The active layer in gravel-bed rivers: An empirical appraisal

The vertical position of the streambed–water boundary fluctuates during the course of sediment transport episodes, due to particle entrainment/deposition and bedform migration, amongst other hydraulic and bedload mechanisms. These vertical oscillations define a topmost stratum of the streambed (i.e. the ‘active layer or active depth’), which usually represents the main source of particles entrained during long and high-magnitude bedload transport episodes. The vertical extent of this layer is hence a capital parameter for the quantification of bedload volumes and a major driver of stream ecology in gravel-bed rivers. However, knowledge on how the active depth scales to flow strength and the nature of the different controls on the relation between the flow strength and the active depth is still scarce. In this paper we present a meta-analysis over active depth data coming from ~130 transport episodes extracted from a series of published field studies. We also incorporate our own field data for the rivers Ebro and Muga (unpublished), both in the Iberian Peninsula. We explore the database searching for the influence of flow strength, grain size, streambed mobility and channel morphology on the vertical extent of the active layer. A multivariate statistical analysis (stepwise multiple regression) confirms that the set of selected variables explains a significant amount of variance in the compiled variables. The analysis shows a positive scaling between active depth and flow strength. We have also identified some links between the active depth and particle travel distances. However, these relations are also largely modulated by other fluvial drivers, such as the grain size of the bed surface and the dominant channel macro-bedforms, with remarkable differences between plane-bed, step-pool and riffle-pool channels. © 2020 John Wiley & Sons, Ltd.     

Comparing single-phase,non-Newtonian approaches with experimental results: Validating flume-scale mud and debris flow in HEC-RAS

Post-fire debris flows and tailing impoundment failures destroy lives and property. These geologic hazards – and other similar processes – fall on a continuum between classic Newtonian flood analyses and geotechnical stability analyses. The US Army Corps of Engineers (USACE) is developing a non-Newtonian library (DebrisLib) that includes a suite of rheological and clastic approaches to hyper-concentrated, mudflow and debris flow dynamics. The Hydrologic Engineering Center (HEC) has implemented these non-Newtonian methods into the widely used, public-domain open-channel hydraulics and morphodynamic software, HEC-RAS (river analysis system). This work presents part of the verification and validation of these non-Newtonian approaches, applying several rheological equations to published laboratory results high-concentration flume experiments. This study tested the linear Bingham model as well as the turbulent and Bagnold quadratic terms of the O'Brien equation. HEC-RAS also includes the non-linear Herschel–Bulkley (HB) approach, which quantifies shear-thickening or shear-thinning processes. The study used these non-Newtonian models in HEC-RAS to simulate 10 of Parsons et al.’s (2001) flume experiments, which measured the snout and plug velocity of fluids with high solid concentrations (C_v = 68–74%) and a broad range of material gradations (d₅₀ = 0.05–1 mm, d₁₅ = 0.006–0.1 mm). The experiments also measured and back-calculated Bingham and HB parameters of the materials, finding HB powers between 0.45 and 1.25 (i.e. fluids that are dilatant, pseudo-plastic and visco-plastic). The rheological models incorporated into DebrisLib and implemented in HEC-RAS reproduce experimental data well for most experiments. The Bingham model generated a plug velocity root-mean-square error (RMSE) of 0.21 m/s using standard flow parameters and Parsons et al.’s calibrated parameters, a substantial improvement over the unmodified shallow water flow equations (RMSE 0.77 m/s). Experiments with strong snout effects tended to generate higher residuals, especially in the snout velocity. The RMSE associated with the O'Brien equation was larger with the Parsons et al. fit parameters, but similar (0.23 m/s) with measured parameters. The turbulent parameter was the largest (often the dominant) parameter in most O'Brien simulations, with the dispersive stress only proving significant for the coarsest material. DebrisLib had to use a modified version of the dispersive term to simulate these concentrations. Both the 2D depth-averaged shallow water equation (SWE) and diffusion wave equation were used to simulate the experiments. The best results were obtained with the SWE with horizontal mixing.     

Three-dimensional tomographic inversion of the seismic velocity structure of the crust and upper mantle of northwestern Nicaragua

Journal of Seismology - A three-dimensional tomographic image of the seismic velocity structure in the crust and upper mantle of northwestern Nicaragua was performed using the back-projection...     

CoRoT/ESTA–TASK 1 and TASK 3 comparison of the internal structure and seismic properties of representative stellar models

We compare stellar models produced by different stellar evolution codes for the CoRoT/ESTA project, comparing their global quantities, their physical structure, and their oscillation properties. We discuss the differences between models and identify the underlying reasons for these differences. The stellar models are representative of potential CoRoT targets. Overall we find very good agreement between the five different codes, but with some significant deviations. We find noticeable discrepancies (though still at the per cent level) that result from the handling of the equation of state, of the opacities and of the convective boundaries. The results of our work will be helpful in interpreting future asteroseismology results from CoRoT.     

Fragmentation of the Current Sheet,Anomalous Resistivity,and Acceleration of Particles

The evolution of the current sheet in the electric current direction (in the guiding magnetic field direction) is studied numerically in the 3-D particle-in-cell model with two current sheets and periodic boundary conditions. In the regime with (where v _D and are the electric current drift and electron thermal velocities, respectively) the current sheets are unstable owing to the Buneman and kink instabilities and become strongly fragmented. During their evolution, in addition to an increase of the energy of the electric field component in the guiding magnetic field direction, the energies of the electric field components in the perpendicular direction are even more enhanced. In the current sheet the anomalous resistivity (η _anom/η _C∼7×10⁵, where η _C is the classical resistivity) is generated and thus the magnetic field dissipates. Most of the dissipated magnetic energy is transformed into the electron kinetic energy in the direction of the electric current. The associated electric field accelerates the electrons from the tail of the distribution function.     

A Flare-Generated Shock during a Coronal Mass Ejection on 24 December 1996

We present a multiwavelength study of the large-scale coronal disturbances associated with the CME?–?flare event recorded on 24 December 1996. The kinematics of the shock wave signature, the type II radio burst, is analyzed and compared with the flare evolution and the CME kinematics. We employ radio dynamic spectra, position of the Nançay Radioheliograph sources, and LASCO-C1 observations, providing detailed study of this limb event. The obtained velocity of the shock wave is significantly higher than the contemporaneous CME velocity (1000 and 235 km?s^?1, respectively). Moreover, since the main acceleration phase of the CME took place 10?–?20 min after the shock wave was launched, we conclude that the shock wave on 24 December 1996 was probably not driven by the CME. However, the shock wave was closely associated with the flare impulsive phase, indicating that it was ignited by the energy release in the flare.     

Exceptional Fireball Activity of Orionids in 2006

We report exceptional fireball activity of the Orionid meteor shower in 2006. During four nights in October 2006 the autonomous fireball observatories of the Czech part of the European Fireball Network (EN) recorded 48 fireballs belonging to the Orionids. This is significantly more than the total number of Orionids recorded during about five decades long continuous operation of the EN. Based on precise multi-station photographic and radiometric data we present accurate atmospheric trajectories, heliocentric orbits, light curves and basic physical properties of 10 Orionid fireballs with atmospheric trajectories that were long enough and, with one exception, were observed from at least three stations. Seven were recorded in within a 2-h interval in the night of 20/21 October. Their basic parameters such as radiant positions and heliocentric orbits are very similar. This high fireball activity originated from a very compact geocentric radiant defined by α = 95.10° ± 0.10° and δ = 15.50° ± 0.06°. These fireballs most likely belonged to a distinct filament of larger meteoroids trapped in 1:5 resonance with Jupiter. From detailed light curves and basic fireball classification we found that these meteoroids appertain to the weakest component of interplanetary matter.     

Orbital Evolution of Příbram and Neuschwanstein

The orbital evolution of the two meteorites Příbram and Neuschwanstein on almost identical orbits and also several thousand clones were studied in the framework of the N-body problem for 5,000 years into the past. The meteorites moved on very similar orbits during the whole investigated interval. We have also searched for photographic meteors and asteroids moving on similar orbits. There were five meteors found in the IAU MDC database and six NEAs with currently similar orbits to Příbram and Neuschwanstein. However, only one meteor 161E1 and one asteroid 2002 QG46 had a similar orbital evolution over the last 2,000 years.     

Large Dust Grains Around Cometary Nuclei

Large amounts of particles ejected from the nucleus surface are present in the vicinity of the cometary nuclei when comets are near the Sun (at heliocentric distances ≤2 AU). The largest dust grains ejected may constitute a hazard for spatial vehicles. We tried to obtain the bounded orbits of those particles and to investigate their stability along several orbital periods. The model includes the solar and the cometary gravitational forces and the solar radiation pressure force. The nucleus is assumed to be spherical. The dust grains are also assumed to be spherical, and radially ejected. We include the effects of centrifugal forces owing to the comet rotation. An expression for the most heavy particles that can be lifted is proposed. Using the usual values adopted for the case of Halley’s comet, the largest grains that can be lifted have a diameter about 5 cm, and the term due to the rotation is negligible. However, that term increases the obtained value for the maximum diameter of the lifted grain in a significant amount when the rotation period is of the order of a few hours.     

Local and global diffusion in the Arnold web of a priori unstable systems

Using the numerical techniques developed by Froeschlé et al. (Science 289 (5487): 2108–2110, 2000) and by Lega et al. (Physica D 182: 179–187, 2003) we have studied diffusion and stochastic properties of an a priori unstable 4D symplectic map. We have found two different kinds of diffusion that coexist for values of the perturbation below the critical value for the Chirikov overlapping of resonances. A fast diffusion along some resonant lines that exist already in the unperturbed case and a slow diffusion occurring in regions of the phase space far from such resonances. The latter one, although the system does not satisfy the Nekhoroshev hypothesis, decreases faster than a power law and possibly exponentially. We compare the diffusion coefficient to the indicators of stochasticity like the Lyapunov exponent and filling factor showing their behavior for chaotic orbits in regions of the Arnold web where the secondary resonances appear, or where they overlap.