Sediment development observations, made in a man-entry combined trunk sewer equipped with non-separating gullies, enable us to describe the principles of deposit formation phenomena related to the storm and dry weather flows, and to the characteristics of this particular material. Measurements of sediment depth taken three times a week, each ten metres along the 460 metres clogged up sections of the sewer, are completed by hydraulic measurements and sediment characterization. Sewer sediment in the upstream network is also regularly observed. The rains may cause a sudden and heavy increase of sediment volume in the trunk sewer as well as local erosion according to the hydraulic singularities. The dry weather flow grades the surface of the sediment and creates a new gradient profile for the bed, which overtime becomes steeper with sediment deposition. The general trend of the curve of the development in time of the volume of sediment deposited is asymptotic, so it seems possible to reach an “equilibrium situation” being a function of the size of the entrance material. Limit conditions of deposition are studied by using Shields' relation, and principles of preventive or curative solutions are proposed.

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