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A field‐based project was initiated in order to characterize velocities and sediment entrainment in a forced‐pool and riffle sequence. Three‐dimensional velocities and turbulence intensities were measured with an acoustic Doppler velocimeter at 222 different points at three similar flows that averaged approximately 4·35 m3 s−1 within a large pool–riffle unit on North Saint Vrain Creek, Colorado. Sediment‐sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location. Tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow, which reached a maximum level of 14·8 m3 s−1. Velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools. Patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle. Flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing, and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply. Jet flow, flow separation, vortex scour and turbulence generation enhance scour in the centre of pools. Ultimately, multiple processes appear to play some role in maintenance of this forced pool and the associated riffle. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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郭遂红 《广东海洋大学学报》2002,22(2):78-81
随着我国改革开放的进展 ,跨国度、跨文化之间的交往越来越频繁。由于文化因素上的差异 ,交际行为在不同的文化中往往产生不同的意义 ,语用迁移难免会造成理解失误或交际失败。因而外语教学在培养学生整体语言技能的同时 ,应注意文化因素的渗透 ,提高学生的跨文化交际能力 相似文献
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"干中学"与产业集群核心能力的形成 总被引：2，自引：0，他引：2
本文构建了产业集群核心能力的分析框架,从理论上说明学习效应是产业集群发展的内生动力,产业集群的学习是一个“干中学”的过程. 相似文献
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《Marine Policy》2017
The increased maritime accidents attributable to communication barriers have raised great concern about international shipping safety. The requirement for effective communication has been contained in the compulsory part of the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers in 2010. However, many Chinese seafarers still experience difficulty in communicating with foreign seafarers at sea even though they passed the English exams required for a Certificate of Competency. Literature argues that the outcomes of maritime English education are not satisfactory. However, this claim is too general and has not been substantiated with empirical evidence. Using a mix of qualitative and quantitative methods, this paper reveals that English communicative incompetence of Chinese seafarers was attributable to the maritime English examinations system, teaching materials, teaching methods and teachers and students themselves. Recommendations were made to improve maritime cadets’ English communicative competence. 相似文献
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Although the Shields relation was developed for rivers, it has been applied to sediment transport by overland flow. According to the Shields relation, where the critical boundary Reynolds number Re*c exceeds 40, the critical Shields number F*c is independent of both Re*c and the ratio of the critical flow depth to particle diameter dc/D. Analyses of data collected from runoff plots in southern Arizona reveal that F*c is positively correlated with both Re*c and dc/D. Thus the Shields relation does not apply to overland flow on debris-covered desert hillslopes. Multiple regression analysis is employed to develop alternative threshold relations in which critical boundary shear stress τc is related to D and dc/D (R2 = 0.782) and to D and Sc (critical gradient) (R2 = 0.625). The computed R2 values derive in large part from the spurious correlations of dc/D and Sc with τc. Nevertheless, the relations may be utilized to predict τc. In this regard, the latter relation is likely to prove more useful than the former because Sc is generally known, whereas dc is not. An investigation of the functional relation between τc and D reveals that τc varies approximately with D2 for overland flow on the desert hillslopes under study, whereas the Shields relation predicts a linear relation (i.e. a D exponent of 1). This result is consistent with Cheng's data which show that F*c varies with (dc/D)?1 where 0.4 < dc/D < 2 and may be explained in terms of increased energy dissipation both in separation zones downslope of particles and in distortion of the water surface as dc/D decreases. Consequently, larger values of τc, and hence F*c, are required to initiate the transport of particles of a given size D as dc decreases. 相似文献
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根据土建类测量学教学中的具体情况,分析了测量学教学和实习中的问题和困难,提出了应加强学生实践动手能力的培养,以及解决这些问题的方法和措施. 相似文献
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申洪军 《成都信息工程学院学报》2004,19(1):131-134
从教学理论和课程模式方面对英语教学的后发优势进行研究,并详细探讨了后发优势对大学英语教学和学生语言交际能力提高的影响.一切从实际出发,发挥后发优势能促进教学各方面的创新和跨越式发展. 相似文献
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We propose that competent layers composed of silicified rocks in the Cretaceous Ryoke low-pressure/temperature (low- P/T ) metamorphic belt in the Iwakuni–Yanai area, Southwest Japan, may be an exhumed equivalent of a seismically-reflective bright-layer in the middle crust. Silicified rocks crop out as layers or long lenticular bodies several to fifteen meters in thickness, and they are restricted to the area of greenschist facies conditions within a structural thickness of about one kilometer. The silicified rock layers exhibit a sharp contact with subjacent biotite schist but a gradational contact with overlying pelitic schist. Silicified rock is mainly composed of fine-grained quartz and minor muscovite and biotite, and some colored minerals have been variably decolorized by alteration. This implies that silicification turned the color of pelitic schists to pale gray or milky white. Quartz veins cutting schistosity at high angles were preferentially developed in the silicified rocks, while schistosity-parallel quartz veins, which underwent viscous flow, were observed in the underlying biotite schists. En echelon quartz veins and fishnet-like quartz veins are characteristic of silicified rocks. The modes of occurrence of quartz veins indicate that silicified rocks were competent relative to underlying biotite schist. The combination of contact with high-competence contrast adjacent to low-permeability silicified rocks may be a good reflector of seismic waves. If the silicified rocks were distributed in the middle crust to a considerable extent, they may result in seismic bright-layer reflections. 相似文献