A bed‐load transport model for rough turbulent open‐channel flows on plane beds

Data from flume studies are used to develop a model for predicting bed‐load transport rates in rough turbulent two‐dimensional open‐channel flows moving well sorted non‐cohesive sediments over plane mobile beds. The object is not to predict transport rates in natural channel flows but rather to provide a standard against which measured bed‐load transport rates influenced by factors such as bed forms, bed armouring, or limited sediment availability may be compared in order to assess the impact of these factors on bed‐load transport rates. The model is based on a revised version of Bagnold's basic energy equation i_bs_b = e_bω, where i_b is the immersed bed‐load transport rate, ω is flow power per unit area, e_b is the efficiency coefficient, and s_b is the stress coefficient defined as the ratio of the tangential bed shear stress caused by grain collisions and fluid drag to the immersed weight of the bed load. Expressions are developed for s_b and e_b in terms of G, a normalized measure of sediment transport stage, and these expressions are substituted into the revised energy equation to obtain the bed‐load transport equation i_b = ω G ^3·4. This equation applies regardless of the mode of bed‐load transport (i.e. saltation or sheet flow) and reduces to i_b = ω where G approaches 1 in the sheet‐flow regime. That i_b = ω does not mean that all the available power is dissipated in transporting the bed load. Rather, it reflects the fact that i_b is a transport rate that must be multiplied by s_b to become a work rate before it can be compared with ω. It follows that the proportion of ω that is dissipated in the transport of bed load is i_bs_b/ω, which is approximately 0·6 when i_b = ω. It is suggested that this remarkably high transport efficiency is achieved in sheet flow (1) because the ratio of grain‐to‐grain to grain‐to‐bed collisions increases with bed shear stress, and (2) because on average much more momentum is lost in a grain‐to‐bed collision than in a grain‐to‐grain one. Copyright © 2006 John Wiley & Sons, Ltd.     

福建沿海赤潮灾害气象预报

根据福建沿海2001～2008年赤潮灾害资料以及相应时期的气象数据,寻找与赤潮灾害发生密切相关的气象因子,分析赤潮灾害发生期间的地面及不同高度的天气形势,并探讨南方涛动指数与赤潮灾害发生的关系,在此基础上根据前期气温、风、云及海况进行福建沿海赤潮灾害气象预报。研究结果表明,福建沿海的风、气温、湿度、日照和气压等气象因子与赤潮的发生存在着密切的关系,但不同海区影响赤潮发生的主要气象因子不尽相同,低层850 hPa和地面形势能较好地反映福建沿海赤潮发生的天气背景,南方涛动指数与第2年赤潮发生日数有着很好的对应关系,二者相关系数为-0.745。     

中国南方湿润区“荒漠化”问题讨论

荒漠化是当今世界所面临的严重的环境与社会经济问题,推进荒漠化研究工作对于保护生态环境,促进区域可持续发展具有重要意义。本文在对荒漠化内涵进行系统回顾的基础上,认为除了人为干扰叠加气候性干旱作用导致的荒漠化之外,南方湿润区因水土流失叠加物理性干旱作用导致的极端土地退化也是一种荒漠化,并从物质基础、气候条件、影响因子、形成营力、核心作用和地表过程方面进行比较,最后以南方红层区特殊的"红层荒漠化"为案例,利用实地调查和TM影像的植被覆盖度指标VFC对1992-2013年红层土地退化进行分析。结果显示,在人为反复干扰和其他自然因素作用下地表植被持续退化、群落干旱趋势明显,表土层流失,岩石风化加剧,以致形成类似"荒漠"景观的现象。红层区的退化过程可类比于喀斯特石漠化、红壤山地丘陵退化,但这类极端土地退化现象可否视为"荒漠化",有待于进一步的深入研究。     

Transport of moderately sorted gravel at low bed shear stresses: The role of fine sediment infiltration

A reliable estimation of sediment transport in gravel‐bed streams is important for various practical engineering and biological studies (e.g., channel stability design, bed degradation/aggradation, restoration of spawning habitat). In the present work, we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses. The laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses, with detailed monitoring of the bed topography using a laser scanner. Effects of bed surface arrangements were documented by testing loose and packed bed configurations. Effects of fine sediments were examined by testing beds with sand, artificial fine sand or cohesive silt infiltrated in the gravel matrix. Analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement, the bed material properties (e.g., size and shape, consolidation index, permeability) and the concentration of fine sediments within the surface layer of moving grains. This concentration is directly related to the distribution of fine particles within the gravel matrix (i.e., bottom‐up infiltration or bridging) and their transport mode (i.e., bedload or suspended load). Compared to loose beds, the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments; in both cases, the bed shear stress for gravel entrainment increases by about 12%. On the other hand, the mobility of gravel increases significantly (bed shear stress for particle motion decreasing up to 40%) for beds clogged at the surface by non‐cohesive sand particles. Copyright © 2017 John Wiley & Sons, Ltd.     

矮堰基础冲刷研究进展

矮堰基础附近冲刷坑的深度是矮堰设计的重要参数,关系到建筑物的稳定与安全。系统回顾了国内外80余年关于矮堰基础冲刷的研究进展,阐述了矮堰基础冲刷的分类和机理,从物理模型试验、理论分析和数值模拟3个方面评述了研究现状和存在的不足。目前矮堰基础冲刷较为成熟的成果多集中于二维矮堰非淹没式清水冲刷方面,以物理模型试验方法为主,对于三维矮堰结构的动床冲刷、非冲积原状土床沙成分组成的影响、行进沙波与冲刷过程的耦合关系、浑水冲刷过程数值模拟技术等问题研究还有待深入。     

Impact of Montmorillonite and Calcite on Release and Adsorption of Cyanobacterial Fatty Acids at Ambient Temperature

Minerals might act as important sorbents of sedimentary organic matter and reduce biodegradation, which favors the formation of hydrocarbon source rocks in the earth's history. Since most organic matter is degraded during the sinking process, at ambient temperature, it is important to investigate the adsorption capacity of different minerals during this process, to assess the organic loss from primary productivity to sedimentary organic matter. In this study, montmorillonite and calcite have been selected to study the impact of different minerals on the release, adsorption, and deposition of cyanobacterial (Synechococcus elonpata) fatty acids (FAs) at ambient temperature. Gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) have been utilized to detect the variation in fatty acids. Primary results suggest that minerals have a different impact on dissolved organic matter. Montmorillonite can specifically enhance the release of fatty acids from cyanobacterial cells by lowering the pH values of the solution. The adsorption of the dissolved organic matter by montmorillonite will also be enhanced under a lower pH value. Conjunction of fatty acids with montmorillonite to form a complex will favor the sinking and preservation of these organics. Selective adsorption is observed among fatty acids with different carbon numbers. In contrast, calcite does not show any impact on the release and adsorption of organic matter even though it is reportedly capable of acting as a catalyst during the transformation of organic matter at high temperature. The primary data bridge a link between primary productivity and sedimentary organic matter, suggesting the relative importance of claystones in the formation of hydrocarbon source rocks in the earth's history.     

红层软岩压缩破坏的分形特征与级联失效过程

基于不同围压条件下的红砂岩三轴压缩试验结果,采用数字图像相关技术对岩样外表面破坏形态进行全程观测,获得不同阶段岩样外表面破坏图像,分别对岩样破坏后阶段的图像进行分析。通过分形盒维数表征岩样外表面裂纹的变化情况,发现岩样破坏越严重,裂纹分形维数越高,且在最后破坏阶段裂纹分形维数出现突变,揭示岩样的破坏特征。引入级联失效数学模型,结合红层软岩内部的矿物成分与细观结构分布特点,概化出其结构的随机连接模型与破坏方式,分析红层软岩加载作用下的内部节点颗粒“容量-荷载”重分配原则以及节点颗粒失效后其初始荷载传递路径。结果表明,红层软岩在受载时为一个级联失效的分阶段破坏过程,级联因子通过级联路径不断传播、增多,级联失效越发明显,同时结合岩石破坏的变形损伤过程与其轴向应力-应变曲线分段特征,揭示该类岩石破坏的级联失效分级标准,为拓展该领域的研究具备一定的参考价值。     

Two-Dimensional Mathematical Model of Tidal Current and Sediment for Oujiang Estnary and Wenzhou Bay

A 2-D mathematical model of tidal current and sediment has been developed for the Oujiang Estuary and the WenzhouBay. This model accomodates complicated features including multiple islands, existence of turbidity, and significant differ-ence in size distribution of bed material. The governing equations for non-uniform suspended load and bed load transport arepresented in a boundary-fitted orthogonal curvilinear coordinate system. The numerical solution procedures along with theirinitial conditions, boundary conditions, and movable boundary technique are presented. Strategies for computation of thecritical condition of deposition or erosion, sediment transport capacity, non-uniform bed load discharge, etc. are suggested.The model verification computation shows that, the tidal levels computed from the model are in good agreement with the fielddata at the 18 tidal gauge stations. The computed velocities and flow directions also agree well with the values measuredalong the totally 52 synchronously observed verticals distributed over 8 cross sections. The computed tidal water throughputsthrough the Huangda‘ao cross section are close to the measured data. And the computed values of bed deformation fromYangfushan to the estuary outfall and in the outer-sea area are in good agreement with the data observed from 1986 to 1992.The changes of tidal volumes through the estuary, velocities in different channels and the bed form due to the influence of thereclamation project on the Wenzhou shoal are predicted by means of this model.     

浙江近海夜光藻的分布及其生态学特点

本文利用历次夜光藻调查资料，阐述了其在浙江近岸海域的分布特点，丰度高峰主要出现在春秋季，夏季亦存在暴发性增殖的可能性，冬季则基本见不到夜光藻的踪迹。杭州湾口等水体浑浊区是夜光藻的低丰度区，仅在春夏季有零星出现，这与水体透明度低、悬浮泥沙含量高及强烈的水动力因子等密切相关。在时间尺度上，夜光藻丰度呈上升趋势，根本原因是海域富营养化加剧的结果。     

Corrected Formula of Bed Resistance Coefficient for Plane Numerical Simulation of Tidal Current

By means of a logarithm law for the velocity profile,a corrected formula of bed resistancecoefficient,which involves many factors such as gradient of still water depth,variation of surfaceelevation,flow direction,and so on,is derived from the 3D governing equations of tidal current by aver-aging over the whole water depth.Theoretical analysis and application have shown that the 2D plane tidalcurrent numerical model would be more reasonable and could be applied to steep bottom topographywhen the corrected bed resistance coefficient is used,therefore the results of reproduction simulation andengineering calculation would be more scientific and reasonable.