraulic risk.
The full paper will develop the different mentioned steps and will highlight essential ones for a sensible mastering of natural risk.
Keywords: Reunion Island, La Rivière des Galets, cooperation for risk management
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