Morphodynamic effects of vegetation life stage on experimental meandering channels

Previous studies have demonstrated that riparian vegetation leads to channel transformation from a multi-bar to a single-thread channel planform. However, it still remains unclear how the presence of pioneer and mature vegetation affects the morphodynamics of single-thread meandering rivers. In this study, we therefore investigated the effects of vegetation strength on the morphodynamic evolution of an experimental meandering channel. Three physical laboratory experiments were conducted using alfalfa sprouts in different life stages – no vegetation, immature vegetation, and mature vegetation – to simulate different floodplain vegetation strengths. Our results demonstrate that vegetation plays a key role in mediating bank erosion and point-bar accretion, and that this is reflected in both the evolution of the channel bed as well as the sediment flux. The presence of mature vegetation maintained a deep, single-thread channel by reducing bank erosion, thereby limiting both channel widening and sediment storage capacity. Conversely, an unvegetated floodplain led to channel widening and high sediment storage capacity. Channel evolution in the unvegetated scenario showed that the active sediment supply from outer bank erosion led to slightly delayed point-bar accretion on the inner banks due to helical flow, deflecting the surface flow toward the outer banks and causing further erosion. In contrast, in the immature vegetation scenario, the outer banks were also initially eroded, but point-bar accretion did not clearly progress. This led to a greater width-to-depth ratio, resulting in a transition from a single- to a multi-thread channel with minor flow paths on the floodplain. The experimental results suggest that the eco-morphodynamic effects of young (low-strength) and mature (high-strength) vegetation are different. Notably, low-strength, early-stage vegetation increases channel complexity by accelerating both channel widening and branching, and therefore might promote the coexistence of multi-bars and pioneer vegetation.     

分层气流条件下地形降水的二维理想数值试验

利用WRF v3.5中尺度数值模式,在条件不稳定层结下,针对分层气流(基本气流风速和大气湿浮力频率呈二层均匀分布)过山时,地形对降水的影响进行了多组二维理想数值试验,以研究不同高度、尺度山脉和不同方向基本气流对降水形态和分布的影响。模拟结果表明,地形重力波触发对流是地形降水的主要机制之一,地形波的特征(波长、振幅)和传播均受到地形和基本气流的影响,其中,强基本气流流经高而陡峭的山脉时,更容易在其背风坡捕捉到重力波,地形降水呈现多种模态,反之亦然;当改变基本气流与山脉交角时,主要通过影响地形强迫抬升速度、基流对波动稳定性发展来进一步影响地形降水的强度和分布。     

Interferometric Measurements of the Interaction of Two Plasmas in a Transverse Magnetic Field

Presented are interferometric results of the interaction of two plasmas without and with a magnetic field. This study is based on the collision-free interaction of two millimetre-scale, counter-streaming plasmas – a proposed experimental simulation of shock production in a supernova remnant. This collision-free interaction is compared with a separate set of experiments with an external 7.5 T magnetic field applied. The interaction dynamics are inferred from spatially and temporally resolved electron density measurements, and the effect of the magnetic field on the plasma interaction is discussed.     

Quantitative infrared spectra of hydrosilicates and related minerals

Absorption coefficients associated with atomic motions of species expected in astronomical environments are determined from infrared measurements of various hydrosilicates, hydrated magnesium oxide, and the Al-bearing chain silicate, sapphirine. Band types measured include O–H stretching modes near 3 μm, Si–O stretching motions near 10 μm, Si–O–Si bends near 14 μm, O–Si–O bends near 20 μm, and translations of cations such as Mg and Ca near 50–200 μm. We obtain data from films of varying thickness and use a ratioing method. First, bandstrengths of O–H fundamentals were determined from spectra obtained from films of controlled thicknesses, generally 6 μm. The O–H absorbance strength was then used to accurately determine thickness for a thinner film of each mineral (found to be  <1 μm  ), thus providing bandstrengths of all other absorptions. Thin films were prepared such that the fundamental lattice modes showed intrinsic behaviour (i.e. band shapes were unchanged upon further thinning) and O–H modes are well resolved above the spectral noise. Bandstrengths were found to depend weakly on structure and should be applicable to other silicate minerals, allowing estimation of elemental concentrations independent of knowing the speciation of dust in astronomical environments. Comparison with observational data of NGC 6302 suggests that lizardite and saponite could be present in addition to refractory minerals.     

The role of laboratory experiments in the characterisation of silicon–based cosmic material

Abstract. Silicate grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, silicate grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of silicate grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of silicate grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by silicate grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about silicates in space and to provide to the reader some indication of the future developments in the field. Received 25 April 2002 / Published online 14 November 2002 Send offprint requests to: L. Colangeli e–mail: colangeli@na.astro.it     

Subsurface storm flow formation at different hillslopes and implications for the ‘old water paradox’

Although many studies over the past several decades have documented the importance of subsurface stormflow (SSF) in hillslopes, its formation is still not well understood. Therefore, we studied SSF formation in the vadose soil zone at four different hillslopes during controlled sprinkling experiments and natural rainfall events. Event and pre‐event water fractions were determined using artificially traced sprinkling water and ²²²Rn as natural tracer. SSF formation and the fraction of pre‐event water varied substantially at different hillslopes. Both intensity of SSF and fraction of pre‐event water depended on whether SSF in preferential flow paths was fed directly from precipitation or was fed indirectly from saturated parts of the soil. Soil water was rapidly mobilized from saturated patches in the soil matrix and was subsequently released into larger pores, where it mixed with event water. Substantial amounts of pre‐event water, therefore, were contained in fast flow components like subsurface storm flow and also in overland flow. This finding has consequences for commonly used hydrograph separation methods and might explain part of the ‘old water paradox’. Copyright © 2007 John Wiley & Sons, Ltd.     

Experimental studies on pathway patterns of secondary oil migration

The oil migration patterns in porous medium are discussed based on the results of diphasic expulsing experiments. In the experiments, dyed kerosene was used as expulsing phase to expulse water saturated in a glass cylinder filled with sorted glass beads. Two series of experiments were done: one with different initial oil column heights and the other with variable oil injection rates. Three secondary migration patterns, including stable displacement pattern, fingering pattern and stringing pattern may be recognized in each series of experiments. Two classic dimensionless parameters, the modified Bond number and the Capillary number, were used to construct a phase diagram that may characterize the patterns. A new method was proposed to calculate the Darcian velocity of moving kerosene in the cylinder when only buoyancy force acts. It is found that when Darcian velocity is used to get the two dimensionless parameters, the data from both series of experiments, as well as those obtained from literatures, may be reasonably drawn in a phase diagram to identify the three migration patterns.     

What can We Learn about Atmospheric Meteor Ablation and Light Production from Laser Ablation?

Laboratory based laser ablation techniques can be used to study the size of the luminous region, predict spectral features, estimate the luminous efficiency factor, and assess the role of chemically differentiated thermal ablation. A pulsed Nd:YAG laser was used to ablate regions from ordinary and carbonaceous chondrite meteorites. CCD cameras and a digital spectroscope were used to measure the size and spectrum from the cloud of vaporised material. Scanning electron microscope (SEM) based energy dispersive x-ray spectroscopy (EDS) provided elemental abundance values in ablated and unablated regions. These results indicated some degree of differential ablation, with the most significant effect being significant loss of carbon from carbonaceous chondrites. This work suggests that a carbon matrix may play the role of the glue in the two component dustball model.