Incipient motion of a bimodal mixture of gravels and silts: A laboratory experimental study.

Emeline Perret, Albert Herrero, Céline Berni, Kamal El kadi Abderrezzak, Benoît Camenen

Monday 29 june 2015

17:00 - 17:15h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 3A. Sediment - Erosion

Water management requires a good understanding of sediment transport and associated river morphodynamics. In mountain and piedmont hydro-systems, gravel-bed rivers often transport large amounts of fine (silt-sized) sediment. The bed material can be regarded as a mixture of coarse (gravels) and fine particles (silt, clay, very fine sand). Whether they are transported separately or not cannot be answered simply, because complex interactions between these two classes of sediment exist. As a consequence, the riverbed is usually made of heterogeneous layers of gravel particles clogged by fine sediment infiltrated in the pores of the matrix. The hydraulic conditions for the deposition and erosion of such sediment mixtures are poorly known, although they are highly related to ecological (affection of river’s habitat) and socio-economic issues (dam management). In this paper, new laboratory experiments are presented to characterize the influence of fine particles on sediment transport. These experiments are conducted in the HHLab of Irstea in Lyon-Villeurbanne (France). Transport of fine (silt, 10 to 30 _m in diameter) and coarse (gravels, 5 to 12 mm in diameter) particles are studied in this controlled environment under conditions close to the incipient motion. Both gravel and silt are moderately sorted. The incipient motion conditions are quite difficult to measure. Several methods for determining the critical shear stress are proposed, but most of them suffer from uncertainty or incomplete descriptions of the experimental parameters. In this paper, we show that according to the chosen methods the value of the critical discharge for incipient motion can vary significantly (23 %). Three kinds of experiment have been achieved: one with a loose gravel bed, one with a well-settled bed, and one with a loose gravel bed clogged with silt-sized sediment. The critical discharge for incipient motion of the bed material was observed to be larger for the gravel-silt mixture. The initial arrangement of the bed appeared to be a crucial parameter, affecting directly the critical discharge