复式钢管混凝土外肋环板节点抗剪承载力研究

在已有理论和试验研究的基础上,对复式钢管混凝土外肋环板节点的抗剪受力性能进行分析。建立了节点核心区的抗剪受力模型,将节点域抗剪贡献分为三部分:节点域内外钢管腹板的抗剪贡献、节点主要连接件竖向肋板与锚固腹板的抗剪贡献以及节点域混凝土的抗剪贡献,推导了复式钢管混凝土柱节点屈服抗剪承载力和极限抗剪承载力的计算公式,为复式钢管混凝土柱节点的工程设计提供承载力计算方法。理论得到的节点屈服剪力和极限剪力值与试验结果进行了对比,并提出抗剪能力储备系数这一新指标反映节点的抗剪切破坏能力,量化地解释了节点发生梁铰破坏后抗剪能力的安全储备。得出此类新型节点在破坏时抗剪储备能力充足,可保证节点达到良好延性的破坏模式,说明节点设计符合强剪弱弯的抗震设计原则。     

Quantifying sediment storage on the floodplains outside levees along the lower Yellow River during the years 1580–1849

The lower Yellow River channel was maintained by artificial levees between 1580 and 1849. During this period, 280 levee breaches occurred. To estimate sediment storage on the floodplains outside the levees, a regression model with a decadal time step was developed to calculate the outflow ratio for the years when levee breaching occurred. Uncertainty analysis was used to identify the likely outflow ratio. Key variables of the model include annual water discharge, a proxy for levee conditions, and potential bankfull discharge of the channel before flood season. Uncertainty analysis reveals an outflow ratio of 0.35–0.56. We estimate that during this period, 18.8–30.1% of the total ~312 Gt of sediment load was deposited on the floodplains outside the levees. Human-accelerated erosion in the Loess Plateau caused a 4-fold increase in sediment delivery to the lower Yellow River, which could not be accommodated by channel morphodynamic changes. As a result, 21.2–27.5% of the total sediment load was deposited within the levees, creating a super-elevated channel bed that facilitated an uncommonly high breach outflow ratio. Hence, the factor of a large super-elevation relative to the mean main channel depth should be considered when designing diversions to restore floodplains. © 2018 John Wiley & Sons, Ltd.     

长江中下游湖泊沉积物氮磷形态与释放风险关系

运用聚类分析、主成分分析和相关矩阵的统计分析手段,对长江中下游湖群共18个湖泊的沉积物氮磷释放风险以及湖泊沉积物、间隙水和上覆水中氮磷形态以及其他相关地球化学参数进行分析。草型和藻型湖泊的环境差异是造成氮磷释放风险的主要原因。氮磷释放风险与铁磷、藻类可利用磷、总氮、总磷、上覆水氮磷含量、间隙水氮含量、孔隙度和有机质含量间的关系最为密切。决定磷酸盐释放风险的主要形态磷是藻类可利用磷和铁磷,其他形态磷或者含量较低或者不易被转化释放,对磷酸盐释放风险影响较小。有机磷含量对磷的释放风险没有直接决定作用,但它与有机质含量间呈显著正相关。     

Age determination of paleotsunami sediments around Lombok Island,Indonesia, and identification of their possible tsunamigenic earthquakes

Age determination of paleotsunami sediment from Lombok Island, Indonesia, and surrounding area has been carried out using the 210 Pb method in BATAN Jakarta. The basic theory of this method assumes that weathering of sediments, including paleotsunami sediments, will result in 210 Pb enrichment. The principle of this method is to calculate 210 Pb contents accumulation in a particular sedimentation interval from the surface to the deeper buried sediments. The results are then converted into age or depositional time in years ago unit. The dating results from the paleotsunami sediments of the Gawah Pudak(S8°46’2.91’’, E115°56’34.23’’) and Gili Trawangan areas(S8°21’1.38’’, E116°2’36.6’’) indicate the Gawah Pudak sediments were deposited 37 years ago(c. in 1977)and 22 years ago(c. in 1992). Three paleotsunami sediments from Gili Trawangan were deposited 149 years ago(c. in 1865), 117 years ago(c. in 1897) and 42 years ago(c. in 1972). These results are then compared to the available Indonesian earthquake catalogue data. This study reveals that paleotsunami sediments around Lombok Islands, from older to younger, were caused by the 1857 earthquake(epicentre in Bali Sea; M7; S8°00’09.45’’,E115°29’56.41’’), 1897 earthquake(epicentre in Flores Sea;M5.5; S6°47’59.62’’, E120°48’03.5’’ or Sulu Sea earthquake; M8.5; 70 km NW of Basilan Island), the 1975 earthquake(Nusa Tenggara; S10°6’16.61’’, E123°48’09.39’’), 1977 earthquake(in Waingapu, Sumba; M8.0;S11°5’39.34’’, E118°27’50.86’’) and the 1992 earthquake(Flores; M7.8; S8°28’52.11’’, E121°53’44.3’’).     

南海北部雾状层分布和特征的地震海洋学研究

利用地震海洋学方法在南海北部陆架和上陆坡区域发现了15个雾状层.这些雾状层的延伸长度从几千米到几十千米,厚度十几米到一百米,其顶界所处水深在135 m至715.5 m范围之间.雾状层在地震海洋学剖面上表现为强反射特征.不同于其他传统声学或光学方法,地震海洋学方法分辨率高,且能在短时间内对整个水体进行成像,可以记录到雾状层的时空变化特征,实现对雾状层的"四维"观测.南海北部上陆坡区域是内孤立波浅化、能量耗散集中的区域,在此过程中内孤立波会导致较大的波致流速,侵蚀海底使得表面沉积物再悬浮,进入水体,形成和维持雾状层的存在.     

地球内核及其边界的结构特征和动力学过程

现代地震学展现出了一个复杂的地球内核内部和表面结构.地球内核内部结构的主要特征表现为其地震波速度和衰减呈现各向异性,且各种结构（速度、衰减和各向异性）均呈现东西半球差异,而内核表面的新发现则包括其局部区域存在起伏的地形和固液并存的糊状层.地球内核压缩波速度和衰减均呈现以地球旋转轴为轴的柱对称各向异性,沿地球旋转轴方向传播的压缩波比沿赤道方向传播的压缩波传播更快且衰减更强烈.同时,内核各向异性结构随深度而变化：内核顶部约100~400 km接近各向同性,而在内核最深处300~600 km内则可能存在一个具有不同各向异性特征的内内核.地球内核的东西半球差异表现在多方面：在内核顶部~100 km厚度内,东半球的各向同性速度比西半球快约0.8%,东半球具有较强的衰减（Q=250）,而西半球则具有较弱的衰减（Q=600）;西半球的顶部各向同性层厚度约为100 km,而东半球顶部各向同性层厚度则约为400 km;在各向同性层底下,西半球具有较强的各向异性（~4%）,而东半球则具有较弱的各向异性（~0.7%）.地球内核边界在菲律宾海、黄海、西太平洋以及中美洲下方存在1~14 km高的地形起伏,在鄂霍次克海西南部下方存在4~8 km厚的糊状层.地球内核的这些新发现引发了对许多可能的新物理机制的探讨,也促使我们重新评估我们对外核成分、外核热化学对流、内核凝固过程和地球磁场驱动力的认识.这些结果表明内核凝固过程和地球磁场的热和化学驱动力远比传统观念认为的横向均匀分布复杂得多.内核西半球可能不断凝固并释放潜热和轻元素,而东半球则可能不断熔化并吸收潜热和轻元素,外核对流的驱动力在东西半球可能截然不同,甚至呈现相反方向.这些凝固与熔化交替过程也发生在局部地形起伏区域.在糊状层区域,地球内核凝固释放潜热和化学能,而在大部分无糊状地区,内核凝固只释放潜热.     

Numerical simulation of dam-break flow and bed change considering the vegetation effects

Existing numerical investigations of dam-break flows rarely consider the effects of vegetation.This paper presents a depth-averaged two-dimensional model for dam-break flows over mobile and vegetated beds.In the model,both the consequences of reducing space for storing mass and momentum by the existence of vegetation and dragging the flow are considered:the former is considered by introducing a factor (1-c) to the flow depth,where c is the vegetation density;the later is considered by including an additional sink term in the momentum equations.The new governing equations are discretized by the finite volume method;and an existing second-order central-upwind scheme embedded with the hydrostatic reconstruction method for water depth,is used to estimate the fluxes;the source terms are estimated by either explicit or semi-explicit methods fulfilling the stability requirement.Laboratory experiments of dam-break flows or quasi-steady flows with/without vegetation effects/sediment transport are simulated.The good agreements between the measurements and the numerical simulations demonstrate a satisfactory performance of the model in reproducing the flow depth,velocity and bed deformation depth.Numerical case studies of six scenarios of dam-break flows over a mobile and vegetated bed are conducted.It is shown that when the area of the vegetation zone,the vegetation density,and the pattern of the vegetation distribution are varied,the resulted bed morphological change differs greatly,suggesting a great influence of vegetation on the dam-break flow evolution.Specifically,the vegetation may divert the direction of the main flow,hindering the flow and thus result in increased deposition upstream of the vegetation.     

A probabilistic model for sediment entrainment:The role of bed irregularity

A generalized probabilistic model is developed in this study to predict sediment entrainment under the incipient motion, rolling, and pickup modes. A novelty of the proposed model is that it incorporates in its formulation the probability density function of the bed shear stress, instead of the near-bed velocity fluctuations, to account for the effects of both flow turbulence and bed surface irregularity on sediment entrainment. The proposed model incorporates in its formulation the collective effects of three para-meters describing bed surface irregularity, namely the relative roughness, the volumetric fraction and relative position of sediment particles within the active layer. Another key feature of the model is that it provides a criterion for estimating the lift and drag coefficients jointly based on the recognition that lift and drag forces acting on sediment particles are interdependent and vary with particle protrusion and packing density. The model was validated using laboratory data of both fine and coarse sediment and was compared with previously published models. The study results show that all the examined models perform adequately for the fine sediment data, where the sediment particles have more uniform gra-dation and relative roughness is not a factor. The proposed model was particularly suited for the coarse sediment data, where the increased bed irregularity was captured by the new parameters introduced in the model formulation. As a result, the proposed model yielded smaller prediction errors and physically acceptable values for the lift coefficient compared to the other models in case of the coarse sediment data.     

Analyses of trends and causes for variations in runoff and sediment load of the Yellow River

Due to the impacts of globe climate change and human activities, dramatic variations in runoff and sediment load were observed for the Yellow River. Analyses of nearly 65 years' data measured at main hydrologic-stations on the Yellow River from 1950 to 2014 indicated that, except for the Tangnaihai station in the head region, sharp downward trends existed in both the annual runoff and annual sedi-ment load according to the Mann–Kendal trend test;and their abrupt changes occurred in 1986 and in 1980, respectively, according to the rank sum test. Factors affecting the changes in the runoff and sediment load were very complicated. Results indicated that the reducing precipitation and the increasing water consumption were the main causes for the runoff decline, while the impoundment of the Longyangxia Reservoir and its combined operation with the Liujiaxia Reservoir exerted a direct bearing on the abrupt change in the annual runoff. In addition to the sediment load decrease associated with the runoff reduction, the reduced storm intensity, the conducted soil erosion control, and the constructed dam buildings all played an important role in the trends and abrupt changes of sediment load decline.     

Morphological reactions of schematic alluvial rivers:Long simulations with a 0-D model

The paper presents a 0-D model of an alluvial watercourse schematized in two connected reaches, evolving at the long time-scale and under the hypothesis of Local Uniform Flow. Each reach is defined by its geometry (constant length and width, time-changing slope) and grain-size composition of the bed, while the sediment transport is computed using a sediment rating curve. The slope evolution is provided by a 0-D mass balance and the evolution of the bed composition is computed by a 0-D Hirano equation. A system of differential equations, solved with a predictor-corrector scheme, is derived and applied to the schematic watercourse to simulate the morphological response to changing initial conditions, and the evolution towards long-term equilibrium conditions. Differently from a single-reach 0-D schematization with uniform grain-size, besides the simplifications adopted, the model proposed here simulates the behaviour of alluvial rivers in a physically-based way, showing a grain-size fining in the downstream direction accompanied by milder slopes, and a tendency to develop concave longitudinal profiles.