Circulation cells topology and their effect on migration pattern of different multi-bend meandering rivers

In meandering rivers, a cross-stream flow, referred to as a secondary current, has important effects on broad spectra of hydraulic/environmental characteristics, running the gamut from river hydrodynamics and geomorphology to stream ecology. The transport equation for vorticity and kinetic energy transfer should be analyzed to specify terms involved in generation of secondary currents. However, there is limited research on scrutinizing these terms in meandering rivers. On the other hand, while rivers are mostly multi-bend, previous studies have been limited to single bends. In the current paper, three physical multi-bend channels representing a strongly curved bend, a mild bend and an elongated symmetrical meander loop are designed in order to unravel mechanisms responsible for forming circulation cells in cross sections. Experiments are carried out in the middle bend of these models. Cross-stream turbulence anisotropy considerably strengthens almost all near bank cells. Moreover, contrary to single sharp bends, multi bend effects hinder the transfer of the kinetic energy in both directions in the entrance section of the strongly curved bend.     

辽南中生代造山期缩短滑脱与晚造山伸展拆离构造

该区的构造格局主要由早期近东西向紧闭的褶皱带和晚期北北东向构造组成。早期的南北向缩短构造以龙王庙平卧褶皱和大小长山岛的直立紧闭褶皱为代表,分别具有扇状间隔性压溶劈理和透入性轴面片理,褶面倒向以北为主。北北东向构造切割近东西向构造,表层表现为北西西向薄皮逆冲推覆构造,浅层构造具有扇状压溶劈理的紧闭褶皱,深层表现为基底与盖层间的拆离断层及其下的韧性剪切带。早期的研究者将该断层作为辽南推覆构造底部的滑脱面,现今则压倒性地采用变质核杂岩的构造理念。根据相关剪切带早期面内褶皱发育,晚期伸展褶劈理发育,通过运动学涡度和应力状态分析,论证早期滑脱-推覆到晚期伸展拆离的演化过程。野外观测证明,辽南基底变质岩西侧的金州断层为一伸展拆离断层,它切割东侧的董家沟断层,前者平行于下伏糜棱岩中的同向伸展褶劈理,后者平行下伏糜棱岩的糜棱面理。金州拆离断层的形成及其东侧的隆起标志着辽南构造体制从缩短到伸展的转折。根据相关的年代学研究,这一构造体制转化发生在早白垩世(约120～107 Ma)。该区最新的构造事件是北东-南西向的缩短,相关的北北东向的右行走滑断层与晚白垩世以来的郯庐断层活动方式一致。     

无粘性土堤漫顶溃决水流运动试验研究

堤防溃决时溃口附近水流形态及溃决流量变化过程的正确描述是指导溃口堵复及时避险、预测溃堤洪水演进并进一步评估淹没损失的前提条件.通过弯道水槽中的概化模型试验,根据无粘性土堤防漫顶溃决的溃口变化和水流运动特征,将无粘性土堤防溃决过程分为漫流、冲槽、展宽及稳定4个阶段.试验系统研究了堤防溃决后河道以及溃口附近水位的变化过程,研究了河道洪水流量、洪水位和筑堤材料对溃口处水位的影响.基于大量的试验资料,进一步归纳总结出溃决流量与溃口口门宽度及溃口处水头变化之间的关系.     

声速剖面对多波束测深影响的新认识

介绍了声速剖面对多波束测深的影响,并得出了当声速剖面的某一节点偏大时,对边缘多波束的影响使水深偏浅,而对中央部分的波束使水深偏深;当声速剖面的某一节点偏小时,结果反之。该结果对野外资料采集及声速剖面编辑具有特别实际的指导意义。最后指出：当声速剖面出现误差时,中央波束的测深精度并不一定比其他波束的测深精度高。     

Overview of continuum and particle dynamics methods for mechanical modeling of contractional geologic structures

Mechanically-based numerical modeling is a powerful tool for investigating fundamental processes associated with the formation and evolution of both large and small-scale geologic structures. Such methods are complementary with traditional geometrically-based cross-section analysis tools, as they enable mechanical validation of geometric interpretations. A variety of numerical methods are now widely used, and readily accessible to both expert and novice. We provide an overview of the two main classes of methods used for geologic studies: continuum methods (finite element, finite difference, boundary element), which divide the model into elements to calculate a system of equations to solve for both stress and strain behavior; and particle dynamics methods, which rely on the interactions between discrete particles to define the aggregate behavior of the system. The complex constitutive behaviors, large displacements, and prevalence of discontinuities in geologic systems, pose unique challenges for the modeler. The two classes of methods address these issues differently; e.g., continuum methods allow the user to input prescribed constitutive laws for the modeled materials, whereas the constitutive behavior ‘emerges’ from particle dynamics methods. Sample rheologies, case studies and comparative models are presented to demonstrate the methodologies and opportunities for future modelers.     

后插筋旋喷桩-灌注桩复合式基坑支护结构设计方法

后插筋旋喷桩－灌注桩复合式基坑支护结构是根据实际工程需要设计出的一种新型结构，具有抗弯能力强，止水效果好，造价低廉和施工方便等优点，具有很好的应用前景。提出了该基坑支护结构截面抗弯计算的力学模型和设计计算方法。     

后弯管式波力发电装置气室结构的试验研究

后弯管式波力发电装置作为一种漂浮式振荡水柱式装置,具有可靠性强、安装维护方便、工作地点灵活的特点。本文在前人研究基础上,以10kW级后弯管式漂浮型发电装置为研究对象,以装置的水动力学性能为主要研究内容,以气室内相对波高作为主要表征参数,通过断面水槽水工物理模型试验,考察了气室内水柱振荡幅度与入射波波高和入射波周期之间的关系,分析了气室完全开敞、输气管添加负载以及弯管横管长度对气室内水柱振荡的影响,为进一步的大功率装置设计与试验提供了理论依据。     

Experimental study on local scour around single oblong pier fitted with a collar in a 180 degree flume bend

In this work, investigation on the development of local scour around an oblong pier in a 180 degree flume bend is presented. Scour hole can cause failure of the bridge especially during the river floods. In this study, the use of oblong collars for reducing the effects of local scour at a bridge pier is presented together with the time aspect of the scour development. Tests were conducted using one oblong pier in positions of 60degree under one flow conditions. The study was conducted using a physical hydraulic model operated under clear-water conditions in cohesionless bed material. In this study, the time development of the local scour around the oblong pier fitted with and without collar plates was studied. Investigated was the effect of size and elevation collar on the time development of scour and its efficacy at preventing scour at a bridge pier. The time development of the scour hole around the model pier with and without a collar installed was compared with similar studies on bridge piers. The results of the model study indicated that the maximum depth of scour is highly dependent on the experimental duration. It was observed that, as the minimum depth of scour occurs for the square collar at width of 3B placed at elevation of 0.1B below the bed and the size of a collar plate increases, the scour decreases. Measuring depth of scouring based on experimental observation, an empirical relation is developed with regression coefficient 95%.     

EXPERIMENTAL STUDY ON BED SCOUR IN A 90° CHANNEL BEND

1 INTRODUCTION Scouring in the bend ways leads to deep sections at the toe of the outer bank of the bend. The presenceof secondary currents and the greater depths at the outer bank cause high velocity along the outer bank.The high velocity and shear stres…     

Insights on the kinematics of deep-seated gravitational slope deformations along the 1915 Avezzano earthquake fault (Central Italy), from time-series DInSAR

Small meandering channels of about 1 m wide on an intertidal mudflat in the Westerschelde estuary the Netherlands) were studied with the aim to improve understanding of the effect of highly cohesive bed and bank sediment on channel inception and meander geometry and dynamics. The study is supported by experiments and modelling. The estuarine meandering channels are less dynamical than alluvial meandering rivers, and the dynamics are more localised. Moreover, the high thresholds for bed sediment erosion and for bank failure lead to two processes, uncommon in larger rivers, that cause most of the morphological change. First, the beds of the channels are eroded by backward migrating steps under hydraulic jumps, while the remainder of the bed surface along the channel is hardly eroded. Second, channel banks erode i) where eroding steps locally cause undercutting of otherwise stable channel banks and ii) in very sharp bends where the flow separates from the inner-bend channel boundary and impinges directly on the bank on the opposite side of the channel. Further morphological change is probably induced by rainfall splash erosion and by storm waves that weaken the mud, and by large mud fluxes from the estuary. The steps were successfully reproduced in laboratory flume experiments. An existing model for step migration predicted celerities consistent with field and laboratory observations and demonstrated a strong dependence on the threshold for erosion. Bank stability models confirm that banks and steps only fail when undercut and weakened by waves, rain or excess pore pressure in agreement with observations. The effects of a high threshold for bank erosion was implemented in an existing meander simulation model that reproduced the observed locations of bank erosion somewhat better than without the threshold, but flow separation and its effect on meander bends remains poorly understood.