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Theses

Influence d`un fossé sur les écoulements rapides au sein d`un versant

Abstract : The subject of this thesis is related to ditches as hydrological discontinuities and to their influence (drainage, infiltration, flow towards the downstream) on pesticide transfers between the agricultural field and surface waters. Ditches can have different behaviours according to the slope and especially to the compared levels of groundwater and water in the ditch (drainage or infiltration function). So far, most knowledge about the behaviour of ditches in transient flow is dedicated to structures located at the bottom of talwegs : for these ditches, one can suppose a no flux boundary under their bed.. This is not the case for ditches located in slopes, which have however a significant influence on the transfer of pollutants and thus on the quality of water in agricultural environment. Two ditches of quite different characteristics were instrumented on the experimental site of La Jaillière, (managed by Arvalis, North-West of France) : a ditch perpendicular to the slope, which maximizes the interception of the flow coming from the upstream, and a ditch located in the direction of slope, for which the transfer of collected water is the dominating role. A hydrodynamic characterization of the soils supporting the ditch across the slope transversely (where the study is concentrated) was carried out. The site is characterized by two inter-connected aquifers, whose dynamics and interactions were studied, on a seasonal scale and on an event scale. Data analysis (from September 2000 to April 2003) allowed us to identify three zones with well typified behaviours, and to distinguish three periods on an annual scale, where the dynamics of the aquifers are different (start period / high waters period / recession period). A fully saturated flow modelisation was developed with the software VisualMODFLOW on the scale of the slope. This enabled us to quantify the flow exchanges between the two aquifers, to validate the assumptions of suggested behaviours and to identify the boundary conditions of the ditch system. Once the ditch system was isolated, we undertook in the framework of the DEA of F Branger a variably saturated flow modelisation with the software Hydrus 2D. That enabled us to improve the modelisation of the local behaviour of the surface aquifer and of the ditch. With regard to the boundary conditions, the data provided by MODFLOW appeared very satisfactory. However the hydrodynamic parameters resulting from MODFLOW, in particular the saturated hydraulic conductivity, appeared unsuited for Hydrus-2D, undoubtedly due to the changing scale. This work allowed us to understand the coupled dynamics of both the aquifers on the experimental slope and to estimate the influence of a ditch perpendicular to the slope on their local behaviour. Orders of flow magnitude drained by the ditch, according to the considered periods were obtained.
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  • IRSTEA : PUB00014560

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V. Adamiade. Influence d`un fossé sur les écoulements rapides au sein d`un versant. Sciences de l'environnement. Université Pierre et Marie Curie, Spécialité Géosciences et Ressources Naturelles, Paris VI, 2004. Français. ⟨tel-02583265⟩

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