Changes in the saltation flux following a step‐change in macro‐roughness

The effect of a step change in macro‐roughness on the saltation process under sediment supply limited conditions was examined in the atmospheric boundary layer. For an array of roughness elements of roughness density λ = 0.045 (λ = total element frontal area/total surface area of the array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness element heights into the array. This matches the value predicted using an empirical design model and provides confidence that it can be effectively used to engineer roughness arrays to meet sand flux reduction targets. Measurements of the saltation flux characteristics in the vertical dimension, including: saltation layer decay (e‐folding) height and particle size, revealed that with increasing distance into the array, the rate of mass flux change with increasing height decreased notably, and (geometric) mean particle diameter decreased. The distribution of the saltation mass flux in the vertical remains exponential in form with increasing distance into the roughness array, and the e‐folding height increases as well as increasing at a greater rate as particle diameter diminishes. The increase in e‐folding height suggests the height of saltating particles is increasing along with their mean speed. This apparent increase in mean speed is likely due to the preferential removal, or sequestration, of the slower moving particles across the size spectrum, as they travel through the roughness array. Copyright © 2018 John Wiley & Sons, Ltd.     

COMPARISON OF METHODS FOR DISTINGUISHING DIFFERENT GRADES OF GEOMORPHOLOGIC SURFACES BASED ON SEDIMENT PARTICLE SIZE FEATURES: TAKING THE QINGYIJIANG RIVER BASIN AS AN EXAMPLE

When using river geomorphology to study tectonic deformation, it is often difficult to distinguish the same level geomorphology in areas with severe weathering. In this paper, we take the geomorphologic surfaces of the Qingyijiang river basin as an example and try to distinguish the geomorphic surfaces by the sediment features that make up them. In order to distinguish different geomorphic surfaces, the traditional particle-size analysis method, SOFM network method and system clustering analysis method are taken to classify 29 samples from different geomorphic surfaces. The classification results of the three methods are different to a certain extent. We analyzed and compared the classification results of the three methods in detail. The results show that the traditional particle size analysis method, SOFM network method and cluster analysis method all can distinguish the geomorphic surface of different genesis, besides, they also can distinguish low-level terraces(T₁, T₂)and high-level terraces(T₃, T₄)for different grades of river terraces. Furthermore, the results also show that SOFM network method and cluster analysis method can make a certain distinction for the low-level terraces(T₁, T₂), while the traditional particle size analysis method is difficult to distinguish them. In addition, we analyzed and compared the three methods from the classification results, the results presentation, the operation process, and the error transmission. The results suggest that the advantages and disadvantages of the three methods are obvious. From the perspective of the classification results, the three methods all can distinguish the river terraces and alluvial fans and can make certain discrimination for different levels of river terraces. From the presentation of the results, the result of SOFM network is simple and clear. From the operation process, the traditional particle-size analysis method is relatively cumbersome, and the SOFM network method and the cluster analysis method are relatively simple to operate. From the perspective of error transmission, the traditional particle-size analysis method calculates the partial particle size feature value of the sample, which has a certain loss for the particle size distribution information of the whole sample. The error of the clustering analysis method has cumulative features and the influence exists consistently. The classification results of the SOFM network are independent of each other, which effectively avoids the problem of such error transmission of clustering analysis method. Overall, the classification results of the SOFM network method are simple and clear, the operation is simple, and the error is small. It has stronger adaptability to identifying different levels of different geomorphic surfaces. The results of this study will provide a simple and effective means for distinguishing different levels of geomorphic surfaces.     

考虑抗震性约束的建筑空间结构优化

当前对建筑空间结构进行优化时,所采用的算法趋同性高,无法实现多目标种群优化,易陷入局部最优解,存在寻优质量低、优化成本高、抗震性能低的问题。针对上述问题,提出一种基于改进粒子群算法的建筑空间结构优化方法。该方法以空间结构的抗震性能、工程造价为优化目标,来优化建立建筑空间结构设计;引入多子群协同进化机制解决建筑空间结构抗震优化设计中多目标间的种群优化问题,同时引入外部档案和精英学习策略改进粒子群算法,筛选出满足目标函数的最优设计方案,完成抗震性约束的建筑空间结构优化。实验结果表明:所提方法对建筑空间结构优化时的特点为寻优质量高、优化成本低、抗震性能高。     

Semiactive control of rigid blocks under earthquake excitation

This paper investigates the usefulness of a semiactive control to reduce the overturning vulnerability of a rigid block on a rigid plane under earthquake excitation. The proposed feedback law is used to set the stiffness of restraints placed at the 2 lower corners of the block. The performance of the semiactive control is numerically validated by subjecting the block to 100 recorded accelerograms. Specific simulations are performed to study the effect of different anchorage design parameters on the utility of the control. Finally, the robustness of the proposed control is addressed with respect to typical issues of the real‐world implementation.     

超低频波粒相互作用分析器

地球磁层的超低频(ULF)波是导致能量带电粒子快速加速的主要原因,同时这些粒子又可以使ULF波发生阻尼,它们之间的相互作用的过程可以通过星载仪器来定量观测.作者根据波与粒子相互作用的原理,结合中高轨道三维空间电场测量系统DC和低频部分的频率响应特点,设计了一种针对ULF波的波-粒相互作用分析器,并进行了模拟实验和验证.该仪器具有快速、精确、结构简单的特点,在中高轨道卫星观测时能对ULF波与粒子的相互作用的过程进行实时的定量测量.     

A numerical study on the cumulative out‐of‐plane damage to church masonry façades due to a sequence of strong ground motions

Seismic shocks occur sometimes as a sequence, close in space and time, of destructive events of comparable intensity. In these cases, a significant portion of the damage to historical buildings can be related with the cumulated damage on structures that become progressively more vulnerable. This research investigates the specific increase of damage determined by a sequence of strong ground motions, focusing the interest on the out‐of‐plane response of 2 church masonry façades. The dynamic analyses were performed by a specific rigid body and spring model RBSM, which only accounts for out‐of‐plane damage mechanisms. Two idealized models of façade, each made of 2 different masonry bonds, have been studied by applying various sequences of recorded accelerograms. The results highlighted a complex relationship between the spectral content of the seismic shocks and the characteristics of the structures that change in the course of the loading sequence due to the development of damage. The Housner spectral intensity proved to be a reliable scalar measure of the ground motion destructiveness for these façades. Moreover, when considering a design‐consistent accelerogram that causes a relevant damage pattern, ie, with a significant elongation of the effective first period of vibration, the numerical results indicated a possible spectral intensity threshold below which the occurrence of repeated seismic shocks, both before and after the reference design shock, can be considered as irrelevant. On the other hand, a catastrophic increase of damage should be expected when this threshold is overcome.     

RAPTOR‐UAV: Real‐time particle tracking in rivers using an unmanned aerial vehicle

River system measurement and mapping using UAVs is both lean and agile, with the added advantage of increased safety for the surveying crew. A common parameter of fluvial geomorphological studies is the flow velocity, which is a major driver of sediment behavior. Advances in fluid mechanics now include metrics describing the presence and interaction of coherent structures within a flow field and along its boundaries. These metrics have proven to be useful in studying the complex turbulent flows but require time‐resolved flow field data, which is normally unavailable in geomorphological studies. Contactless UAV‐based velocity measurement provides a new source of velocity field data for measurements of extreme hydrological events at a safe distance, and could allow for measurements of inaccessible areas. Recent works have successfully applied large‐scale particle image velocimetry (LSPIV) using UAVs in rivers, focusing predominantly on surficial flow estimation by tracking intensity differences between georeferenced images. The objective of this work is to introduce a methodology for UAV based real‐time particle tracking in rivers (RAPTOR) in a case study along a short test reach of the Brigach River in the German Black Forest. This methodology allows for large‐scale particle tracking velocimetry (LSPTV) using a combination of floating, infrared light‐emitting particles and a programmable embedded color vision sensor in order to simultaneously detect and track the positions of objects. The main advantage of this approach is its ability to rapidly collect and process the position data, which can be done in real time. The disadvantages are that the method requires the use of specialized light‐emitting particles, which in some cases cannot be retrieved from the investigation area, and that the method returns velocity data in unscaled units of px/s. This work introduces the RAPTOR system with its hardware, data processing workflow, and provides an example of unscaled velocity field estimation using the proposed method. First experiences with the method show that the tracking rate of 50 Hz allows for position estimation with sub‐pixel accuracy, even considering UAV self‐motion. A comparison of the unscaled tracks after Savitzky–Golay filtering shows that although the time‐averaged velocities remain virtually the same, the filter reduces the standard deviation by more than 40% and the maxima by 20%. Copyright © 2017 John Wiley & Sons, Ltd.     

Estimating sand concentrations using ADCP-based acoustic inversion in a large fluvial system characterized by bi-modal suspended-sediment distributions

Quantifying sediment flux within rivers is a challenge for many disciplines due, mainly, to difficulties inherent to traditional sediment sampling methods. These methods are operationally complex, high cost, and high risk. Additionally, the resulting data provide a low spatial and temporal resolution estimate of the total sediment flux, which has impeded advances in the understanding of the hydro-geomorphic characteristics of rivers. Acoustic technologies have been recognized as a leading tool for increasing the resolution of sediment data by relating their echo intensity level measurements to suspended sediment. Further effort is required to robustly test and develop these techniques across a wide range of conditions found in natural river systems. This article aims to evaluate the application of acoustic inversion techniques using commercially available, down-looking acoustic Doppler current profilers (ADCPs) in quantifying suspended sediment in a large sand bed river with varying bi-modal particle size distributions, wash load and suspended-sand ratios, and water stages. To achieve this objective, suspended sediment was physically sampled along the Paraná River, Argentina, under various hydro-sedimentological regimes. Two ADCPs emitting different sound frequencies were used to simultaneously profile echo intensity level within the water column. Using the sonar equation, calibrations were determined between suspended-sand concentrations and acoustic backscatter to solve the inverse problem. The study also analyzed the roles played by each term of the sonar equation, such as ADCP frequency, power supply, instrument constants, and particle size distributions typically found in sand bed rivers, on sediment attenuation and backscatter. Calibrations were successfully developed between corrected backscatter and suspended-sand concentrations for all sites and ADCP frequencies, resulting in mean suspended-sand concentration estimates within about 40% of the mean sampled concentrations. Noise values, calculated using the sonar equation and sediment sample characteristics, were fairly constant across evaluations, suggesting that they could be applied to other sand bed rivers. © 2018 John Wiley & Sons, Ltd.     

Response of soil erosion and sediment sorting to the transport mechanism on a steep rocky slope

A deeper understanding of the sediment characteristics associated with rock fragment content can improve our knowledge of the erosional processes and transport mechanisms of sediments on steep rocky slopes. This research used simulated rainfall experiments lasting for 1 h at a rate of 90 mm h⁻¹ and employed 5 × 1 × 0.4 m parallel troughs filled with purple soils with different rock fragment volumetric contents (0, 5, 10, 20, 30 and 40%) on a 15° slope gradient. For each simulated event, runoff and sediment were sampled at 1- and 3-min intervals, respectively, to study, in detail, the temporal changes in the size distributions of the eroded sediments. The results show that sediment concentrations, soil erosion rates and soil loss ratios significantly decreased as rock fragment content increased for rock fragment contents from 0 to 40% in purple soils. During the transportation process, clay particles often formed aggregates and were then transported as larger particles. Silt particles were more likely to be transported as primary particles with a low degree of sediment aggregation. Sand-sized particles, which constituted a greater proportion of the original soil than the eroded sediments, were formed from other fine particles and transported as aggregates rather than as primary particles. Suspension-saltation, which mainly transports fine particles of 0.02–0.05 mm and coarse particles larger than 0.5 mm in size, was the most important transport mechanism on steep rocky slopes. The results of this study can help to explain the inherent laws of erosional processes on steep rocky slopes and can provide a foundation for improving physical models of soil erosion. © 2019 John Wiley & Sons, Ltd.     

Laboratory and theoretical evaluation of impact of packing density,particle shape,and uniformity coefficient on erodibility of coarse-grained soil particles

Sedimentation – including erosion, transport, and deposition of coarse-grained particles – is a primary and growing environmental, engineering, and agricultural issue around the world. Soil erosion occurs when the hydrodynamic force induced by flowing water exceeds the geotechnical resistance of soils, as measured by critical shear stress for initiation of soil-particle motion. Even though various quantitative methods have been suggested with respect to different types of soil, the most widely accepted formula to estimate critical shear stress for coarse-grained soil is a direct function of the median grain size of the soil particles; however, the erosion resistance of soils also varies with other geotechnical properties, such as packing density, particle shape, and uniformity coefficient. Thus, in this study, a combined rolling–lift model for particle detachment was derived based on theoretical analysis. A series of experimental flume tests were conducted with specimens prepared with standard soil types, as well as laboratory-prepared mixtures of coarse-grained soil to validate the theoretical model and determine the effect of other geotechnical properties on the erosion characteristics of coarse grains, coupled with the effect of median particle size. The results indicated that the median grain size is the primary variable determining the resistance of coarse grains, but the critical shear stress also varies with the packing density of the soil matrix. In addition, angular particles show more erosion resistance than rounded particles, and the erosion potential of a soil decreased when the grain is well graded (higher value of uniformity coefficient). Additionally, regression analysis was performed to quantify the effect of each parameter on the critical shear stress of coarse grains. © 2020 John Wiley & Sons, Ltd.