Simulation of flow over pool-riffle topography: A consideration of the velocity reversal hypothesis     

P. A. Carling  N. Wood 《地球表面变化过程与地形》1994,19(4):319-332

Computer flow simulations using the HEC-2 step-backwater routine are used to demonstrate the effect of systematically varying river channel width, riffle spacing and channel roughness on the shear velocity, section-mean velocity and energy slope in fixed-bed pool-riffle sequences. Initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the River Severn. Subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored. The purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence ‘reversal’, such that pools scour at high flow whilst deposition occurs on riffles. It is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal. For prescribed conditions, the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width. Downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool-riffle topography.  相似文献   

关于地球磁场极性反转机制的思索     

杨合群 《西北地质》2002,35(3):126-130

地球磁场极性在地质历史中发生过相当频繁的倒转。作者将地球内部划分为岩石圈－软流圈－中圈－液圈－固核等5个动力学圈层，认为中圈与固核间可异步旋转；地球偶极磁场由中圈与固核异步旋转时所驱动的液圈中的封闭涡流与系磁场作用产生；该偶极子场极性由地球所通过的银道面上侧或下侧磁场方向及液圈涡流的方向共同决定，二者之一反向，极性发生倒转。  相似文献   

Reversal of monthly East Asian winter air temperature in 2020/21 and its predictability          下载免费PDF全文

Hongqing Yang  Ke Fan 《大气和海洋科学快报》2022,15(1):42-48

在全球变暖的背景下,近年来东亚冬季气温存在复杂的季节内变化.本文研究了2020/21年东亚冬季气温的月际转折及可预测性.结果 表明,2020/21年东亚冬季气温前冬(2020年12月-2021年1月中旬)偏冷,后冬(2021年1月中旬-2月)偏暖.西伯利亚高压强度在前冬和后冬也出现转折变化.在前冬,由于2020年9月巴...  相似文献   

东南极普里兹湾NP93－2柱样古地磁结果          下载免费PDF全文

王保贵  侯红明  汤贤赞  袁友仁 《极地研究》1996,8(1):34-40

（王保贵）（候红明）（汤贤赞）（袁友仁）ＰａｌｅｏｍａｇｎｅｔｉｃｒｅｓｕｌｔｓｏｆＣｏｒｅＮＰ９３－２ｆｒｏｍｔｈｅＰｒｙｄｚＢａｙ，ＥａｓｔｅｒｎＡｎｔａｒｃｔｉｃａ￥ＷａｎｇＢａｏｇｕｉ；ＨｏｕＨｏｎｇｍｉｎｇ；ＴａｎｇＸｉａｎｚａｎａｎｄＹｕ...  相似文献   

长江中游反向过程——来自四川盆地东部的构造地貌指示     

王平  郑洪波  刘少峰 《第四纪研究》2013,33(4)

万里长江的形成经历了数次大规模的河流袭夺事件,现今的长江中游河段(川江)曾经作为古长江的西支,自东向西汇入南流的古金沙江,后来由于不断的反向才使其与古长江的东支,即现今的长江下游得以贯通.针对这一河流演化模式,学术界普遍将长江三峡的黄陵背斜视为古长江东支和西支的分水岭,然而却一直未能获得有效的地貌证据,以致于无法解释具体的河流袭夺和反向过程.为此,本文从川东-湘鄂西弧形断褶带的构造地貌特征入手,深入分析了不同区段长江及其支流与构造线的交切关系,提出古长江东支和西支的分水岭应位于弧形带的“中线”附近的向斜谷地内,而并非前人认为的黄陵背斜.通过对向斜谷地内正交型水系样式下的河流袭夺案例分析,结合先进的河流纵剖面分析技术,揭示出由于基准面下降而造成的纵向河段被袭夺并最终反向的机制.另外,根据江汉盆地西缘(宜昌地区)的白垩纪-新生代地层的沉积学和物源分析,进一步限定了长江中游的袭夺和反向开始于始新世,直到中新世才形成现今贯通的长江.  相似文献   

Sediment routing hypothesis for pool‐riffle maintenance     

David J. Milan 《地球表面变化过程与地形》2013,38(14):1623-1641

This paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool‐riffle maintenance in sinuous upland gravel‐bed streams. The findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments, if little or no sediments are fed into them. A combination of clast tracing using passive integrated transponder (PIT) tagging and bedload traps (positioned along the thalweg on the upstream riffle, pool entrance, pool exit and downstream riffle) are used to provide information on clast pathways and sediment sorting through a single pool‐riffle unit. Computational fluid dynamics (CFD) is also used to explore hydraulic variability and flow pathways. Clast tracing results provide a strong indication that clasts are not fed through pools, rather they are transported across point bar surfaces, or around bar edges (depending upon previous clast position, clast size, and event magnitude). Spatial variations in bedload transport were found throughout the pool‐riffle unit. The pool entrance bedload trap was often found to be empty, when the others had filled, further supporting the notion that little or no sediment was fed into the pool. The pool exit slope trap would occasionally fill with sediment, thought to be sourced from the eroding outer bank. CFD results demonstrate higher pool shear stresses (τ ≈ 140 N m^–2) in a localized zone adjacent to an eroding outer bank, compared to the upstream and downstream riffles (τ ≈ 60 N m^–2) at flows of 6 · 2 m³ s^–1 (≈ 60% of the bankfull discharge) and above. There was marginal evidence for near‐bed velocity reversal. Near‐bed streamlines, produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg. Some streamlines appear to brush the outer edge of the pool for the 4 · 9 m³ s^–1 to 7 · 8 m³ s^–1 (between 50 and 80% of the bankfull discharge) simulations, however complete avoidance was found for discharges greater than this. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

地球磁极倒转的分形混沌研究          下载免费PDF全文

申维  房丛卉  张德会 《地学前缘》2009,16(5):201-206

地球磁场多次发生南北(正负)磁极位置的变换,这已为大家所公认。但造成这种异常现象的原因,则是迄今未能很好解答的一个难题。应用分形混沌理论对地球磁极倒转进行了分析,认为发生地球磁极倒转时间段在时间轴上具有分形分布的性质;地球磁场系统通过不断与外界交换物质和能量,维持一种空间或时间的有序结构。地球磁场极性的随机倒转具有混沌运动的自逆转特性,混沌理论给地磁极性倒转提出了一个简明的动力机制解释。  相似文献   

虾夷扇贝不同性别类型2 个Dmrt基因DM 结构域分析     

周丽青  杨爱国  王清印  郑利兵  刘志鸿  侯丫  郑言鑫 《海洋科学》2015,39(3):19-25

为探讨虾夷扇贝(Patinopecten yessoensis)性别决定和雌雄同体形成的分子机制,利用兼并引物,以闭壳肌基因组DNA为实验材料,扩增和克隆了虾夷扇贝Dmrt基因的DM结构域,雌性、雄性和雌雄同体3种性别共获得2个具有不同DM序列的克隆,序列长度均为141bp,分别命名为Py Dmrt3和Py Dmrt4。Py Dmrt3在3种性别类型中均有克隆,而雄性中还克隆出Py Dmrt4。结果表明,在虾夷扇贝不同性别中,Dmrt基因家族的成员可能会有不同;雄性py Dmrt3的DM结构域核苷酸和氨基酸变异频率高,其次是雌雄同体,雌性的较保守,与其他软体动物DM结构域比对,该基因家族在进化上仍然高度保守,由此推测,部分雄性可能是发生了性逆转的雌雄同体,这种较高的变异性可能也是雌雄同体的一个特点,Py Dmrt4可能参与调控雄性性别的形成。  相似文献   

Prevalence of the terrain reversal effect in satellite imagery     

Miguel-Angel Bernabé-Poveda 《International Journal of Digital Earth》2015,8(8):640-655

The terrain reversal effect is a perceptual phenomenon which causes an illusion in various 3D geographic visualizations where landforms appear inverted, e.g. we perceive valleys as ridges and vice versa. Given that such displays are important for spatio-visual analysis, this illusion can lead to critical mistakes in interpreting the terrain. However, it is currently undocumented how commonly this effect is experienced. In this paper, we study the prevalence of the terrain reversal effect in satellite imagery through a two-stage online user experiment. The experiment was conducted with the participation of a diverse and relatively large population (n = 535). Participants were asked to identify landforms (valley or ridge?) or judge a 3D spatial relationship (is A higher than B?). When the images were rotated by 180°, the results were reversed. In a control task with ‘illusion-free’ original images, people were successful in identifying landforms, yet a very strong illusion occurred when these images were rotated 180°. Our findings demonstrate that the illusion is acutely present; thus, we need a better understanding of the problem and its solutions. Additionally, the results caution us that in an interactive environment where people can rotate the display, we might be introducing a severe perceptual problem.  相似文献   

导线加测陀螺方位角的过程简化探讨     

程飞  张丽娜  赵志红 《测绘工程》2014,(6):56-58

针对地下陀螺定向时间较长、作业强度大、陀螺仪电量等问题,现就陀螺仪传统测量过程（主要是逆转点法）提出简化.通过已知坐标反算操作获得粗北方向的方法取代传统的粗定向过程,可以大大提高定向效率和经济效益,并且能够节省电池电量、节省作业人员体力.文中对比新旧方法在寻找粗北方向的测量结果,从而分析方法的可行性.  相似文献