12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
SEDITEF: SIMULATION OF EROSION AND DEPOSITION PROCESSES INDUCED BY TORRENT EXTREME FLOODS
NEW SOFTWARE FOR TORRENT HYDRAULICS ENGINEERING
Jean-Marc Tacnet1, Dominique Laigle2, Didier Richard3 and Christophe Peteuil4INTRODUCTION
Mountain streams and rivers are subject to strong variations in topography due to sediment transport caused by torrent floods. The longitudinal and lateral erosion of the stream bed and banks leads to destabilization of slopes, foundations, and levees. Deposition raises the bed level and causes overflow of water and solids. In a torrent watershed, preventive management actions aim at reducing the intensity of torrential phenomena and/or minimize their effects on risk issues. Scientific understanding of phenomena related to torrent hydraulics remains very partial and expertise plays a major role in engineering this type of natural hazards. The characterization of the phenomena and the quantification of debris volumes is the first issue for engineers.
On this basis, bedload transport simulation remains an important issue for engineers and technicians:
where will the torrent overflow? How deep will the bed be incised? What is the influence of check- dams? Despite recent research on bedload transport and sediment transport modelling (Papanicolaou, 2008), no easy-to-use software is able to simulate the evolution of a torrent under heavy solid transport conditions and to analyze trends in the evolution of its longitudinal profile during a flood. The paper presents a new development resulting from collaboration between Cemagref and ONF/RTM, whose aim was to develop an easy-to-use tool for engineering but also for technical training of ONF/RTM staff.
This application is designed on the basis of relatively simple assumptions how to help quantifying the volumes and the flow heights during floods. Hydraulic assumptions are: steady flow (at each time step) without full computation of the flow propagation; input of liquid hydrographs and sedimentographs upstream of the segment and at nodes; consideration of singularities (narrowing, abrupt enlargement, concrete floor, torrent check-dams).
TECHNICAL AND NUMERICAL FEATURES
The hydraulics hypotheses of the software are based upon a 1D numerical model, designed to solve coupled equations of hydrodynamics and solid transport. The hydrodynamics set of equations is given by:
0 . .
2 2 0
v h x l
h v C
g x h x g z x v v
For the solid phase, a simple Exner’s type equation for the mass conservation is combined with a bed- load transport equation like the one proposed by Rickenmann (1991) and presented below, but other equations have been implemented:
1 PhD., Eng., Jean-Marc Tacnet. Cemagref, ETGR, Saint-Martin d’Hères, France (e-mail: jean-marc.tacnet@cemagref.fr)
2 PhD., Eng., Dominique Laigle, Cemagref, ETGR, Saint-Martin d’Hères, France
3 Didier Richard. Cemagref, ETGR, Saint-Martin d’Hères, France
4 Christophe Peteuil, RTM/ONF, Grenoble, France
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q q J s
J d
d q
q
q x l t n z
cr s
s
1 1 6
. 12
0 1
6 . 1 2
30 90
2 . 0
0
Where qcr
s
gd150.5J 1.1267 .
1 1
065 .
0
With: v: mean velocity in the cross section, h: flow depth, z0: bed elevation, C: Chezy coefficient, l:
channel width, n: porosity of the bed material, qs, q: respectively, solid and liquid discharge per unit of channel width, qcr: critical liquid discharge per unit of channel width, s: specific gravity of the solid material, d30, d50, d90: parameters of the grain size distribution.
J: is either the bed slope or the energy slope defined as
g h v x z
J 2
2 0
The set of equations is solved using a finite difference scheme, with a regular grid mesh and a classical but robust second-order Crank-Nicolson implicit time scheme.
Fig. 1 Example of user-interface for torrent profile input (including variable thickness and grain size distribution of parts of the torrent bed)
The application is designed to be evolutive and able to integrate new bedload transport laws, to simulate a multi-layer torrent bed. An easy to use interface (Fig. 1) has been designed and developed in order to allow easy data input for practitioners. Several levels of users have been considered and complex options are hidden but accessible. Training courses are planned to complete specific documentation included in the on-line help of the software.
REFERENCES
Rickenmann, D. (1991). Hyperconcentrated flow and sediment transport at steep slopes. Journal of Hydraulic Engineering 117(11): 1419-1439.
Papanicolaou, A.N., Elhakeem, M., Krallis, G., Prakash, S., Edinger, J. (2008). Sediment Transport Modelling Review – Current and Future Developments. Journal of Hydraulic Engineering 134(1):
1-14.
Keywords: bedload transport, torrent risk management, decision making, simulation
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