海岸裂流的浮子示踪法实验测量

研究模型实验中采用浮子示踪法测量沙坝海岸裂流的方法,讨论了该方法的具体技术实现和测量结果的精度。实验中裂流由波浪在垂直岸线的直墙上的反射所形成的两列交叉的波浪生成。通过处理CCD图像的浮子轨迹得到了流场的详细速度分布,如旋涡分布等,显示了该方法优于流速仪定点测量的一些特点。与ADV流速仪测量结果的对比表明,由浮子轨迹得到的表层平均流速具有可靠的精度,特别是也可以应用于波浪破碎产生的裂流这样复杂流场的测量。     

Short-term Water Mass Movements in Lake Michigan: Implications for Larval Fish Transport

Water mass movement within the Great Lakes may rapidly transport fish larvae from favorable nursery areas to less favorable habitats, thereby affecting recruitment success. During 2001 and 2002, we released satellite-tracked drifting buoys in eastern Lake Michigan to follow discrete water masses, and used ichthyoplankton nets to repeatedly sample larval fish within these water masses. Observed nearshore water currents were highly variable in both direction and velocity. Current velocities far exceeded potential larval fish swimming speeds, suggesting that currents can potentially rapidly advect fish larvae throughout the lake. Evidence suggests that while paired drifters released during 2002 were able to track relatively small alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens) larvae within an alongshore coastal current, paired drifters released during 2001 failed to track larger alewife larvae when flow was more offshore and highly variable. These results are consistent with the decorrelation scales associated with alongshore and offshore transport.     

3D-modelling of Lake Kivu: Horizontal and vertical flow and temperature structure under spatially variable atmospheric forcing

With the increasing extraction of methane from Lake Kivu, there is a growing need to evaluate the effect of such operations on the lake’s permanent density stratification. This requires understanding of the spatial structure and variability of flow velocities and constituents in Lake Kivu. In this study, we develop a 3D hydrodynamic model of Lake Kivu, set-up within DELFT3D at a 750 m grid spacing and forced by COSMO-CLM atmosphere model results at a 2.8 km grid spacing. Validation shows that the model correctly reproduces the generation and breakdown of the temperature stratification in the upper mixed layer and predicts flow velocity magnitudes and directions similar to measurements both at the surface and at greater depth. Analysis of currents reveals a surface current pattern with two clockwise circulations, one around the whole lake and a smaller one in the northern part, with velocities around 0.1 m/s. This pattern is consistently present over an (ensemble-)averaged day, both in the wet and in the dry season, while day-by-day variations are large. Time-averaged deep currents are found to be a few mm/s at maximum. However, the variations can be substantial, with standard deviations up to 2 cm/s for the currents at 220 m depth, attributed to internal seiches. The temperature stratification, present during the entire wet season, is found to first break down in the dry season in the southern part of the lake. This is explained by the spatial differences in the wind stress and the evaporation heat fluxes during the dry season.     

Tracking River Plumes with Volatile Halocarbon Contaminants: The Niagara River—Lake Ontario Example

The concentrations of six predominant volatile halocarbon contaminants CCl₄, total and industrial CHCl₃, CHBrCl₂, CH₂Cl₂, CFCl₃, and C₂Cl₄ in epi- and hypolimnetic lake waters are used to demonstrate tributary plumes and nearshore currents on the Lake Ontario/Niagara River example. The relative speed and sensitivity of the analysis for volatile halocarbons provides for a synoptic view of the major currents at the time of sampling. For example, the individual distributions of several contaminants indicate the existence of a Niagara River surface water plume in northerly direction for approximately 8 km into the lake. In contrast, the predominantly east-west isostats of the contaminant concentrations in the hypolimnetic water indicate little direct influence by the Niagara River and show the existence of other contaminant sources in these areas.     

流对浅水波在缓变地形上传播的影响

孤立波在缓变地形上传播,将发生折射。在这一过程中加入流的影响将使传播过程更加复杂。本文采用有流影响的变系数二维KdV方程来描述这一现象。通过数值计算,文中给出了孤立波在斜坡海岸、水下圆岛等两种地形上有流及无流情况下,和旋涡流场中的折射的时间历程。并讨论了流对孤立波分裂、折射及传播速度的影响。     

Lake trout in northern Lake Huron spawn on submerged drumlins

Recent observations of spawning lake trout Salvelinus namaycush near Drummond Island in northern Lake Huron indicate that lake trout use drumlins, landforms created in subglacial environments by the action of ice sheets, as a primary spawning habitat. From these observations, we generated a hypothesis that may in part explain locations chosen by lake trout for spawning. Most salmonines spawn in streams where they rely on streamflows to sort and clean sediments to create good spawning habitat. Flows sufficient to sort larger sediment sizes are generally lacking in lakes, but some glacial bedforms contain large pockets of sorted sediments that can provide the interstitial spaces necessary for lake trout egg incubation, particularly if these bedforms are situated such that lake currents can penetrate these sediments. We hypothesize that sediment inclusions from glacial scavenging and sediment sorting that occurred during the creation of bedforms such as drumlins, end moraines, and eskers create suitable conditions for lake trout egg incubation, particularly where these bedforms interact with lake currents to remove fine sediments. Further, these bedforms may provide high-quality lake trout spawning habitat at many locations in the Great Lakes and may be especially important along the southern edge of the range of the species. A better understanding of the role of glacially-derived bedforms in the creation of lake trout spawning habitat may help develop powerful predictors of lake trout spawning locations, provide insight into the evolution of unique spawning behaviors by lake trout, and aid in lake trout restoration in the Great Lakes.     

Bioavailability of particle‐associated nutrients as affected by internal regeneration processes in the Nyanza Gulf region of Lake Victoria

Land‐use changes have been implicated a lot in the eutrophication of many lakes while forgetting the role of internal loading as influenced by the hydrological regimes of a given lake. The phosphorus loading of Nyanza Gulf is influenced both by internal and external loading with the internal loading playing a greater role in its eutrophication. The shear on the Apatite Phosphorus (AP) rich residual rock in the western end of the gulf through strong currents across the Rusinga Channel erodes the rock into non‐apatite inorganic phosphorus (NAIP) which is readily available for primary productivity. The current suspends the phosphorus rich apatite sediment together with the reserved phosphorus within sediments to the water column. The NAIP concentration on the western end of the gulf is exceptionally high, >1500 mg kg^?1, and together with the hydrological forcing; is believed to be the driving force of Nyanza Gulf eutrophication. External loading through rivers and municipal discharges exacerbates the problem. The external loading mainly influences the inner gulf on the eastern shore while the internal loading affects mainly the western end of the gulf. Nyanza Gulf eutrophication can be managed by adopting the following measures: (i) the Mbita Causeway needs to be opened and a bridge erected in its place in order to reduce the strong current through the Rusinga Channel and the residence time within the gulf, by increasing the flushing rates; (ii) the farming communities within the basin need to be sensitized on the controlled use of fertilizers; (iii) the municipal wastes should be treated to tertiary level before discharge into the lake; and (iv) reduce erosion within the basin through re/afforestation.     

THREE-DIMENSIONAL NUMERICAL MODELLING OF THE WAVE-IN- DUCED RIP CURRENTS UNDER IRREGULAR BATHYMETRY

A process-based 3-D hydrodynamic model is established to simulate the rip current structures under irregular bathymetry.The depth-varying wave-induced residual momentum,the surface rollers,the turbulent mixing and the wave-current interactions are considered.Experimental datasets are used to validate the model,and it is shown that the model can effectively describe the 3-D structures of the rip currents in both normal and oblique wave incident cases.The flow patterns of the rip currents see various characteristics for different incident wave directions.In the normal incident case,pairs of counter-rotating primary circulation cells are formed,and an offshore rip flow occurs in the embayment troughs.The peak seaward velocities occur at the top of the bed boundary layer,and the undertow is incorporated in addition to the rip currents.In the oblique incident case,the longshore currents are dominant,which result in a meandering flow along the depth contour,and the undertow is weaker compared to that in the normal incident condition.     

水库异重流的三维数值模拟及影响因素分析

不同水沙条件下异重流的潜入位置以及潜入后流速和含沙量的垂向分布规律是研究异重流运动规律的关键要素,对优化水库异重流的排沙调度、提高水库运用效益等至关重要。本文利用SCHISM三维水沙数学模型对水库异重流水槽试验进行了数值模拟,结果表明模型能够得到符合实际情况的垂线流速和含沙量分布,且异重流潜入后的平均厚度、平均流速和平均含沙量均与实测值吻合较好。对异重流模拟结果的分析发现,入口流量和含沙量的增加造成异重流运动速度越快,而入口含沙量的减小和流量的增加会造成异重流厚度增加。异重流垂向流速和含沙量的无量纲化分布受到入口水沙条件的影响,表现在其与断面Froude数的相关性,Froude数越大,则最大流速点与河床的距离越小,最大流速越大;含沙量致密层的厚度越小,对应的含沙量也越大。此外,潜入点位置也受入口水沙条件的影响,表现在潜入水深与入口水深之比随入口Froude数增加而增加的定量关系,由此可以利用入口水沙条件预报异重流潜入的位置。研究结果不仅验证了SCHISM模型用于模拟水库异重流运动的优势,而且丰富和完善了关于异重流潜入点位置及流速和含沙量垂向分布受入口水沙条件影响的变化规律。     

Rip currents near coastal structures in Lake Michigan: Characterization and assessment for warnings

Rip currents near coastal structures commonly occur in Lake Michigan in the Great Lakes region of the United States. Lack of timely warning due to undocumented characteristics of rip currents and no assessment tool can contribute to tragic drownings incidents. In this paper, we characterized rip current occurrences near breakwater structures and developed an assessment tool for providing timely rip current warnings to beachgoers at the study site, City of Port Washington, WI. Characteristics of rip currents near the structure were observed from field measurements or visual images. Deflection rip currents had speeds of ～ 0.2 m/s and lasted for several hours. The rip current occurrences were associated with environmental proxies. It was found that rip currents can occur even when the water appears calm near the structure. A Structure Rip Checklist and Assessment Matrix (SRiCAM) with a four-tiered risk was developed and validated using observations. Furthermore, the SRiCAM was integrated into cyberinfrastructure with a data contingency plan to provide real-time warnings to the public. The applicability of the SRiCAM to other locations across Lake Michigan was further tested and results are promising. Overall, the SRiCAM has the potential to be widely extended to foster recreational water safety and resilience to rip current hazards in the Great Lakes.     

Relationships between wind-driven and hydraulic flow in Lake St. Clair and the St. Clair River Delta

A three-dimensional hydrodynamic forecasting model of the Great Lakes Huron-Erie Corridor is used to investigate mixing and the relationship between hydraulic and wind-induced currents in a shallow lake system in which lake inflows come through several channels of a river delta. The hydrodynamics in Lake St. Clair and the channels of the St. Clair River Delta are evaluated for (1) a one-year simulation from 1985 including water age calculation, (2) 8 different wind direction scenarios, and (3) a storm event. Observations and model simulations show distinct regions in the lake in which currents are forced by either hydraulic flow from the river system or from wind stress over the lake. However, during severe storm events, these regions are found to shift or even disappear due to changes in the delta channel inputs into the lake. These changes underscore the need for realistic, unsteady river flow boundary conditions at interfaces between a shallow lake and river delta. Steady inflow conditions will not allow for potential shifting of these current zones, and will also fail to resolve flow retardation or reversals during storm events.     

The Problem of Coordination of Levels of Short-Circuit Currents in Power Systems

The problem of coordination of short-circuit (fault) currents is considered. Data on the dynamics of variation of the levels of single-phase and three-phase short-circuit currents (their highest values) in networks rated for different voltage are presented for a long-term period. The main factors (integral parameters of the networks) affecting the values of short-circuit currents are listed. Statistics of variation of rated parameters of electrical equipment in past years and in the recent period is presented and predictions for the future are made. Basic methods and means for limiting short-circuit currents are discussed. A method of automatic and stationary network separation and a method of circuit design are described in detail. The efficiencies of different methods are compared. The effect of integral parameters of the network on the maximum level of short-circuit currents is shown. The aspects of the switching life of breakers and of allowance for probabilistic characteristics of faults and for the risk factor of decision-making are considered. It is shown that coordination of the levels of short-circuit currents is a possible means for raising the reliability of power installations and systems.__________Translated from Elektricheskie Stantsii, No. 4, April 2005, pp. 19 – 32.     

Storm-induced Redistribution of Deepwater Sediments in Lake Ontario

High-resolution seismic reflection profiles, side-scan sonar profiles, and surface sediment analyses for grain size (% and, silt & clay), total organic carbon content, and carbonate content along shore-perpendicular transects offshore of Olcott and Rochester in Lake Ontario were utilized to investigate cm-thick sands or absence of deepwater postglacial sediments in water depths of 130 to 165 m. These deepwater sands were observed as each transect approached and occupied the “sills,” identified by earlier researchers, between the three deepest basins of the lake. The results reveal thin (0 to 5-cm) postglacial sediments, lake floor lineations, and sand-rich, organic, and carbonate poor sediments at the deepwater sites (> 130 m) along both transects at depths significantly below wave base, epilimnetic currents, and internal wave activity. These sediments are anomalous compared to shallower sediments observed in this study and deeper sediments reported by earlier research, and are interpreted to indicate winnowing and resuspension of the postglacial muds. We hypothesize that the mid-lake confluence of the two-gyre surface current system set up by strong storm events extends down to the lake floor when the lake is isothermal, and resuspends and winnows lake floor sediment at these locations. Furthermore, we believe that sedimentation is more likely to be influenced by bottom currents at these at these sites than in the deeper basins because these sites are located on bathymetric highs between deeper depositional basins of the lake, and the bathymetric constriction may intensify any bottom current activity at these sites.     

Stabilization of Short-Circuit Current Levels in Grids Rated 100 kV and Above

Actual values of short-circuit currents in power systems are presented. Levels of short-circuit currents and extent of reengineering or replacement of breakers in elimination of stationary separation nodes in a large power system are described. Stationary separation of the grid and circuit design solutions are shown to be technically and economically beneficial solutions.     

泉州湾潮汐、潮流及浅滩干湿变化的数值研究

本文建立了一种适应POM模型特点的干湿点数值处理方法，将其镶嵌于二维POM模型中用以模拟泉州湾的潮汐潮流，两个站点上的模拟结果与实测数据吻合较好。计算结果还显示：泉州湾的潮汐潮流均以半口潮为主；除在湾口中部呈现出较强的旋转流外，湾内大部海域以往复流为主；另外，计算得到的泉州湾内平均潮差约为4m，如此大的潮差是导致湾内浅滩和暗礁在落潮时干出、涨潮时重新被漫过的根本原因。     

水库异重流淤积成因分析及前锋运动规律

汛期暴雨冲刷河道两岸形成浑水入库,在一定条件下会潜入水库底部形成异重流。若上游降雨历时较短,形成的异重流因失去后续浑水补给而停止运动进而造成水库淤积。针对这一问题,基于水库异重流挟沙能力的计算公式,分析异重流造成水库淤积的原因,即异重流的超饱和输沙和沿程流量损失。针对异重流因沿程流量损失而发生的水库淤积,对失去后续浑水补给的异重流进行水槽试验,分析坡度和浑水入流流速对异重流前锋厚度及运动速度的影响。试验结果表明:坡度或浑水入流流速越小,失去浑水补给的异重流前锋运动速度越慢,前锋厚度减小越快。     

基于ARGO浮标资料的太平洋三维流场估算

针对实际海洋中三维海流观测困难和流场资料欠缺的问题,通过ARGO浮标观测资料与WOA09背景场资料进行最优插值融合,得到较精细的温盐网格化资料,并在此基础之上通过引入P矢量方法,对太平洋海区的三维流场进行了诊断推算。分析表明:ARGO浮标再分析融合数据能够较好表现太平洋温盐场分布结构,在此基础之上诊断推算的三维流场能够合理刻画太平洋流场的基本特征,为获取大尺度海洋三维流场信息提供了有效的途径。     

Experimental study of the flow structure of decelerating and accelerating flows under a gradually varying flume

The turbulence characteristics of both decelerating and accelerating flows under a gradually varying flume are investigated by using a three-dimensional down-looking acoustic Doppler velocimeter(ADV). The time-averaged velocity profiles are flatened except for the central parts, and fairly fit into logarithmic laws and those in the plane circulation under the gradual expansion are more likely to be negative. The complex secondary currents are identified under the present gradual transition attributed to the combination of driving forces induced by both the boundary configuration variation and the unbalanced turbulence: a circulation on each side of the expansion and a pair of circulations on each side of the contraction. One sees an anisotropy in the turbulence intensities, the turbulence intensities increase or level out with the flow depth except those under expansion, and the V component of the turbulence intensity typically outweighs that in the streamwise direction. Apart from the above results, the respective particular distributions of the primary Reynolds shear stresses(xy? and xz?) under the gradual expansion and contraction can account for the patterns of the secondary currents in this investigation.     

Simultaneous Process-Response Study on the East and West Coasts of Lake Michigan

A 15-day time-series study was conducted simultaneously on both the east and west sides of Lake Michigan during the summer of 1974 at Zion, Illinois and South Haven, Michigan. The primary objective of the study was to determine the nature of process-response activities as a single weather system moves offshore and onshore as it crosses the lake. Observed patterns in the weather, waves and currents are quite similar at both sites. Changes in processes and the topographic response occurred simultaneously on both sides of the lake, at least within the limits of sampling intervals. The conceptual model developed by the authors for Lake Michigan coastal processes, was found to be applicable to this study.     

Lake Turkana,major Omo River developments,associated hydrological cycle change and consequent lake physical and ecological change

This study aimed to explore Lake Turkana's ecological reliance on hydrology and to determine the hydrological changes and consequences arising from the major hydropower and irrigation developments in the lake's basin. The major developments on Ethiopia's Omo River are especially significant as this river provides over 80% of the lake's annual freshwater influx and associated nutrients. The cascade of hydropower dams permanently dampens the natural hydrological cycles and lake level variability. The driving force of the flood influx to the lake is curtailed and the pattern of lake currents will adjust. Ultimately 80% of the river inflow to the lake will be regulated. Large volumes of water are required to initially fill the hydropower dam reservoirs. During 2015–16 when the huge Gibe III reservoir was filled, Lake Turkana's water level declined 2?m.The study has shown that large-scale irrigation schemes in the Lower Omo can potentially abstract 50% of the Omo River water, and that this would cause the lake level to shrink permanently to the detriment of the lake ecology. Possible lake level drops of over 15?m are demonstrated. The basin's natural capital is being replaced by large-scale plantation developments. The hydrological changes are drastic and the ecological consequences on Lake Turkana have not been fully understood. Without serious mitigation measures, Lake Turkana is a potential African Aral Sea disaster in the making, emulating what has happened to other great lakes such as Lake Chad.