séminaire "Matière molle et complexe" : Adrien Guerin (MSC Paris)

Adrien Guérin (laboratoire Matière et Systèmes Complexes, Université Paris 7) donnera un séminaire intitulé : "Formation of rain flutes over soluble rocks by a water flow"

Erosion by dissolution is a decisive process shaping small-scale landscape morphology [1]. On the surface of soluble rocks like gypsum, salt (halite) or limestone, characteristic patterns known as rain flutes or Rillenkarren can be observed. These patterns occur when the dissolving surface is inclined and subjected to a thin runoff flow. The rock surface then erodes into nearly parallel channels (rills) directed along the main slope and regularly spaced. Although these patterns are commonly observed, the conditions of their occurrence remain incompletely understood to our knowledge [2]. Here, we study in a laboratory experiment the dissolution patterns appearing at the surface of inclined blocks of soluble materials (salt and plaster of Paris) submitted to a thin free-surface flow. Rectangular blocks of salt or of plaster of Paris (10 x 20 cm and 3 cm in thickness) are tilted at a controlled angle. A constant-head reservoir supplies water at the top of the slope. Water flows down by gravity in a thin film of water uniformly spread over the salt block (typical depth 100 - 500 micrometres). The top surface of this film is a free surface, and the flow naturally adapts its velocity and the depth of the film to the two control parameters of the experiment (the flow-rate and the slope). First, we observe that the average erosion rate of a block increases with the square root of the flow velocity. A simple model of solute transport (advection diffusion) explains this scaling law, which verifies on salt as well as plaster. Second, approximately 1 mm-wide parallel rills spontaneously develop on the initially flat surface of salt (resp. plaster), at the time scale of a minute (resp. x30 min). The pattern wavelength and amplitude then increase through time. We propose that these patterns are created by the interaction between the rock surface dissolution and the flow. This interaction induces a heterogeneous velocity field, which in turn induces heterogeneous solute concentration and dissolution rate.

 [1] P. Meakin and B. Jamtveit, Geological pattern formation by growth and dissolution in aqueous systems. Proc. R. Soc. A, 466 659-694, 2010.

 [2] M. Perne and Franci Gabrovsek, Theproblem of rillenkarren development : a modelling perspective. In Karst Rock Features, Carsologica 9, pp. 55-61, 2009.