Plenary Lecture

Modeling Muddy Flash Floods and Debris Flows

Professor Blaise Nsom
Université Européenne de Bretagne
Université de Brest
LIME/LBMS/IUT de Brest. BP 93169. Rue de Kergoat. 29231. BREST CEDEX 3
France
E-mail: Blaise.nsom@univ-brest.fr

Abstract: After long and intense rains in a mountainous region, large quantities of water flow in the torrents. For some reason, this flow can be obstructed by cross-linked branches and debris (natural dam). When the hydrostatic pressure exerted by the fluid exceeds a given yield value, the dam collapses and the fluid is released inside and outside the torrent bed, as well. Such scenario known as a dam-break flow can describe the initiation of certain geological flows, (debris flows, mudflows, etc.)

As for any gravity current, the flow description depends on the time scale. Immediately after the dam collapse, the inertial forces are the dominant ones and this configuration can model a flah flood. Flash floods develop at time and space scales that conventional observation systems are not able to monitor, so reliable modelling remains a crucial step. At larger time scale, a viscous regime takes place where the viscous forces become the dominant ones and this configuration can model a classical debris flow. Debris floods develop in a long domain, i.e. a domain of space that is much longer than it is wide. They generally erode their bed and transport much energy and can move rocks and boulders upon very long distances.

Both, the flash floods and the debris floods constitute dangerous phenomena for public safety and quality of life. The originality of the present approach is to consider these two flood waves as special cases of a single global model of a dam-break flow of a muddy fluid; depending on the time scale. The study was experimental, analytical and numerical, as well. The experimental study consisted in designing model fluids to be used in the laboratory experiments, characterizing these synthetic muds and monitoring the corresponding dam-break flows in the laboratory. While the theoretical study consisted in stating the equations of motion governing the different flows studied, and solving them in their non dimensional form, both analytically and numerically.


Brief Biography of the Speaker: Professor of Mechanics at the University of Brest (France).
Obtained one Ph.D. in Aerothermics at the University of Toulouse in 1981 and another Ph.D. in Hydrodynamics at the University of Metz in 1987. He obtained the “Prix des Sciences de l’Académie Nationale de Metz” in 1988.

Nsom has been Assistant Professor at Metz and Associate Professor at Chambéry/Grenoble in 1994 and became Professor in 2000 at Brest. He is Director of Laboratory of Mechanical and Electrical Engineering, Member of the University Senate, Member of the Steering Committee of the Technopôle Brest Iroise in charge of the “Mechanics and Materials” panel. Nsom is presently the President of the Commission charged to appoint the Assistant Professors and the Professors in Mechanics, Engineering Mechanics and Civil Engineering at University of Brest.

Nsom has been Expert Evaluator of EU projects, he is presently Coordinator of the EU sub-project “Coatal Risks” in the “InterMareC” programme. He also coordinated a Report to the EU Commission on “Rheology of Debris Flows” and he is participant in national projects as well. He has supervised 7 Ph.D. theses and he has chaired national conferences and co-organized a IUTAM symposium.

His field of research concerns modelling and experimental investigations in complex fluid flows, with application to natural hazards and process. He is author or co-author of about 90 publications and communications in national and international conferences. His 2 last papers were published this year (2007) in Physics of Fluids (Stability of Couette Flow of a Stratified Fluid) and in Applied Rheology (Physical and Mechanical Characterization of Soya, Colza and Rye Seeds).