The impact of hydrograph variability and frequency on sediment transport dynamics in a gravel-bed flume

A laboratory study was undertaken to investigate how changes in flow regime and hydrograph shape (number of cycled hydrographs and duration of each hydrograph) together impact bedload transport and resulting bed morphology. Three hydrologic conditions (experiments) representing different levels of urbanization, or analogously different flow regimes, were derived from measured hydrometric field data. Each experiment consisted of a series of hydrographs with equal peak discharge and varying frequency, duration and flashiness. Bedload transport was measured throughout each hydrograph and measurements of bed topography and surface texture were recorded after each hydrograph. The results revealed hysteresis loops in both the total and fractional transport, with more pronounced loops for longer duration hydrographs, corresponding to lower rate of unsteadiness until reaching the peak discharge (pre-urbanization conditions). Shorter duration hydrographs (urban conditions) displayed more time above critical shear stress thresholds leading to higher bedload transport rates and ultimately to more variable hysteresis patterns. Surface textures from photographic methods revealed surface armoring in all experiments, with larger armor ratios for longer duration hydrographs, speculated to be due to vertical sorting and more time for bed rearrangements to occur. The direction of bed surface adjustment was linked to bedload hysteresis, more precisely with clockwise hysteresis (longer hydrographs) typically resulting in bed coarsening. More frequent and shorter duration hydrographs result in greater relative channel adjustments in slope, topographic variability and surface texture. (c) 2019 John Wiley & Sons, Ltd.

Slow magnetic dynamics and hysteresis loops of the bulk ferromagnet Co-7(TeO3)(4)Br-6

Helmuth Berger, Ivica Zivkovic

Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound Co-7(TeO3)(4)Br-6, was studied by ac susceptibility and related techniques. Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt time (2 x 10(-4) s) span the full spectrum of magnetic dynamics inside a convenient frequency window, offering a rare opportunity for general studies of magnetic dynamics. Within the experimental window, the ac susceptibility data build almost ideally semicircular Cole-Cole plots. A comprehensive study of experimental dynamic hysteresis loops of the compound is presented and interpreted within a simple thermal-activation-assisted spin-lattice relaxation model for spin reversal. Quantitative agreement between the experimental results and the model's prediction for dynamic coercive field is achieved by assuming the central physical quantity, the Debye relaxation rate, to depend on frequency, as well as on the applied field strength and sample temperature. Crossover between minor to major hysteresis loops is carefully analyzed. Low-frequency limitations of the model, relying on domain-wall pinning effects, are experimentally detected and appropriately discussed.

2011

Sediment Transport and Flow Conditions in Steep Rivers with Large Immobile Boulders

Tamara Ghilardi

Recent flood events in Switzerland and across Europe pointed out several deficiencies in hazard assessment, planning, and prediction methods for flood risk mitigation. A good understanding of the dynamics of mountain rivers, grounded on a sound physical-based theoretical framework, is primordial both from an environmental and a safety point of view. Although mountain rivers control sediment supply to lowland rivers, relatively few studies have been carried out on steep mountain channels, mainly during the last two decades. While these studies provide a multitude of sediment transport equations, generally with similar forms, most of them does not take into account the extreme conditions characterizing alpine torrents. For these latter, the presence of macro-roughness elements, such as large relatively immobile boulders, disrupts the flow and alters channel roughness. Moreover, bedload fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions, even under constant sediment feeding and water discharge. This research project investigates the impact of randomly distributed boulders (cascade morphology) on the sediment transport capacity and bedload fluctuations in steep channels. This is done by means of 41 laboratory experiments, carried out on a tilting flume at the Laboratory of Hydraulic Constructions (LCH) at Ecole Polytechnique Fédérale de Lausanne (EPFL). The influence of several boulder sizes and distance between roughness elements is investigated for three flume slopes (S=6.7%, 9.9% and 13%). Sediment transport, bulk mean flow velocities and variables describing the morphology were assessed regularly during the experiments. Firstly, the detailed analysis of a 13 hours laboratory experiment is presented. Periodical bedload pulses are clearly visible on this long duration experiment, along with correlated flow velocity and bed morphology fluctuations. The relation among bulk velocity, morphology variables time evolution and bedload transport is investigated by correlation analysis, showing that fluctuations are strongly linked. Visual observations indicate that the detected periodical fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, varying in time and space, clearly influences these bedload pulses. For all the experiments, bedload pulses were then characterized by their amplitude and period. It is shown that for higher stream power the fluctuations decrease, both in duration of a cycle and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediments, thus decreasing the fluctuations. The impact of increasing channel slopes on sediment transport is well known. The present research shows that it is also indispensable to take into account the presence of boulders in the estimation of the sediment transport capacity, since it is strongly decreasing with dimensionless boulder distance. Sediment transport capacity is better estimated when taking the liquid discharge as basis parameter instead of bed shear stress. The critical discharge for incipient motion is shown to be dependent not only on the channel slope but also on the dimensionless distance between boulders. A sediment transport formula based on excess discharge and taking into the presence of boulders is herein developed.

EPFL2013

Magnetodynamical response of large-area close-packed arrays of circular dots fabricated by nanosphere lithography

Dirk Grundler

We report a combined experimental and theoretical study of the quasistatic hysteresis and dynamic excitations in large-area arrays of NiFe nanodisks forming a hexagonal lattice with the lattice constant of 390 nm. Arrays were fabricated by patterning a 20-nm-thick NiFe film using the etched nanosphere lithography. We have studied a close-packed (edge-to-edge separation between disks dcp = 65 nm) and an ultraclosed packed (ducp = 20 nm) array. Hysteresis loops for both arrays were qualitatively similar and nearly isotropic, i.e., independent on the in-plane external field orientation. The shape of these loops revealed that magnetization reversal is governed by the formation and expulsion of vortices inside the nanodisks. When we assumed that the nanodisks’ magnetization significantly decreases near their edges, micromagnetic simulations with material parameters deter-mined independently from continuous film measu-rements could satisfactorily reproduce the hysteresis. Despite the isotropic hysteresis, significant in-plane anisotropy of the dynamic response of the ultraclose-packed array was found experimentally by the all-electrical spin-wave spectroscopy and Brillouin light scattering. Dynamical simulations could successfully reproduce the difference between excitation spectra for fields directed along the two main symmetry axes of the hexagonal lattice. Simulations revealed that this difference is caused by the magnetodipolar interaction between nanodisks, which leads to a strong variation of the spatial distribution of the oscillation power both for bulk and edge modes as a function of the bias field orientation. Comparison of simulated and measured frequencies enabled the unambiguous identification of experimentally observed modes. Results of this systematic research are relevant both for fundamental studies of spin-wave modes in patterned magnetic structures and for the design of magnonic crystals for potential applications as, e.g., spin-wave guides and filters.