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1.
STEFAN LÜTHI 《Sedimentology》1981,28(1):97-105
A theoretical consideration of two dimensional underflows and surge-type turbidity currents results in a general momentum equation. A number of formulae in current use are special cases of this equation, among which are the modified Chézy equation and Bagnold's criterion for autosuspension. Five dimensionless parameters are included: the Richardson number Ri (defined as the inverse square of the Froude number), the friction coefficient cf, the slope β, the dimensionless settling velocity of the sediment Vs/u and the changes in flow height with distance dD/dx. The latter is mainly a measure of the dilution by entrainment of ambient water. For chalk powder experiments on surge type turbidity currents and on the initial front of continuous underflows the momentum equation is shown to be correct. Values for Ri range from about 1.5 at 0° slope to about 0.75 at 5° and are slightly to substantially lower than values from earlier authors. The two types of turbidity currents investigated show close similarity. A surprising attribute is their strong dilution even at very low-angle slopes. Pelitic sedimentation is possible from the upper, dilute part of the currents, graded intervals found at the base of turbidites can be explained as bedload deposits from the lowermost, concentrated layer of the current; hydraulic jumps are expected to be rare in surge-type turbidity currents and fronts of incipient underflows. 相似文献
2.
采用变坡实验水槽研究了坡面薄层水流水动力学特性——流态、流速、水深及阻力系数随流量和坡度的变化规律,实验坡度5°～25°、单宽流量为0.625×10-3～12.5×10-3m3/(s·m).研究结果表明,坡面薄层水流流态与水深密切相关,坡面流的流态基本上呈过渡流和紊流;薄层水流平均流速、水深和Darcy-Weisbach阻力系数主要受流量控制,坡度的影响并不显著,其关系均可用流量和坡度的幂函数形式模拟,相关系数(R2)分别为0.98、0.97和0.95,去掉坡度后相关系数分别下降7%、16%和3%.随着雷诺数的增大,Darcy-Weisbach阻力系数下降.当单宽流量小于0.008m3/(s·m)时,坡度对阻力系数的影响较为显著;当流量大于0.008m3/(s·m)时,阻力系数基本受流量控制.随着流量增大,阻力系数呈幂函数形式递减. 相似文献
3.
Sediment deposition from turbidity currents in simulated aquatic vegetation canopies 总被引：1，自引：0，他引：1 
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下载免费PDF全文 Marianna Soler Jordi Colomer Teresa Serra Xavier Casamitjana Andrew M. Folkard 《Sedimentology》2017,64(4):1132-1146
A laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l?1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, CDaxc (where CD is the array drag coefficient, at the frontal area of cylinders per unit volume, and xc is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until CDaxc = 5. For CDaxc > 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles. 相似文献
4.
Modelling of turbidity currents on Navy Submarine Fan, California Continental Borderland 总被引：2，自引：1，他引：2
Several Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain-size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep-tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain. On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s?1 and flow velocities of about 0.75 m s?1. Using this flow velocity and channel dimensions, sediment concentration (～2×10?3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2. On the mid-fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross-flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand-rich late Pleistocene and early Holocene turbidity currents on the fan. 相似文献
5.
EMPLACEMENT OF FLYSCH-TYPE SAND BEDS 总被引：1，自引：0，他引：1
PH. H. KUENEN 《Sedimentology》1967,9(3):203-243
Recently several attempts have been made to explain deep-sea sands or flysch-type sandstone beds by normal currents, instead of by turbidity currents. The arguments that are offered against turbidity currents and those in favour of normal currents are inconclusive. Current measurements and calculations indicate 1 m from the bottom on abyssal plains velocities are less than 30 cm/sec. The ubiquitous structures: sole markings, graded bedding, fine-grained ripple mark between a lower and a covering set of horizontal laminae, and convolution, are shown each in turn to be inexplicable on the basis of normal traction currents and the same holds for the uniform bed thickness. On the other hand these features develop readily in a circular flume from overloaded suspension currents. These experiments show that to support a heavy charge of fine sand in a clay suspension a current must exceed 100 cm/sec, and in clear water double that amount is needed. The inadequacy of normal currents both in velocity and kind is thus established. This lends powerful support to the case for turbidity currents. Many authors claim to have found evidence for the deflection of turbidity currents or for currents flowing across the paleo-slope. Explanations offered include the Coriolis force, normal currents, multiple turbidity currents, or surge waves. Analysis shows that all are open to serious doubts. The author suggests, quite tentatively, that the deflections may be only simulated by the development of lamination and grain orientation oblique and perpendicular to the current direction. Sagging of the trough floor may also play a part by confusing the determination of paleo-slope. Another possibility is that the turbidity current deviated from its original direction by “internal slope”, by momentum, by centrifugal force, or by lack of space. Admittedly, a problem remains, for the swift deposition deduced from the climbing ripples is in contradiction with the supposed stretching of the turbidity current inferred from grading. 相似文献
6.
泥沙淤积是影响多沙河流水库寿命的一大难题,而异重流排沙是减少库区淤积的重要措施之一。异重流的潜入现象是异重流开始形成的直观标志,研究异重流潜入条件的判别方法有助于掌握异重流在库区内的演进规律。总结了水库异重流潜入条件的定性描述及定量计算方法,指出已有的潜入点判别公式的优缺点及适用范围,改进了描述异重流运动的动量方程,同时分析了异重流流速与含沙量沿垂线不均匀分布对动量传递的影响;在此基础上提出新的异重流潜入条件判别式,并用多组室内及野外实测资料对该判别条件进行率定与验证。分析结果表明,新的计算公式可用于判别小浪底库区异重流的潜入条件。 相似文献
7.
李利阳 《沉积与特提斯地质》2015,35(4):106-112
本文在总结前人对浊流沉积研究的基础上,分析前人对浊流与浊积岩、浊流沉积与浊流相模式的对应关系之间的认识,并对鲍马序列进行重新审视。在海底扇研究过程中,鲍马序列已经不能充分反映浊流沉积的全过程。鲍马序列所反应的沉积模式其实是由碎屑流、浊流、底流等多种形式流体组合和改造后的结果,海底扇沉积模式不能笼统归结为浊流沉积作用的结果。在完善重力流、底流等沉积作用的同时,建立一个与沉积作用相互联系的深海沉积系统,以对深海研究提供更好地指导和预测。 相似文献
8.
基于双强度折减策略的边坡稳定性分析方法探讨 总被引：3，自引：0，他引：3
基于能耗分析理论,假定黏聚力和内摩擦角按不同折减系数进行强度折减,采用不同边坡综合安全系数定义方式,根据虚功原理推导了各自综合安全系数的目标函数表达式。采用序列二次规划法和内点法编制了非线性规划迭代程序,对综合安全系数目标函数进行优化求解。依据算例结果探讨了双强度折减技术评价方法对边坡安全系数计算的影响规律。研究结果表明,边坡稳定性影响因素众多,抗剪强度参数c与φ的折减系数之间不存在惟一确定的函数关系,现有预先假定黏聚力和内摩擦角按某一比例进行强度折减的处理方法尚难以完全合理解释;通过强度折减的最短路径来定义边坡综合安全系数的方法物理意义更为明确。研究结论可为工程技术人员加深采用强度折减技术进行边坡稳定性分析的认识提供有益参考。 相似文献
9.
In September 1984, the freshwater input to the Eastmain River (James Bay, Canada) was increased by a factor of 50 over a 6-d period during a controlled reservoir discharge. Changes to the current, salinity and turbidity regimes were monitored during the peak runoff. Estuarine salinity values fell rapidly with increasing mean flow, as did the amplitude of the semi-diurnal tidal currents. A large increase in bottom shear stress dispersed the settled suspension layer into the water column, raising concentrations of suspended matter in the estuary by a factor of 4 in 3 d. The peak values exceeded 150 mg I?1. This led to erosion of the river silt deposits, with the export of an estimated 6 × 104 metric tons of sediments. After the reduction of discharge, current values returned to their normal range within a day, whereas upstream salinity intrusion occurred at a slower rate. Horizontal diffusivities of about 100 m2 s?1 were required to match the observed lag. 相似文献
10.
海底浊流在坡道转换处的流动及沉积的数值模拟 总被引：2，自引：1，他引：1
根据一经多项试验数据验证的基于三维不可压缩流体Navier-Stokes方程和湍流 k-ε 模型的重力流数值计算的数学模型,模拟并分析了单粒径沉积物的海底浊流沿不同斜坡流至近似平坦坡的流动及沉积特征。模拟结果显示了有关海底浊流的一些重要特征:连续入流的浊流在斜坡上的流速随着斜坡的增大而增大,同时浊流厚度由于对环境水体的夹带而渐渐增厚,坡度越大,增厚越快;流至近水平坡时,流速均有明显的降低,但大斜坡入流依然保持相对较高的流速。在沉积方面,初步的模拟结果显示对给定的沉积物来说存在一相对应的临界坡度:当坡度小时,坡上沉积多,坡下少,这样整体的坡度有逐渐增大之势;当坡度大时,坡上沉积少或为侵蚀,而坡下沉积相对较多,坡度有整体减小之势。了解了不同坡度转换的浊流沉积的上述特点,对于我们根据实测的浊流沉积的剖面特征推测其形成的环境,进而推测相关油气储层的分布状况会有一定的参考作用。 相似文献
11.
MATRIX OF TURBIDITES: EXPERIMENTAL APPROACH 总被引：2，自引：0，他引：2
PH. H. KUENEN 《Sedimentology》1966,7(4):267-297
The matrix (< 40 μ) of turbidites forms a possible clue to the density of turbidity currents and the origin of the graywacke matrix. Experiments in a circular flume provide a mechanism to study the relation between composition of suspensions at various speeds and their deposits. There is a close analogy to the lower part of turbidity currents. The lutum content of samples with median diameters greater than 400 or 500 μ is found to correspond to the suspended load of the pore water. The higher value for finer deposits can be recalculated to suspension concentration by use of the “sedimentation factor”. Hence, each turbidite carries, as it were, a sample of its depositing current. The lutum content depends not on the ratio of sand to lutum in the current, as tacitly assumed by many authors, but mainly on the ratio lutum to water, although also influenced by velocity. The average lutum density of coarser recent deep-sea sands is 1-2%. This indicates turbidity currents with 5-10% lutum by weight (density 1.03–1.07). The sand must be added to ascertain the current density. In first approximation turbidity currents tend to have densities at their nose of 1.1–1.2, but higher and much lower values also occur. The maximum original lutum percentage of coarse turbidites is below 10%. Higher values are very scarce and are due to post-depositional mixing, or we are dealing with slides. However, in fine-grained turbidites there is more matrix up to 20% for a median of 100 p. Hence, coarse graded marine graywackes with 20 or more per cent matrix are presumably weakly metamorphic turbidites, that originally held the same modest amount of lutum as recent turbidites of the same grain size. The Trask sorting of the experimental deposits is very good, like the average of natural turbidites. Most cumulative curves of turbidite grain-size analyses on arithmetic probability paper show a characteristic bend in fine sand or silt sizes. 相似文献
12.
Alexandre Normandeau Daniel Bourgault Urs Neumeier Patrick Lajeunesse Guillaume St-Onge Louis Gostiaux Cédric Chavanne 《Sedimentology》2020,67(2):1045-1068
The monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0·5 m sec−1 and 2·0 m sec−1, respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0·5 m sec−1 occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km h−1 and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism. 相似文献
13.
14.
IAN STATHAM 《Sedimentology》1974,21(1):149-162
Rotating drum experiments on the repose angles of mixtures of glass spheres have shown that φr (angle of shear) is strongly influenced by the proportions of the mixture. It was found that φr reached a peak value for the minimum porosity mixture; where the pore spaces between the large particles were just filled with small material; which was attributed to increased dilatation on the shear plane during avalanching. The geomorphic significance of these observations, in terms of slope development, is discussed. Secondly, the results of the experiments showed that, although more constant than φi (limiting angle of repose), φr was subject to some variation. Thus φr, as measured in a rotating drum, is not a true constant and can not be exactly analogous to φ'cv (angle of internal sliding friction at constant volume) as measured in a shearbox test—as has been previously suggested. It is tentatively suggested that at least some of the variability in φr is attributable to the magnitude of the immediately preceding value of φi, in that an unusually high value of φi, favours a lower value of φr due to the greater amount of kinetic energy released on failure. 相似文献
15.
A. I. REES 《Sedimentology》1966,6(3):209-240
Four flume experiments have been performed in which the behaviour was investigated of a bed of fine silt under running water with and without an excess of material introduced into suspension. In the presence of excess material, bed features were formed in the tangential stress range 0.1 < θ < 0.4, and the bed was found to remain plane in the ranges 0 < θF < 0.1 and 0.4 < θF < 1.3. In the absence of excess material the stable condition over the entire range appeared to be that of the plane bed. Dual values of the threshold stress for grain movement were postulated to explain these results, and a relationship has been found between the magnitude of the tangential stress and the upstream slope of the features. 相似文献
16.
Deposits and transport processes resulting from the resedimentation of cold, unconsolidated ignimbrite into water were simulated by flume experiments. The ignimbrite sample used was poorly sorted (σ = 2·4–3), fine ash‐rich (< 63 μm, 17–30 wt%) and included both dense lithic clasts (> 2000 kg m?3) and pumice (500 to ca 1300 kg m?3). As a result of the binding forces of the ash matrix, the experiments involved resedimentation from a steep front onto the floor (with or without an initial ramp) of the water‐filled tank under both still and wave‐generated conditions. Larger discrete collapse events were induced by oversteepening the sample front and by undercutting from wave action. The mass of the collapse and proportion of pore–space water strongly influenced the style of resedimentation and the deposits. Initial collapse events were from the top of the steep front and fell onto the floor. The largest, densest clasts were deposited as a lithic lag in a proximal sediment wedge or rolled down to a break‐in‐slope. Fine ash was transported in dilute turbidity currents, and coarse unsaturated pumice clasts floated off. Moderate collapse events generated high‐density turbidity currents, trapping pumice in the flow, causing them to saturate. These low‐density pumice clasts were easily remobilized by wave activity and passing currents and accumulated on the gentle slope at the bottom of the resedimented deposit. Large collapse events slumped, producing poorly sorted mounds similar in texture to the original starting material. As the matrix of the ignimbrite sample became saturated with water, moderate and large collapse events generated debrisflows and slurries that deposited massive, poorly sorted deposits. Furthermore, once more gentle slopes were established between the sample and deposit, small cascading grainflows deposited lithic clasts on the upper slopes and levees of pumice at the terminus of low‐relief, ash channels. The experiments show that, excluding large collapse events and debrisflows, resedimenting ignimbrite in water is effective at segregating low‐density pumice clasts from dense lithic clasts and fine ash. Experiments using fine‐ash poor ignimbrite and well‐sorted quartz sand for comparison formed an inherently unstable initial steep front that immediately collapsed by continuous grain avalanches. The grainflow deposits had textures similar to the fines‐poor starting material. 相似文献
17.
Processes of late Quaternary turbidity current flow and deposition on the Var deep-sea fan, north-west Mediterranean Sea 总被引：3，自引：1，他引：3
High-resolution seismic boomer profiles, with a vertical resolution of less than 1 m, together with piston cores and previous side-scan sonar data, are used to describe late Quaternary sedimentation on the Var deep-sea fan. Chronological control is provided by foram biostratigraphy and radiocarbon dating in cores, and is extended over the fan by seismic correlation. Regional erosional events correspond to the oxygen isotopic stage 2 and 6 glacial maxima. Cores and seismic data define a widespread surface sand layer that is correlated with prodelta failure in 1979 and subsequent submarine cable breaks. Numerical modelling constrains the character of this 1979 turbidity current. It originated from a relatively small slide on the upper prodelta that put sufficient material in suspension to form an accelerating turbidity current which eroded sand from the Var Canyon. The turbidity current was only 30 m thick on the Upper Valley, but experienced significant flow expansion in the Middle Valley to thicknesses of more than 120 m, where it spilled over the eastern Var Sedimentary Ridge at a velocity of about 2·5 m s?1. Other Holocene turbidity currents (with a recurrence interval of 1000 years) were somewhat muddier and thicker, but also deposited sand on the levees of the Middle Valley, and are inferred to have had a similar slide-related origin. Late Pleistocene turbidity currents deposited thick mud beds on the Var Sedimentary Ridge. The presence of sediment waves and the mean cross-flow slope inferred from levee asymmetry indicates that some of these flows were many hundreds of metres thick and flowed at velocities of about 0·35 m s?1. This contrast with Holocene turbidites suggests that a slide origin is unlikely. Estimated times for deposition of thick mud beds on the levees are many days to weeks. The Late Pleistocene flows may therefore result from hyperpycnal flow of glacial outwash in the Var River. The variation in the Late Pleistocene to Holocene turbidite sedimentation is controlled more by variations in sediment supply than by sea-level change. 相似文献
18.
使用自主研发的旋转水槽制作稳定循环流动的泥石流,通过实测泥石流阻力坡降和流动速度,探索泥石流阻力坡降的速率效应。在旋转水槽试验中,通过控制线速度v,制作出稳定循环运动的泥石流,其阻力坡降J等于槽底坡降tanθ,可以实测。试验再现了野外的泥石流运动特性,观察到“龙头”、颗粒弹跳、低阻力坡降、固液相分离、剪切速率效应等现象。试验结果表明,在一定的流动速度范围内（例如试验中1.88m·s-1≥v>0.47m·s-1）,泥石流的流动速度越高,阻力坡降越大;如果泥石流运动速率过低（例如试验中v<0.47m·s-1）,泥石流会出现固液两相分离现象,砾石（粒径>2mm）运动明显落后于液相运动。本项目的试验研究成果为泥石流制作和阻力坡降测量提供新途径,为泥石流阻力坡降的速率效应理论研究提供实验支持。 相似文献
19.
不同安全系数对应的边坡滑动面位置分析 总被引：3，自引：0，他引：3
为了探讨强度折减计算边坡稳定性过程中安全系数和滑动面之间的关系,针对一均质边坡算例,通过理论分析和FLAC3D强度折减法数值计算,分析当黏聚力 和内摩擦角 变化时,边坡安全系数和滑动面位置的响应情况,结果表明：(1) 边坡的安全系数受到黏聚力或内摩擦角中任一值的影响,而滑动面位置受到两者组成的函数 （ ）的影响;(2) 当 为定值时,改变抗剪强度参数 和 并不能引起滑动面位置的变化;当 增大时,边坡破坏模式由浅层破坏转变为深层破坏,滑动面越来越缓,其上缘逐渐远离坡顶,滑体的体积增大。 相似文献
20.
Various deep-water deposits developed in the Yinggehai Basin and Qiongdongnan Basin(Ying-Qiong Basin)in the northern South China Sea,making the two basins significantly hydrocarbon-producing areas and ideal for studying the genetic mechanism and sedimentary characteristics of deep-water clastic rocks.Using cores,image well logging,heavy mineral assemblages,and seismic data,we thoroughly studied the geometry,tectonic background,driving mechanism and source-to-sink process of deep-water deposits in the study area.The results showed that:(1)there were five genetic mechanisms of deep-water clastic rock,i.e.,slides,slumps,debris flows,turbidity currents,and bottom currents.(2)The sliding deposits were distributed from the delta front to the continental slope toe.The slumping deposits were mainly distributed at the continental slope toe or the basin’s central area,far from the delta front The turbidity deposits were widely developed in the deep-water area,but with huge differences in thickness.The bottom currents mainly reworked previous deposits far from the slope.(3)Slip and extension along the preexisting fault zone were the main structural factors that drive the axial channel formation at the slope foot.(4)The sand-rich gravity sediment flows in the Ying-Qiong basin were primarily caused by the direct supply of terrigenous debris into the marine environment over the slope break. 相似文献