共查询到19条相似文献，搜索用时 175 毫秒
1.
基于Laio土壤水分动态随机模型(Laio模型),利用2006-2010年5~9月土壤水分连续监测数据及日降水资料,分析科尔沁沙地固定沙丘和沙质草地生长季根系层土壤水分动态及其与降水格局的关系,研究点尺度土壤水分概率密度函数,并对Laio模型涉及的13个参数进行了敏感性分析。结果表明:① 研究区年降水的季节分配极不均匀,主要集中在4~10月的生长季,占全年降水量的93%;0~5 mm降水事件占全年降水事件的73%,但其降水量只占全年降水量的25%;降水间隔期以0~10 d为主,占全年无降水期的38%,其频数最高,占全年间隔期频数的87%。② 固定沙丘和沙质草地根系层厚度分别为0~100 cm和0~70 cm,沙质草地根系层土壤水分显著高于固定沙丘;两类沙地7月份的土壤水分都显著高于生长季其他月份。③ 两类沙地生长季根系层土壤水分均服从正态分布;通过Laio模型得到了两类沙地生长季根系层土壤水分概率密度函数p(s),其峰值及峰值出现的位置和峰的阔度均与观测结果很接近,说明Laio模型能对科尔沁沙地土壤水分概率密度函数进行较好的模拟。④ Laio模型涉及的13个参数中,对p(s)最为敏感的参数是降水频率λ、平均降水量α、最大蒸散量Emax、水分胁迫点s*和凋萎系数sw,主要影响p(s)曲线的峰值。 相似文献
2.
异重流的形成是影响活性污泥系统二沉池水力学性能的主要因素,形成异重流的主要原因包括沉淀池内不均匀的污泥分布和温度分布。采用多相流欧拉模型、标准k-ε湍流模型和Boussinesq假设研究了二沉池中活性污泥和表面散热引起的异重流。表面散热的计算采用经验方程,数值计算结果同实验结果作了比较,结果表明模型能较好地预测辐流式二沉池的流场、温度场和污泥分布。研究还表明,Frp2Gr/Re2表示了不均匀温度分布引起的浮升力与不均匀污泥分布引起的作用力的比值,用它可以判断两种作用力在异重流的形成当中的相对大小。 相似文献
3.
为研究河型转化过程机理,建立了考虑弯道二次流影响与边岸崩塌过程的平面二维河流数学模型,包括水流模型、泥沙模型和边岸崩塌模型。通过在水流动量守恒方程中增加弥散应力项以考虑弯道二次流的影响,并采用室内水槽实验结果对水流模型进行了验证;利用上荆江沙市至石首天然长河段的水沙过程和河道演变资料,对泥沙模型进行了验证;结果表明本模型数值计算量合适,有较好的适应范围。模型中提出了边岸崩塌过程的模拟技术,相对于传统平面二维水沙模型而言,可以更好地模拟天然河道的横向摆动以及洲滩消长过程。 相似文献
4.
为研究淤泥质河口的水沙运动规律,建立了用于模拟淤泥质河口水沙运动的二维数学模型。该模型采用基于无结构三角网格下的有限体积法对方程组进行离散,结合Roe-MUSCL方法及时间方向的预测-校正格式,使模型在时空方向具有二阶计算精度。模型中分别采用不同方法计算粘性和非粘性泥沙的输移源项,并引入粘性泥沙的起动流速和冲刷率计算公式。采用已有的概化水槽试验数据对模型进行了初步验证。然后模拟了1995年10月小潮及大潮期间海河口的潮流运动与泥沙输移过程,计算得到的潮位、潮流速及含沙量过程与实测过程符合较好,结果表明模型能够用来模拟淤泥质河口粘性和非粘性泥沙的不平衡输移过程。同时还比较了泥沙输移源项的不同处理方式对计算结果的影响,计算表明在淤泥质河口水沙运动数学模型中必须同时考虑粘性和非粘性泥沙的输移。 相似文献
5.
河北平原地下水锶同位素形成机理 总被引：5，自引：0，他引：5
为了研究河北平原地下水锶同位素的来源与形成机理, 对所采水样进行了分析.研究了87Sr/86Sr比值“时间积累效应”: 随着地下水年龄和埋深的增大而增大; 与地下水中过剩4Heexc呈正相关关系, 与δ18O和δD呈负相关关系.探讨了Sr2+与87Sr/86Sr比值的关系, 将地下水分为3类: (1) 中等Sr2+含量与高87Sr/86Sr比值水(Ⅰ类水); (2) 低Sr2+含量与高87Sr/86Sr比值水(Ⅱ类水); (3) 高Sr2+含量与低87Sr/86Sr比值水(Ⅲ类水), 即热水.通过综合分析认为: (1) 河北平原第四系地下水中的放射成因Sr是由富含Na和Rb的硅酸盐矿物风化作用提供的, 主要矿物为斜长石; (2) 黄骅港热水中的放射成因Sr是由碳酸盐溶解形成的, 87Sr/86Sr比值低, Sr/Na比值大; (3) 补给区地下水是由流经火成岩和变质岩区地下水的侧向补给的, 87Sr/86Sr比值中等.第三系地下水放射成因Sr的来源及形成机理尚须进一步研究. 相似文献
6.
为探寻全吸力范围内土壤水分特征曲线的可靠测定方法,采用沙箱排水法、Hyprop仪蒸发法、压力膜仪排水法和露点水势仪蒸发法分吸力段测定盐亭紫色土耕地表层2~7 cm和亚表层7~12 cm土壤的水分特征曲线,对比测定结果的方法间差异,并分析其原因。结果表明:对于表层和亚表层土壤,低吸力段(h >-100 cm)水分特征曲线的沙箱法和Hyprop仪法的均方根误差ERMS (θ)均较小,在0.026~0.082 cm3/cm3范围内,确定系数R2均大于0.962,说明这两种方法测定结果之间差异不大。高吸力段(h <-330 cm)的压力膜仪法与露点仪法测定结果之间的差异较大,ERMS (θ)为0.062~0.097 cm3/cm3,R2较低,为0.775~0.952。因此,全吸力范围内水分特征曲线测定方法的选择与组合应考虑土壤孔径分布特征和研究目的。 相似文献
7.
水文响应单元(Hydrological Response Units,HRU)是SWAT模型模拟的基本单元,传统方法划分的水文响应单元在空间分布上不连续且难以确定其明确的空间位置,不能反映HRU间的相互作用和进行精确空间分析。利用GIS工具对土地利用和土壤类型数据进行概化处理,提出了HRU空间离散化的方法,实现了水文响应单元在空间上的准确定位。在此基础上,针对SWAT模型中同一子流域所有HRU采取相同延迟的弱点进行改进,并选择太湖地区西苕溪流域对改进的SWAT模型进行水文模拟验证。改进后,校正期港口站Nash效率系数ENS(Nash-Sutcliffe Efficiency)从0.64提高到0.67,验证期ENS系数从0.70提高到0.76。研究表明:修正后的SWAT模型更能反映流域的水文特征,可以达到非常好的效果,考虑到HRU距离因素的径流延迟更为准确地刻画径流过程。实现HRU空间离散化将为模型改进和更小尺度的空间分析提供数据基础。 相似文献
8.
西准噶尔地区晚古生代岩浆活动剧烈,地壳的垂向和侧向增生显著,地壳生长和演化存在多阶段性。本文重点通过Sr-Nd-Pb同位素填图研究,发现西准噶尔地区εNd(t)值为2.29~8.75,(87Sr/86Sr)i值为0.697 397~0.708 336,(206Pb/204Pb)i值为17.4975~19.0352,整体表现为高正εNd(t)、低(87Sr/86Sr)i和年轻的地壳模式年龄特征,源区以古生代新生地壳为主,地幔贡献值整体大于50%,深部地壳几乎不存在古老的结晶基底,可以与区域构造地质、地球物理资料作较好匹配。区域晚古生代主要经历3个时期的造山阶段,分别对应造山带演化的第一阶段(中晚石炭世,岛弧为代表的侧向生长为主)、第二阶段早期(晚石炭世—早二叠世,后碰撞阶段的垂向生长为主)和第二阶段晚期(早二叠世—早三叠世,壳幔混源背景下的垂向生长),区域造山作用结束于早三叠世。 相似文献
9.
澜沧江-湄公河是中国与东南亚地区重要的跨境河流，其泥沙问题涉及流域内各国的生态、农业、渔业、航运、海岸侵蚀等诸多领域，近年来下游输沙量持续减少，受到广泛关注。综述了澜沧江-湄公河泥沙研究的争论焦点，包括全流域主产沙区的位置，湄公河干流水文站点间实测输沙量异常变化关系以及人类活动对泥沙过程的影响。系统梳理了现有研究中存在的主要问题，包括下湄公河泥沙观测数据质量不高、全流域总输沙量1.60亿t/a的数据基础存疑、水文模拟的时空分辨率不足、未考虑土地利用变化的影响和悬移质之外的泥沙研究不充分等，在此基础上提出了进一步研究建议。 相似文献
10.
澜沧江-湄公河是中国与东南亚地区重要的跨境河流，其泥沙问题涉及流域内各国的生态、农业、渔业、航运、海岸侵蚀等诸多领域，近年来下游输沙量持续减少，受到广泛关注。综述了澜沧江-湄公河泥沙研究的争论焦点，包括全流域主产沙区的位置，湄公河干流水文站点间实测输沙量异常变化关系以及人类活动对泥沙过程的影响。系统梳理了现有研究中存在的主要问题，包括下湄公河泥沙观测数据质量不高、全流域总输沙量1.60亿t/a的数据基础存疑、水文模拟的时空分辨率不足、未考虑土地利用变化的影响和悬移质之外的泥沙研究不充分等，在此基础上提出了进一步研究建议。 相似文献
11.
建立了基于库区不规则断面的一维非恒定异重流数学模型,并采用明流与异重流水沙输移模型交替运算的两步模式,即用潜入条件动态判别异重流计算的上游边界位置,将潜入点上游的明流浑水段与下游异重流段计算连接起来。水流运动、泥沙输移与河床变形过程完全耦合,采用TVD(Total Variation Diminishing)形式的MUSCL-Hancock格式进行数值求解。将该模型应用于恒定流量与释放定量悬沙两种条件下的异重流水槽实验模拟,比较了有无水面梯度项对模拟精度的影响,计算结果表明该模型能较为准确地预测异重流的厚度、含沙量分布及传播过程。 相似文献
12.
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
13.
The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post‐depositional consolidation and soft‐sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain‐support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non‐cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle‐support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain‐to‐grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle‐support mechanism depends upon flow conditions, particle concentration, grain‐size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse‐grain tail (or dense‐grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large‐scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensu 4 ), in which fluid turbulence is the main particle‐support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge‐like flows and quasi‐steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge‐like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi‐steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening‐up units capped by fining‐up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near‐steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co‐exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits. 相似文献
14.
OCTAVIO E. SEQUEIROS BENOIT SPINEWINE RICK T. BEAUBOUEF TAO SUN MARCELO H. GARCIA GARY PARKER 《Sedimentology》2010,57(6):1463-1490
Turbidity currents in the ocean are driven by suspended sediment. Yet results from surveys of the modern sea floor and turbidite outcrops indicate that they are capable of transporting as bedload and depositing particles as coarse as cobble sizes. While bedload cannot drive turbidity currents, it can strongly influence the nature of the deposits they emplace. This paper reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud which is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream‐migrating antidunes and downstream‐migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open‐channel flows and, in fact, is fitted well by an existing relation determined for open‐channel flows. In the case of dunes and downstream‐migrating antidunes, for which flow separation on the lee sides was observed, form drag falls in a range that is similar to that due to dunes in sand‐bed rivers. This form drag can be removed from the total bed shear stress using an existing relation developed for rivers. Once this form drag is subtracted, the bedload data for these cases collapse to follow the same relation as for plane beds and upstream‐migrating antidunes, for which no flow separation was observed. A relation for flow resistance developed for open‐channel flows agrees well with the data when adapted to density underflows. Comparison of the data with a regime diagram for field‐scale sand‐bed rivers at bankfull flow and field‐scale measurements of turbidity currents at Monterey Submarine Canyon, together with Shields number and densimetric Froude number similarity analyses, provide strong evidence that the experimental relations apply at field scale as well. 相似文献
15.
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
16.
Suspended-load fallout rate as an independent variable in the analysis of current structures 总被引：4，自引：0，他引：4
DONALD R. LOWE 《Sedimentology》1988,35(5):765-776
The dynamic interpretation of most current-structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size. The data underlying existing stability-field diagrams were collected largely from the study of flows carrying coarse-grained sediment entrained through particle-by-particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed-load layers derive sediment from the collapsing suspended-sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity-grain-size relationships from those beneath flows deriving sediment from bed erosion alone. Useful stability-field diagrams for turbidity currents must include suspended-load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability-field diagram of this type indicates that Bouma Tabc sequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed-load layers are fed from above rather than below. These include shallow-shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls directly into moving water. 相似文献
17.
Continental slope sediment failures around the epicentre of the 1929 'Grand Banks' earthquake have been imaged with the SAR (Système Acoustique Remorqué) high-resolution, deep-towed sidescan sonar and sub-bottom profiler. The data are augmented by seismic reflection profiles, cores and observations from submersibles. Failure occurs only in water depths greater than about 650 m. Rotational, retrogressive slumps, on a variety of scales, appear to have been initiated on local steep areas of seabed above shallow (5–25 m) regional shear planes covering a large area of the failure zone. The slumps pass downslope into debris flows, which include blocky lemniscate bodies and intervening channels. Clear evidence of current erosion is found only in steep-sided valleys: we infer that debris flows passed through hydraulic jumps on these steep slopes and were transformed into turbidity currents which then evolved ignitively. Delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps provide a mechanism to produce a turbidity current with sustained flow over many hours. 相似文献
18.
CHARLOTTE GLADSTONE DAVID PRITCHARD† Associate Editor – Jess Trofimovs 《Sedimentology》2010,57(1):53-84
Turbidity currents are turbulent, sediment‐laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep‐water abyssal plains. Because of the natural stratification of the oceans and/or fresh water river inputs to the source area, the interstitial fluid within which the particles are suspended will often be less dense than the deep‐water ambient fluid. Consequently, a turbidity current may initially be denser than the ambient sea water and propagate as a ground‐hugging flow, but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water. When this occurs, all or part of the turbidity current lofts to form a buoyant sediment‐laden cloud from which further deposition occurs. Deposition from such lofting turbidity currents, containing a mixture of fine and coarse sediment suspended in light interstitial fluid, is explored through analogue laboratory experiments complemented by theoretical analysis using a ‘box and cloud’ model. Particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit. A range of beds can be deposited by bimodal lofting turbidity currents. Lofting may encourage the formation of tabular beds with a rapid pinch‐out rather than the gradually tapering beds more typical of waning turbidity currents. Lofting may also decouple the fates of the finer and coarser sediment: depending on the initial flow composition, the coarse fraction can be deposited prior to or during buoyancy reversal, while the fine fraction can be swept upwards and away by the lofting cloud. An important feature of the results is the non‐uniqueness of the deposit architecture: different initial current compositions can generate deposits with very similar bed profiles and grading characteristics, highlighting the difficulty of reconstructing the nature of the parent flow from field data. It is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep‐water massive sands, thinly bedded turbidite sequences and linked debrites, depending on the parent flow and its subsequent development. For example, a lofting flow may lead to a well sorted, largely ungraded or weakly graded bed if the fines are transported away by the cloud. However, a poorly sorted, largely ungraded region may form if, during buoyancy reversal, high local concentrations and associated hindered settling effects develop at the base of the cloud. 相似文献
19.
Depositional turbidity currents have filled many intraslope minibasins with sediment creating targets for petroleum exploration. The dynamics of sustained turbidity currents and their depositional characteristics are investigated in a scaled physical model of a minibasin. Each turbidity current deposited a downstream thinning wedge of sediment near the inlet. Farther downstream the turbidity current was ponded by a barrier. The ponded part of the turbidity current was separated from the sediment‐free water above by a relatively sharp, horizontal settling interface indicating highly Froude‐subcritical flow. The very slow moving flow within the ponded zone created conditions for the passive rainout of suspended sediment onto the bed. In the lower part of the ponded zone, the concentration and mean grain‐size of the sediment in suspension tended to be relatively uniform in both the vertical and streamwise directions. As a result, the deposit emplaced in the ponded zone showed only a weak tendency toward downstream fining and was passively draped over the bed in such a way that irregularities in the inerodible bed were accurately reflected. The discharge of suspended sediment overflowing the downstream end of the minibasin was significantly less than the inflow discharge, resulting in basin sediment trapping efficiencies >95%. A simple model is developed to predict the trapping of sediment within the basin based on the relative magnitudes of the input discharge of turbid water and the detrainment discharge of water across the settling interface. This model shows a limiting case in which an intraslope basin captures 100% of the sediment from a ponded turbidity current, even through a succession of sustained flow events, until sediment deposition raises the settling interface above the downstream lip of the minibasin. This same process defines one of the mechanisms for minibasin filling in nature, and, when this mechanism is operative, the trap efficiency of sediment can be expected to be high until the minibasin is substantially filled with sediment. 相似文献