Abstract
In this paper, we present some first attempt to measure graded suspension of sands in an alpine river. It is a real challenge to measure gradedsuspensions of sands in such dynamic rivers. Specific measurements of the graded suspension in the middle of a secondary channel were attempted using a sampler that allows simultaneous sampling at three different elevations (5 cm, 10 cm, and 20 cm) above the bed. Very large concentrations (up to C = 40 g/l) of fine sediments in suspension were measured during a dam reservoir flushing event. It included large amounts of sand (from 10 to 60% of the measured concentration). A substantial graded suspension of sand was measured even for relatively low velocities. Sand fluxes estimates for varying bed shear stresses during the event are discussed. In particular, based on the measured bed shear stress, a semi-empirical formula developed for sand suspension is applied to test its ability to predict sand flux in alpine rivers. The sensibility of such formula to the grain size is very high whereas large uncertainties exist in the measurement (d50 varies from 0.1 to 0.3 mm depending on the sample), which makes its application difficult at the moment.
Highlights
A substantial amount of fine sediments can be found in piedmont gravel bed rivers
It is a real challenge to measure graded-suspensions of sands in such dynamic rivers since gravel bedload may occur at the same time
It appears that the model cannot reproduce the scatter in experimental data by using a single grain size, but it is very sensitive to the grain size as it could be expected from the settling velocity dependency
Summary
A substantial amount of fine sediments can be found in piedmont gravel bed rivers. It remains very difficult to estimate the total amount of fine sediments since they are washed out and can infiltrate the gravel matrix. Very different dynamics can be observed from one class of sediments to another, i.e. washload made of silts versus graded-suspension of sands. It is a real challenge to measure graded-suspensions of sands in such dynamic rivers since gravel bedload may occur at the same time. Measurements are generally limited to washload (through surface sampling and/or turbidity measurement) or bedload of coarse particles (by using bedload sampler with a coarse net or by using tracers).
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