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This paper presents continuous field experiments carried out during 4 years in a man-entry egg-shaped combined sewer in Lyon, France in order to contribute to the knowledge and the modelling of sediment accumulation and sediment removal by means of a Hydrass flushing gate. The 250 μm sediments are mainly mineral, and their physical and chemical characteristics appear as rather stable in time and space. Long-term sediment monitoring reveals: i) a regular asymptotic increase of both the sediment mass and the slope of its longitudinal profile, ii) a clear correlation between local sediment profile irregularities and sewer ancillaries but without significant influence on the global and long term accumulation. Simple sediment accumulation modelling shows: i) a good suitability of a three parameters conceptual model to reproduce asymptotic sediment volume accumulation, and ii) a good suitability of the Velikanov model to reproduce sediment profiles. Both models reproduce observations with an acceptable margin of uncertainty for operational management purposes but are very sensitive to input data and parameter values. The Hydrass flushing gate is efficient and it appeared that the mass of sediments moves downstream linearly with the number of flushes. [ABSTRACT FROM AUTHOR]

For three years (2001-2003), sediment samples were extracted from about 100 silt traps (STs) spread out all over the combined sewer network of Paris. These STs, whose volume varied from 21 to 325m³, were cleaned out as soon as their filling capacities were reached. All these sediment samples were analysed for physicochemical parameters (pH, organic matter (OM) content, grain size distribution), total hydrocarbons (THs), 16 polycyclic aromatic hydrocarbons (PAHs) selected from the priority list of the USEPA, and heavy metals (Al, Ag, As, Cd, Cu, Cr, Sn, Fe, Mn, Hg, Ni, Pb, Zn). For each silt trap, six sediment samples were extracted before the clean out procedure: three samples were extracted from the sediment surface (5-10 cm depth) and three other samples were extracted from a deeper sediment layer (approximately at 1m depth). The location of these sampling points allowed us to take into account the possible spatial fluctuation of pollutant loads in each ST. The first results showed that there were some important inter-site variations of pollutant contents. These variations have to be taken into account by the sewer manager for the fate of the ST sediments. Therefore, we decided to assess the grain size distribution of some pollutants. OM, heavy metals and PAHs have been investigated on the five grain size fractions (>20 mm, 8-20 mm, 0.5-8 mm, 50-500 µm, <50 µm) for 9 STs, which have been selected on their heavy metal content basis. This work aims at understanding the distribution of the pollutant contents and at improving the knowledge of the ST sediment pollution. [ABSTRACT FROM AUTHOR]