Author(s): A. M. Berta; G. Bianco
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Abstract: During floods the effects of solid transport in river beds cannot be neglected, either by a morphological or an hydrodynamical point of view. Sediment transport is closely related to the “moving layer”, i.e. the water-sediment layer which moves in the lower part of the flow. Moving layer's variations along rivers lead to depositions and erosions and are typically unsteady, but are often tackled with expressions developed for steady (equilibrium) conditions, as a consequence of the still limited knowledge of the sediment transport in strong time-dependent conditions and of the scarcity of measures. This paper investigates both theoretically an experimentally the moving layer. A rapidly changing unsteady flow was in fact reproduced during laboratory tests by the erosion of a saturated steep slope, built with non-cohesive granular materials. Even though water-sediment flows ranged from hyper-concentrated to ordinary bed load transport, we have experimentally quantified and used only moving layer's depths relevant to hyper-concentrated mono-dimensional flows, occurred for slope gradients in the range 3% -20%. The results show that for fixed water discharges, moving layer depths increased from upstream to downstream: the comparison between moving layer measures and relating equilibrium values shows a tendency toward equilibrium conditions. In this study we highlight the difficulty of knowing the equilibrium's achievement, not only due to the choice of the equilibrium's expression, but because strongly dependant on the estimation of the parameters involved (for ex. friction angle..). An in-house developed model is proposed to evaluate moving layer depth from unsteady flows, and is calibrated on our experimental data. The model can be applied both on open channels and on embankments/dams providing that the flows can be modelled as mono-dimensional, and that slopes and applied shear stress levels fall within the proposed ranges.
Year: 2009