Soil detachment by overland flow on steep cropland in the subtropical region of China

Accurate prediction of soil detachment capacity is fundamental to establish process-based erosion models and improve soil loss assessment. Few studies were conducted to reveal the mechanism of detachment process for yellow soil on steep cropland in the subtropical region of China using field experiments. This study was performed to determine soil detachment characteristics and explore the relationships between soil detachment capacity (D _c) and flow rate, slope gradient, mean velocity, shear stress, stream power and unit stream power. Field experiments were conducted on intact soil with flow rates ranging from 0.2 × 10⁻³ to 0.5 × 10⁻³ m⁻³ s⁻¹ and slope gradients varying from 8.8 to 42.4%. The results showed the following. (a) D _c of yellow soil was smaller than other soils because of its high clay content. (b) D _c was more susceptible to flow than to slope gradient. Power functions were derived to depict the relationship between D _c and the flow rate and slope gradient (R² = 0.91). (c) D _c was better simulated by power functions of the stream power (R² = 0.83) than functions of the shear stress or the unit stream power. (d) Considering its accuracy, simplicity and accessibility, the power function based on flow rate and slope gradient is recommended to predict D _c of yellow soil in the field. The results of this study provide useful support for revealing soil detachment mechanism and developing process-based soil erosion models for the subtropical region of China.     

Evolution of pore structure and hydraulic conductivity of randomly distributed soluble particle mixture

The plane strain behavior of particulate mixtures containing soluble particles was investigated by conducting both laboratory tests and numerical analysis. To perform the laboratory experiments, soluble mixtures were prepared using photoelastic disks and ice disks with diameters in the ratios (D_{ice disk}/D_{photoelastic disk}) of 0.5 and 0.7, and the evolution of the force chain and pore structure was monitored during the dissolution of the ice disks. Subsequently, numerical analysis was conducted by using the 2‐dimensional discrete element method for the soluble mixtures, and it was compared with the experimental results. Additionally, parametric studies were implemented by varying the particle size ratios between the soluble and non‐soluble particles and the volumetric fraction of the soluble particles. The results of the laboratory experiments and numerical analysis demonstrate that (1) after the dissolution of the soluble particles, the pore fabric of the specimens changed, resulting in a force chain changes, local void increases, and coordination number decreases; (2) the effects of soluble particles on the macro‐behaviors of the mixtures could be divided into 3 zones based on the particle size ratios between the soluble and non‐soluble particles and volumetric fraction of soluble particles. These zones were as follows: (Zone 1)—with a small total soluble volume, slight decrease in the in situ lateral pressure (K₀), and minor increase in the hydraulic conductivity (k); (Zone 2)—with a moderate soluble particle; the dissolution generated a honey‐comb particle structure; (Zone 3)—the total soluble volume was very large, and the high volumetric fraction of the dissolving particle collapsed the pore structure, decreasing in the in situ lateral pressure (K₀) but increasing the hydraulic conductivity (k). The horizontal stress returned to almost the original level, and the internal arching formation increased significantly with the hydraulic conductivity (k).     

Simulation and statistical modelling approaches to investigate hydrologic regime transformations following Eastern hemlock decline

Ecohydrological processes occurring at or near the Earth's surface are strongly influenced by Eastern hemlock [EH; Tsuga canadensis (L.) Carrière], a foundation tree species of eastern North American forests. EH populations are currently threatened by the invasive hemlock woolly adelgid (HWA; Adelges tsugae Annand). HWA HWA populations have been expanding rapidly throughout the EH's range. Catchment-scale research examining the hydrological consequences of HWA infestation is lacking, and plot-scale studies remain conflicted in their findings. Given the complex relationships between canopy interception, unsaturated and saturated groundwater storage, and root water uptake, it is not immediately clear how EH loss will affect the hydrologic cycle. We investigated the impact of EH mortality on stream discharge characteristics across a regional sample of catchments utilizing both simulation and statistical modelling approaches. We first examined the relationship between various catchment characteristics, including EH health, and three hydrological variables through regression analysis. We then employed a non-parametric statistical test to evaluate differences in hydrologic regime trends between non-infested and infested catchments. Finally, we calibrated a physically based hydrologic model and considered differences in optimal model parameter values and simulated overland runoff between non-infested and infested catchments. HWA presence modified several ecohydrological characteristics and precipitation partitioning between groundwater flows and surface runoff, potentially driving higher stream flashiness and overland flow, lower baseflow contributions and catchment storage, shorter flow-path lengths, and variable source area dilation at infested sites. Our results suggest that EH decline will augment flooding potential associated with the increasing frequency and intensity of Atlantic Basin tropical cyclone events. Further, our physically based simulation provides more determinate results than regression analysis, indicating that a purely statistical methodology, commonly utilized in studying the relationship between landcover characteristics and hydrologic regime, neglects dynamic physical ecohydrologic relationships.     

Seismic rehabilitation performance of steel side plate moment connections

Moment connections in an existing steel building located in Kaohsiung, Taiwan were rehabilitated to satisfy seismic requirements based on the 2005 AISC seismic provisions. Construction of the building was ceased in 1996 due to financial difficulties and was recommenced in 2007 with enhanced connection performance. Steel moment connections in the existing building were constructed by groove welding the beam flanges and bolting the beam web to the column. Four moment connections, two from the existing steel building, were cyclically tested. A non‐rehabilitated moment connection with bolted web‐welded flanges was tested as a benchmark. Three moment connections rehabilitated by welding full‐depth side plates between the column face and beam flange inner side were tested to validate the rehabilitation performance. Test results revealed that (1) the non‐rehabilitated existing moment connection made by in situ welding process prior to 1996 had similar deformation capacity as contemporary connection specimens made by laboratory welding process, (2) all rehabilitated moment connections exhibited excellent performance, exceeding a 4% drift without fractures of beam flange groove‐welded joints, and (3) presence of the full‐depth side plates effectively reduced beam flange tensile strain near the column face by almost half compared with the non‐rehabilitated moment connection. The connection specimens were also modeled using the non‐linear finite element computer program ABAQUS to further confirm the effectiveness of the side plate in transferring beam moments to the column and to investigate potential sources of connection failure. A design procedure was made based on experimental and analytical studies. Copyright © 2009 John Wiley & Sons, Ltd.     

Controls on at‐a‐station hydraulic geometry in steep headwater streams,Colorado, USA

Detailed hydraulic measurements were made in nine step‐pool, five cascade and one plane‐bed reach in Fraser Experimental Forest, Colorado to better understand at‐a‐station hydraulic geometry (AHG) relations in these channel types. Average values for AHG exponents, m (0·49), f (0·39), and b (0·16), were well within the range found by other researchers working in steep gradient channels. A principal component analysis (PCA) was used to compare the combined variations in all three exponents against five potential control variables: wood, D₈₄, grain‐size distribution (σ), coefficient of variation of pool volume, average roughness‐area (projected wetted area) and bed gradient. The gradient and average roughness‐area were found to be significantly related to the PCA axis scores, indicating that both driving and resisting forces influence the rates of change of velocity, depth and width with discharge. Further analysis of the exponents showed that reaches with m > b + f are most likely dominated by grain resistance and reaches below this value (m < b + f) are dominated by form resistance. Copyright © 2010 John Wiley & Sons, Ltd.     

半刚接框架-非加劲钢板墙体系抗震试验研究

半刚接钢框架-内填非加劲薄钢板剪力墙结构是一种新型结构形式,它弥补了传统抗弯钢框架侧向刚度不足的缺点,为采用更加经济的半刚性节点提供了可能,目前国内外对该新型结构体系性能研究属刚起步阶段,因此对该结构体系的研究具有重要的理论和实际意义。文中对一榀两层单跨梁柱平齐端板连接半刚性框架-内填非加劲薄钢板剪力墙结构进行了低周往复循环加载拟静力试验,在试验中,试件破坏过程缓慢,钢板墙拉力带充分发育,试验得出的滞回环饱满,表明该新型结构体系具有优异的抗震性能,从而为今后对该新型结构体系做更深入的分析研究打下了良好基础。     

Inundation flow velocity of tsunami on land

An estimation of tsunami inundation flow velocity is one of the most challenging issues among tsunami research. Based on field data of inundation depth and inundation flow velocity u estimated using Bernoulli's theorem and inundation depth, fundamental characteristics of the relationship between inundation flow velocity and inundation depth are examined. Fundamental characteristics of the velocity coefficient where g is gravitational acceleration, h_f and h_r are inundation depths at the front and the back of structures such as a rectangular building with vertical walls, respectively) implicitly included in the relationship are examined through hydraulic experiments. As a result, C_v = 0.6 is recommended as its simple and practical value. It is confirmed through these examinations that the Froude number, defined by where , ranges 0.7–2.0, and when C_v = 0.6 is adopted this Froude number ranges 0.42–1.2. By using the relationship and C_v = 0.6, two simple and practical relationships are presented for two cases where inundation flow velocity exerts the largest or the smallest fluid force on structures. These relationships can be used to roughly grasp the practical side of tsunami damage, and estimate fluid force acting on individual structures, moving velocity and collision force of floating objects and sediment transport such as boulder and sand. Fundamental characteristics of the waterline (tsunami trace) distribution around/on the typical object model (square pillar, corn and column) are also examined through steady flow experiments, and it is confirmed that the maximum and the minimum values of h_f/h₀ in the full type model of the square pillar are almost the same as those of h_f/h_r obtained by field surveys where h₀ is uniform flow depth. It is also confirmed that h_r ? h₀ when the Froude number, defined by where u₀ is uniform flow velocity, is much less than 1.0. Using a newly defined velocity coefficient, tsunami inundation flow velocity on land can be estimated practically and would be useful for checking proposed sediment transport models that are now being developed by tsunami geologists.     

复合土工膜渗透性能试验研究

以高密度聚乙烯复合土工膜为试验材料,利用柔性壁渗透仪测定一定时间间隔通过复合土工膜的渗透量,通过计算求得复合土工膜的渗透系数,分析渗透系数随渗透压和渗透压应力路径的变化规律。利用扫描电子显微镜拍摄复合土工膜的照片,从微观结构上揭示了渗透系数变化的原因。试验结果表明:柔性壁渗透仪可以精确测定复合土工膜的渗透性能参数;复合土工膜在0.1MPa的有效应力作用下,渗透压从0.1MPa增大到0.6MPa的范围内,渗透系数有3种变化趋势,分别是渗透系数基本不变、渗透系数随渗透压的增大而增大和渗透系数随渗透压的增大而减小。复合土工膜物理性质的不均匀性和渗透通道的形态是影响防渗效果的主要因素。     

压裂过程中地面和井下倾斜场的主控因素研究

水力压裂技术改造储层已经成为非常规油气开发的重要技术手段,以井下测斜仪为代表的远场测量技术进行裂缝形态监测已经逐步应用于压后裂缝效果评价。采用不同的裂缝分析模型对压裂过程中地面和井下倾斜场进行了正演研究,验证了模型的正确性,并采用矩形张裂缝模型对影响压裂倾斜场的地层和裂缝参数进行了定量分析。研究表明,（1）地面倾斜场对于裂缝的方位角和倾角变化较为敏感,对于裂缝的尺寸及裂缝中心深度变化不敏感;（2）井下倾斜场值对于裂缝的尺寸、裂缝中心深度变化较为敏感,对于裂缝的方位角及倾角变化不敏感。研究结果可应用于储层水力压裂优化设计中。     

基于ABAQUS平台的水力裂缝扩展有限元模拟研究

随着扩展有限元理论的深入研究,利用扩展有限元方法模拟水力压裂具有了一定的可操作性。相比于常规有限元方法,XFEM方法具有计算结果精度高和计算量小的优点。但是,如何模拟射孔孔眼、如何模拟流体与岩石相互作用以及分析水力裂缝的扩展规律仍然是难题。以研究水力压裂裂缝扩展规律为目的,建立了岩石多孔介质应力平衡方程、流体渗流连续性方程和边界条件。通过有限元离散化方法对耦合方程矩阵进行处理。通过富集函数定义初始裂缝（射孔孔眼）,选择最大主应力及损伤变量D分别作为裂缝起裂和扩展判定准则,利用水平集方法模拟水力裂缝扩展过程。数值模拟结果显示：增加射孔方位角、压裂液排量和减小水平地应力差,起裂压力上升;黏度对起裂压力无明显影响。增加射孔方位角、压裂液排量、黏度和减小水平地应力差值有助于裂缝宽度的增加。增加水平地应力差值、压裂液排量和减小射孔方位角以及压裂液黏度有助于裂缝长度增加,反之亦然。基于ABAQUS的水力裂缝扩展有限元法可对不同井型和诸多储层物性参数及压裂施工参数进行分析,且裂缝形态逼真,裂缝面凹凸程度清晰,结果准确。此研究可作为一种简便有效研究水力压裂裂缝扩展规律的方法为油田水力压裂设计与施工提供参考与依据。