黄河口最大浑浊带特征及其时空演变

根据汛期黄河口多船同步水文泥沙实测资料,对黄河口最大浑浊带特征及其时空演变进行研究,得出汛期黄河口最大浑浊带在整个潮周期始终存在,其含沙量和范围形态受潮相的控制,在落急和落平时最为发育.文中还探讨了黄河口最大浑浊带的形成机制,指出其形成主要受河流携带大量泥沙、泥沙异重流、河口密度环流及湍流的作用.     

Formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river

Feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors. We describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized Isère River in France. The channel was laterally confined with embankments and mostly straightened. From the beginning of the 1970s to the end of the 1990s, alternate bars were progressively but heavily colonized by vegetation. This context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions. Based on a GIS analysis of aerial photographs (between 1948 and 1996), we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age, size and mobility. Field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars. A highly significant statistical association of geomorphological and vegetation variables (RV of co-inertia analysis = 0.41, p < 0.001) explained 95% of the variability in just one axis, supporting the existence of very strong feedback between geomorphological changes (i.e. the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest, and vegetation succession). Such dynamics reflect the fluvial biogeomorphological successions model, as described by the authors earlier. © 2020 John Wiley & Sons, Ltd.     

三峡工程运用后长江中游典型江心洲植被动态变化趋势及机制探析

受三峡工程运用的影响,长江中游水沙情势剧变,江心洲生境结构发生变化,洲上植被密度、活力和分布情况也随之改变。因此亟需开展针对江心洲植被的长期观测以厘清其对三峡调控的响应机制,从而制定有效的洲滩植被保护和修复策略。卫星遥感技术是开展长时间、长河段地貌观测的常用手段。目前应用卫星遥感技术监测江心洲植被动态的研究,主要聚焦于三峡工程运用对江心洲植被面积和覆盖度的影响,而较少深入探讨植物长势和分布模式的变化趋势。因此,本文以长江中游4个典型江心洲为研究对象,提出了一系列可用于反演江心洲淹没范围和滩面上植被动态的方法,并量化分析三峡工程运用对植被动态的影响。结果表明:(1)三峡工程运用后,江心洲整体淹没频率降低,部分原有边滩逐步满足耐水植物的生长条件,促使江心洲植被面积呈现增长的趋势;原分布在高滩上的不耐水植物逐步蔓延至低滩,导致江心洲植被茂密程度整体上升;(2)三峡工程运行前,2002年含沙量较大的漫滩洪水有促进植物第二年生长的趋势;而三峡工程运行后,2016年具有同样规模但含沙量减少近80%的漫滩洪水则有抑制植物生长的作用;(3)部分形态稳定的江心洲,其高、低滩植被分布模式之间的异质性受三峡...     

Cyclic behaviour of interior beam–column joints reinforced with plain bars

The seismic damages commonly observed on beam–column joints of old reinforced concrete structures, built with plain bars and without proper detailing, justifies the need to further study the behaviour of this type of structures. The response of these structures when loaded cyclically, as occurs during the earthquakes, is partially controlled by the bond properties between the reinforcing bars and the surrounding concrete. This paper presents the results of an experimental campaign of unidirectional cyclic tests carried out on six full‐scale beam–column joints built with plain bars. These joint specimens are representative of existing reinforced concrete structures, that is, built without adequate reinforcement detailing for seismic demands. For comparison, an additional specimen is built with deformed bars and tested. The seven specimens are designed and detailed to allow the investigation of the influence of bond properties, lapping of the longitudinal bars in columns and beams, bent‐up bars in the beams, slab contribution and concrete strength. The lateral force–drift relationships, global dissipated energy evolution, contribution of the joint, beams and columns to the global dissipated energy, ductility, equivalent damping, final damage observed, homogenized reinforced concrete damage index, displacement components, curvature evolutions and Eurocode requirements are presented and discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Alternate bars in a sandy gravel bed river: generation,migration and interactions with superimposed dunes

A field study was carried out to investigate the development of alternate bars in a secondary channel of the Loire River (France) as a function of discharge variations. We combined frequent bathymetric surveys, scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics. The channel exhibited migrating bars, non‐migrating bars and superimposed dunes. Possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods. We propose that the reworking of bar sediments during low flows (high width‐to‐depth ratio β), reinforced by high values of the Shields mobility parameter, can explain the formation or re‐generation of new alternate migrating bars during a subsequent flood. The migration pattern of the bars was found to be cyclic and to depend mainly on (i) channel layout and (ii) the dynamics of superimposed dunes with heights and lengths depending on location and discharge value. For instance, the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events. We compare the findings from the field with results from theoretical studies on alternate bars. This gives insight in the phenomena occurring in the complex setting of real rivers, but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena. Copyright © 2014 John Wiley & Sons, Ltd.     

Wave decomposition phenomenon and spectrum evolution over submerged bars

Wave decomposition phenomenon and spectrum evolution over submerged bars are investigated by a previously developed numerical model. First, the computed free surface displacements of regular waves at various locations are compared with the available experimental data to confirm the validity of the numerical model, and satisfactory agreements are obtained. In addition, variations of decomposition characteristics with incident wave parameters and the change of energy spectrum for regular waves are also studied. Then the spectrum evolution of irregular waves over submerged bars, as well as the influence of incident peak wave period and the steepness of the front slope of the bar on spectrum evolution, is investigated. Wave decomposition and spectral shape are found to be significantly influenced by the incident wave conditions. When the upslope of the bar becomes 1:2, the length of the slope becomes shorter and will not benefit the generation of high frequency energy, so spectrum evolution is not significant.     

Modeling and cyclic behavior of segmental bridge column connected with shape memory alloy bars

This paper examines the quasi-static cyclic behavior,lateral strength and equivalent damping capacities of a system of post-tensioned segmental bridge columns tied with large diameter martensitic Shape Memory Alloy(SMA) link-bars.Moment-curvature constitutive relationships are formulated and analysis tools are developed for the PT column,including a modified four-spring model prepared for the SMA bars.The suggested system is exemplified using a column with an aspect ratio of 7.5 and twelve 36.5 mm diameter NiTi martensitic SMA bars.A post-tensioning force of 40% to 60% of the tendon yield strength is applied in order to obtain a self re-centering system,considering the residual stress of the martensitic SMA bars.The cyclic response results show that the lateral strength remains consistently around 10% of the total vertical load and the equivalent viscous damping ratios reach 10%-12% of critical.When large diameter NiTi superelastic SMA bars are incorporated into the column system,the cyclic response varies substantially.The creep behavior of the superelastic SMA bar is accounted for since it affects the re-centering capability of the column.Two examples are presented to emphasize the modeling sensitivities for these special bars and quantify their cyclic behavior effects within the column assembly.     

Life of a bifurcation in a gravel‐bed braided river

Channel bifurcation is a key element in braided rivers, determining the water and sediment distribution and hence controlling the morphological evolution. Recent theoretical and experimental findings, as well as field observations, showed that bifurcations in gravel‐bed braided rivers are often asymmetrical and highly unstable. In this paper field data are presented on a bifurcation in the Tagliamento River, northeast Italy. The planform configuration of the bifurcation and its temporal evolution was monitored by an automatic digital camera during a series of seven floods with different magnitudes. This remote sensing technique allowed a high temporal resolution (pictures were acquired every hour) that was proved to be essential in a highly dynamic system as the one considered here. Digitized maps of the channels provided information on the location of the bifurcation, the width of the anabranches, the angle between them, along with the occurrence and migration of sediment bars. Data were acquired at two different water levels, giving the possibility to compare low and high flow conditions. The monitored bifurcation is largely unstable and shows sudden changes in the water distribution, mainly driven by the bar migrating in the upstream channel and entering the distributaries. A relationship between width asymmetry and flood magnitude was observed, confirming previous analyses. Moreover, recent theoretical findings were applied, in order to test the possibility to estimate general trends in bifurcation evolution. The analysis pointed out the relevance of a correct assessment of the characteristic temporal scales, as the bifurcation evolves on a timescale similar to that of bar migration and flood duration. Understanding the interactions between these processes is therefore crucial in order to increase the ability to model and predict the morphological evolution of a braided network. Copyright © 2012 John Wiley & Sons, Ltd.     

近百年来长江河口航道拦门沙的形态演变特征

长江河口航道拦门沙对长江航运、泄洪以及上海港的建设具有重大的影响。通过近百年来历史资料和最新图件中北支、北港、北槽和南槽拦门沙河段的主泓剖面与平面图,分析近百年来4 个入海航槽拦门沙的形态演变特征。经分析发现：近100 年来,北支拦门沙由口外向口内逐渐移动并演变为口内巨型沙坎;北港拦门沙滩顶向下游移动了近30 km,2001 年后北港拦门沙河段开始有心滩发育;北槽拦门沙有两个明显的滩顶,但至2010 年这一显著特征消失;南槽拦门沙滩顶呈双峰型-多峰型-单峰型变化趋势,且滩顶向下游移动了约14 km。显然,除了北支拦门沙演变与崇明岛大规模围垦、北槽拦门沙演变与深水航道直接相关外,北港、南槽拦门沙演变特征主要与长江深水航道整治工程、横沙东滩圈围以及青草沙水库等大型工程影响有关。