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Field investigation of bedforms and flow resistance in a large sand river
No full textThe paper refers to a field study of river bedforms and their effect on the flow resistance in a large sand river (i.e. river Po-Italy). Multibeam echosounder surveying was carried out, and different computational methods are applied and compared, in order to calculate bed form pattern (spatial distribution, height, wave length, steepness). The field data in terms of dune geometry have been compared with those calculated by means of different approaches. Linear approaches to flow resistance in presence of bed form are applied and compared with field observed data in terms of energy slope. Good results are obtained ac-counting for a recently proposed semi-analytical model for the bedform drag, considering the effects of a sudden expansion of a free surface flow rather than of a pressure flow
Modeling the effect of sediment concentration on the flow-like behavior of natural debris flow
No full textThe rheological behavior of natural slurries consisting of fine-grained, reconstituted debris-flow deposits on pyroclastic terrains having different solid concentrations (ranging from 30 to 42%) has been investigated using a rotational rheometer equipped with a vane rotor system. Experiments were done by increasing the applied shear stress step by step; then a decreasing stress ramp was applied following the same shear stress levels. The slurry mixtures exhibit a typical yield-stress fluid behavior with a static yield stress larger than the dynamic yield stress. In the range of the shear rate corresponding to the flow-like behavior the slurry mixtures behave as a dilatant fluid at lower grain concentrations and as a pseudoplastic fluid in correspondence with the higher grain content, showing a strong discrepancy from the Bingham idealization. The rheological behavior is better interpreted by a Herschel-Bulkley model, whose rheological parameters strongly depend on the granular concentration. Therefore, a generalized Herschel-Bulkley model accounting for the bulk sediment concentration effect is proposed
Yield Stress Model for Natural Debris Flows in Presence of Fine and Coarse–Grained Sediments
No full textWhen dealing with natural geo–hazards, it is important to understand the influence of sediment sorting on debris flows. The presence of coarse fraction is one of the aspects which affects the rheological behaviour of natural viscous granular fluid mixtures. In this paper, experiments on reconstituted debris flow mixtures with different coarse–to–fine sediment ratios are considered. Such mixtures behave just as non–Newtonian yield stress fluids and their rheological behaviour is largely affected by the presence of coarse fraction. Experimental results demonstrate that yield stress is very sensitive not only to bulk sediment concentration but also to coarse sediment fraction. A novel yield stress model is presented. It accounts for an empirical grading function depending on the coarse–to–fine grain content. The yield stress model performed satisfactorily in comparison with the experiments, showing that it is almost independent of the coarse–to–fine grain fraction in case of dominant coarse sediment content
Thixotropic behavior of reconstituted debris-flow mixture
Full text linkTime-dependent rheological properties and thixotropy of reconstituted debris-flows samples taken from channel bank deposits are examined using a commercial rheometer equipped with a vane rotor geometric system. Sweep tests and creep tests were carried out involving mixtures having different grain concentrations ranging between 50% and 58%. Different initial conditions of the mixtures were considered in order to analyze the effects of aging and rejuvenation (thixotropy) over a short period of time and long period of time. Tested slurries show viscosity bifurcation, yield stress and time-dependent behavior. According to the experimental results, three different regimes were identified: a lower shear rate regime, corresponding to a shear rate lower than the critical value; an intermediate banding shear rate regime characterized by static and dynamic yield stress level; and a higher shear rate regime where the flowing debris behaves as a non-Newtonian fluid characterized by a constant steady state ultimate apparent viscosity. In any case, the initial state of the mixture and the sediment concentration affects the ultimate steady state rheology and the time-dependent (thixotropy) slurries’ behavior
Dune bed statistical analysis using multibeam echosound survey data
No full textIn alluvial river the bed forms represent the most relevant features governing flow resistance and bedload transport, they can have a wide range of geometry patterns corresponding to the same hydrometric condition. Bed forms influences the kinematic field, the flow resistance, the conveyance, and the stability of the fluvial infrastructures. Morphological changes and the bedload component can be evaluated considering the changing flow conditions, the geometry of the bedforms and their downstream migration. Different scales of bedforms may coexist within the channel, and so ripples are superimposed on the back of the primary forms. Bed undulation can be analysed through spectral analyses approach. In this study, Multi-Beam Echo-Sounder (MBES) surveys carried out in Po River (Italy) are considered. Combining bed surface DTM and spectral analysis, the filtered bed undulation is obtained, statistic geometrical properties of the bedforms and their kinematic are analysed
Flow field investigation by means of ADCP for the evaluation of hydropower propellers installed at open channel
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