Delineation of the northern limit of the Congo Craton based on spectral analysis and 2.5D modeling of aeromagnetic data in the Akonolinga-Mbama area,Cameroon

Aeromagnetic data of the Akonolinga-Mbama region are analyzed in order to elucidate the subsurface geology of the area. The available data in the form of a residual aeromagnetic map is interpreted as a vast magnetically quiet region, and complex zones which do not correlate with the surface geology of the region.Within the magnetically quiet zone, a high negative circular elongated anomaly zone seems to represent an intrusion of a plutonic rock into the metamorphic formations of the region. Spectral analysis and two-and-a-half dimensional (2½-D) modeling are used to estimate the depth of the causative bodies and determine the source rocks along three profiles crossing the suspected areas. Models from various zones of granitic intrusions are obtained, thereby proposing some shallow fault lines along zones of contact. This permits us to mark out the northern margin of the Congo Craton, thus enabling us to distinguish the cratonic formations from the Pan African fold belt. Part of the belt has been thrust over the northern portion of the Congo Craton in Cameroon.     

Bidirectional behavior of unreinforced masonry walls

Most of the studies related to the modeling of masonry structures have by far investigated either the in‐plane (IP) or the out‐of‐plane (OP) behavior of walls. However, seismic loads mostly impose simultaneous IP and OP demands on load‐bearing or shear masonry walls. Thus, there is a need to reconsider design equations of unreinforced masonry walls by taking into account bidirectional effects. The intent of this study is to investigate the bidirectional behavior of an unreinforced masonry wall with a typical aspect ratio under different displacement‐controlled loading directions making use of finite element analysis. For this purpose, the numerical procedure is first validated against the results of the tests on walls with different failure modes conducted by the authors. Afterward, the response of the wall systems is evaluated with increasing top displacement having different orientations. A set of 19 monotonic and three cyclic loading analyses are performed, and the results are discussed in terms of the variation of failure modes and load–displacement diagrams. Moreover, the results of wall capacity in each loading condition are compared with those of the ASCE41‐06 formulations. The results indicate that the direction of the resultant force, vectorial summation of IP and OP forces, of the wall is initially proportional to the ratio of stiffness in the IP and the OP directions. However, with the increase of damage, the resultant force direction inclines towards the wall's longitudinal direction regardless of the direction of the imposed displacement. Finally, recommendations are made for applicability of ASCE41‐06 formulations under different bidirectional loading conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling planform evolution of a mud‐dominated meandering river: Quinn River,Nevada, USA

A depth‐averaged linearized meander evolution model was calibrated and tested using the field data collected at the Quinn River in the Black Rock Desert, Nevada. Two approaches used to test the model were: (1) simulating meander evolution and comparing the results with the observed 38 year migration pattern; and (2) fitting the model parameters to present bank asymmetry (the ratio of the maximum bank gradients on opposite sides of the channel). The data required as input were collected in the field during a high flow in May 2011 and from aerial photographs and LiDAR data. Both approaches yielded similar results for the best fit parameter values. The bank asymmetry analysis showed that the bank asymmetry and the velocity perturbation have high correlation at close to zero spatial lag while the maximum correlation between the bank asymmetry and maximum bend curvature is offset by about 25 m. The model sufficiently replicated 38 years of channel migration, with a few locations significantly under‐ or over‐predicted. Inadequacies of the flow model and/or variation in bank properties unaccounted for are most likely the causes for these discrepancies. Flow through the Quinn River was also simulated by a more general 3D model. The downstream pattern of near‐bank shear stresses simulated by the 3D model is nearly identical to those resulting from the linearized flow model. Topographic profiles across interior bends are essentially invariant over a wide range of migration rates, suggesting that the traditional formulation that cut bank erosion processes govern migration rates is appropriate for the Quinn River. Copyright © 2014 John Wiley & Sons, Ltd.     

Peak discharge increase in hyperconcentrated floods

The peak of river floods usually decreases in the downstream direction unless it is compensated by freshwater inflow from tributaries. In the Yellow River (China) the opposite is regularly observed, where the peak discharge of river floods increases in the downstream direction (at a rate far exceeding the contribution from tributaries). This flood peak discharge increase is probably related to rapid morphological changes, to a modified bed friction, or to a combination of both. Yet the relative role of these processes is still poorly understood. This paper aims to analyze the relative contribution of bed erosion and friction change to the peak discharge increase, based on available data and a recently developed numerical model. Using this high-resolution, fully coupled morphodynamic model of non-capacity sediment transport, two hyperconcentrated floods characterized by downstream peak discharge increase are numerically reproduced and analyzed in detail. The results reveal that although erosion effects may contribute to the downstream discharge increase (especially in case of extreme erosion), for most cases the increase must be mainly due to a reduction in bed friction during peak discharge conditions. Additionally, based on the concept of channel storage reduction, the effects of decreasing bed friction and (very strong) bed erosion can be integrated in explaining the peak discharge increase.     

A semi-analytical method for predicting the outflow hydrograph due to dam-break in natural valleys

The paper presents a semi-analytical method for predicting the flow rate hydrograph due to a hypothetical sudden and total dam failure in a natural valley. The method generalizes the approach proposed by Hunt for the dam-break problem in a rectangular frictionless sloping channel to a valley with a cross-section area expressed by a power-law function of water depth, in order to take into account the most common shapes of natural valleys. The parameters of the deriving model can be set by exploiting data usually available concerning the dam section geometry and the reservoir storage-depth curve. The application of the technique to three different reservoirs is discussed. The results show that the flow rate hydrographs obtained at the dam site agree with the ones calculated by means of a finite volume numerical code based on two-dimensional shallow water equations. The method requires moderate computational and data collecting effort, so it can be regarded as a useful alternative to other procedures commonly adopted in the practice.     

How can the uncertainty in the natural inflow regime propagate into the assessment of water resource systems?

The Canadian Rocky Mountain headwaters support the water resource systems of the Canadian Prairies. Significant variations in natural headwater contributions have been observed due to warming climate. Projecting future natural headwater flows under climate change effects, however, has large uncertainty. First, there are difficulties in climate modeling and downscaling in alpine regions. Second, streamflow modeling in mountainous areas is extremely challenging. There is therefore a need to understand the effects of uncertainty in the natural inflow regime, and in particular how this translates into uncertainty in representing the state and the outflow of water resource systems. Considering the Oldman River basin in Alberta, Canada, we synthesized different inflow regimes based on site/inter-site properties of the historical inflow regime. The water resources system was then conditioned on the synthesized inflow regimes to identify the mechanisms of error propagation from the headwater streamflows to the water allocations. The results show that the response of the water resource system to the uncertainty in the generated inflow regime depends on the system state, flow condition and the component of interest. Generally, the response of the reservoirs to the uncertainty in the estimated inflow regime is more significant in dry years, in particular during low flow conditions. The response at the system outlet is rather different, as the propagation of the headwater uncertainty is more significant during high flow conditions. Also, similar inflow estimates in terms of error and uncertainty may result in different error and uncertainty estimates in the simulated outflows; therefore, lower bias and uncertainty in estimating the regional inflow regime does not necessarily mean lower bias and uncertainty in simulating the streamflow at the outlet of the system. Our results provide improved understanding of uncertainty propagation through complex water resource systems, but also portray the need for better climate and hydrological modeling in the Rocky Mountains for improved water management in the Canadian Prairies, particularly in the face of uncertain climate futures. This will be crucial if the natural headwater inflows decline and/or the system faces drought conditions.     

Assessment of the impact of pore-scale mass-transfer restrictions on microbially-induced stable-isotope fractionation

Stable-isotope fractionation has become an established method for the assessment of contaminant biodegradation in groundwater. At the pore scale however, mass-transfer processes can limit the bioavailability of chemical species and therefore affect the observed fractionation. This can challenge the application of stable-isotope analysis in practice. A linear-exchange model provides a computational link between the microbially-induced isotope fractionation, determined under ideal conditions, and the fractionation observed under conditions of limited bioavailability. This simplifying conceptual approach allows for accurately estimating the mass-transfer limited degradation rates but its applicability for stable-isotope fractionation at the pore scale has not been evaluated yet. In this study, we perform high-resolution numerical simulations of microbial degradation and stable-isotope fractionation of a chemical species in a pore-scale model. The numerical results are compared to theoretical predictions derived from the linear-exchange model. Our results show an overall good agreement between numerical simulations and theoretical predictions, which confirms the applicability of the theoretical approach and of the value for the mass-transfer coefficient previously derived from the geometry of the pore space. In addition we provide a quantitative link between the value of the observable fractionation factor and the effective bioavailability of a biodegradable contaminant.     

一阶弹性波方程数值模拟中的混合吸收边界条件

Liu和Sen（2010和2012）在地震波场数值模拟中提出一种混合吸收边界条件. 该方法具有计算量小、容易实现及吸收效果好等优点. 但现有的混合吸收边界条件是针对二阶位移-应力方程设计的,存在稳定性问题. 本文首先推导了两种速度-应力单程波方程：二阶Higdon单程波方程和一阶Higdon单程波方程. 进而提出基于一阶弹性波方程的混合吸收边界条件方法. 在内部区域和边界之间引入一个过渡区域,通过单程波与双程波方程平滑过渡来消除人工边界反射. 为了改善混合吸收边界条件的吸收效果和稳定性,我们采用了能同时吸收纵、横波反射的一阶单程波方程和与变量位置有关的加权系数. 为了验证混合吸收边界条件的有效性,将其与常规分裂完全匹配层（PML）方法进行了比较. 数值模拟结果表明,与PML边界条件相比,混合吸收边界条件在耗用更小计算时间和存储量的前提下,可以获得更好的吸收效果. 另外,本文提出的两种混合吸收边界条件中,混合一阶Higdon吸收边界条件具有更好的稳定性.     

基于OpenSees的混凝土结构连续倒塌分析中的从属节点建模技术

对重大工程结构进行强地震作用下的连续倒塌全过程分析并建立相应的设计与控制方法,已成为当前地震工程领域的发展趋势。目前,由于数值求解方面的困难,绝大多数针对极端作用下的结构连续倒塌的研究止步于数值临界状态的界定,在分析过程中不能实时地对结构构件的损伤状态进行监测并根据构件的损伤状态对分析模型进行修改。为了实现连续倒塌过程中构件的逐步失效,在OpenSees程序中,基于Beam with Hinges Element构建了端部带附属节点的Beam with Hinges Element,并根据构件失效情况对附属节点的多点约束进行控制。采用三次静力凝聚方法和Newmark-beta法,给出了对此改进建模技术可信的理论背景,方法的准确性通过一个简单框架算例得到了验证。