Ecological functioning of the Seine river downstream the wastewater treatment plant "Seine Aval": experimental datas and bidimensional modeling.

Laurent Martin
Abstract : A reach of the Seine river, spreading from upstream the effluents of the treatment plant "Seine Aval", down to the Andrésy and Dénouval's dams was selected as detailed study site for the water quality evolution process of this river. The previous works from the PIREN Seine have shown that the inorganic pollution generated by urban effluents was tightly related to the solid phase. Thus within the water column, the thin particular phase, which is the only one able to be transported, includes very thin suspended material often rich in organic matter, bacterias attached to this particles and phytoplanktonic cells. The sedimentation and erosion processes in fluvial environment have a critical impact on water quality. The biological processes, heavy consumer of dissolved oxygen ( heterotrophic bacterain degradation of the organic matter), strongly depends on time residence of the sediment in the river. Bidimensional modelings were developped in order to estimate this residence time, which depends on hydrodynamic parameters as well as specific particle parameters ( particle size analysis, density, water content, settling rate,...). The hydrodynamic part of the study was realised with the TELEMAC 2D model from the LNHE ( Laboratoire National d'Hydraulique et Environnement, EDF) The solid transport and biological equations were integrated to the SUBIEF model from LNHE. Navigation, the main process of the erosion during low flood period, was explicitly introduced in the model. So was the exchange process between the water column and the superficial sediment layer, wether they are diffusive or constrained by deposit and erosion. The results generated for several low flow period simulations were compared to experimental datas. They demonstrated that the conceptualisation selected for the water quality model matches the simulation of the main processes occuring immediatly downstream of a big urban effleuent. Several timescales of the sediment transport have been identified. This work shows that coupling solid transport processes and biological processes is mandatory in order to accurately study the water quality of a fluvial ecosystem subject to human impact.
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Laurent Martin. Ecological functioning of the Seine river downstream the wastewater treatment plant "Seine Aval": experimental datas and bidimensional modeling.. Sciences of the Universe [physics]. École Nationale Supérieure des Mines de Paris, 2001. English. ⟨pastel-00005463⟩

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