HR: 0800h
AN: H11E-0334    [Abstracts]
TI: Infiltrative Instability Near a Topographic Jump. Implication for the Underground Drainage of Soluble Rocks
AU: * Genthon, P
EM: pierre.genthon@noumea.ird.nc
AF: IRD Paleotropique, BP A5, Noumea, 98848 New Caledonia
AU: Ormond, a
EM: anne.ormond@cnes.fr
AF: OMP Dynamique Terrestre et Planetaire, 14 Ave. E. Belin, Toulouse, 31400 France
AB: The infiltrative instability takes place when an reactive fluid is injected into a soluble porous matrix. It has been widely studied using either experimental or numerical approaches, when the fluid is injected along the axis of a core sample or radially from it's axis of symmetry. However it is known that in the case of a limestone formation, the initiation of a drainage network is associated to topographic gradients due either to a tectonic event or to a sea level change. Recently, the evolution of a fissure network submitted to dissolution by meteoric water has been simulated and compared with observations of speleologists. But in this case channel orientations result largely from the initial fissure distribution. We propose that the infiltration instability should be also studied in a initially homogeneous formation to assess the contribution of initial heterogeneities. Therefore, we have simulated the infiltrative instability near a topography jump, with simplified chemical system and boundary conditions. The dissolution is assumed to occur either instantaneously or with a first order kinetics, and the porous medium is assumed to be permanently saturated by the fluid and to lie above a perfectly impervious medium. Then the characteristic numbers of our study reduce to the P\'{e}clet and Dahmkohler numbers and to the permeability ratio between the initial porous medium and the dissolution channels. The numerical method involves a multigrid solver for the flow equation and a Volume of Fluid method in the case of instantaneous dissolution. Our models produce first a short channel near the edge of the jump, emerging at mid-height of the cliff (non-dimensional time : 0.06) while the formation of the drainage of the whole formation involves a much longer time scale (non-dimensional time : 13 for the drainage of a length of twice the height of the topographic jump). Moreover, the style of dissolution instabilities varies in the upstream direction and tends to the formation of large zones of pervasive dissolution far from the jump. This is viewed as the result of the decreasing filtration velocity inside the initial homogenous porous medium that implies a decrease of the P\'{e}clet number and an increase of the Dahmkohler number. Possible consequences on the drainage structure of carbonated or silicated soluble rocks are discussed.
DE: 3220 Nonlinear dynamics
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1620 Climate dynamics (3309)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting