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Home page > Séminaires > Séminaires 2019 > 13 mai 2019, 11h30, Condorcet 454A. Séminaire Interne MSC. Eduardo Monsalve & Adrien Guérin : "Experimental study of the swash zone dynamics and morphology of a stepped beach" & "Formation of rain flutes over soluble rocks by a water flow".

13 mai 2019, 11h30, Condorcet 454A. Séminaire Interne MSC. Eduardo Monsalve & Adrien Guérin : "Experimental study of the swash zone dynamics and morphology of a stepped beach" & "Formation of rain flutes over soluble rocks by a water flow"

Sauf mention contraire, les séminaires et les soutenances se déroulent à 11h30 en salle 454A du bâtiment Condorcet.


Experimental study of the swash zone dynamics and morphology of a stepped beach

Eduardo Monsalve

In this experimental project, we study the development of a granular cross-shore beach profile under water wave forcing. We generate waves in a narrow flume 3 m long and 8 cm wide, which contains light plastic particles. Waves propagate through a deep water region (10 cm deep) before attaining an erodible beach. The shear stress exerted by the waves on the granular bottom produces transport and modifies the beach profile until an equilibrium is reached.

The beach length, slope and curvature are determined by the several contributions, among which stand out grain density, size, bottom shear stress and wave dissipation. By measuring the surface displacement and beach profile at any time in the swash zone, and by using mass conservation and momentum balance, we estimate the fluid velocity and critical shear stress during breaking. In addition, we measured experimentally the viscous fall velocity of the grains, which together with the wave non-linearity (asymmetry) complete the model of sediment transport. This permit us to predict and verify experimentally the beach profile at the equilibrium. Particular attention is given to the beach step generated by a backwash vortex generated after flow separation under certain physical conditions.

Formation of rain flutes over soluble rocks by a water flow

Adrien Guérin

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 (10×20 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 μm). 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 (advectiondiffusion) explains this scaling law, which verifies on salt as well as plaster. Second, approximately 1mm-wide parallel rills spontaneously develop on the initially flat surface of salt (resp. plaster), at the time scale of a minute (resp. - 30 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.

References [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 Gabrovšek, The problem of rillenkarren development : a modelling perspective. In Karst Rock Features, Carsologica 9, pp. 55-61, 2009.


Contact : Équipe séminaires / Seminar team - Published on / Publié le 9 mai


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