浊流和泥石流的异重流初期潜入点的实验研究

潜入点的水流泥沙条件是异重流的发生条件和持续条件,受到了国内外学者的广泛关注。初期潜入点Fr代表异重流的发生条件,而稳定潜入点Fr则代表异重流的持续条件。通过一系列的低浓度浊流和高浓度浊流及泥石流的异重流潜入点的实验研究,分析对比低浓度浊流和高浓度浊流及泥石流的异重流的关系,得到在均匀顺直水槽中的异重流初期的潜入点Fr规律。提出在一定的水槽宽度、泥沙浓度条件下,异重流的初期潜入点Fr与头部流速成正比。与其它文献不同研究条件的实验结果对比有很好的一致性,说明该研究结论有很好的可靠性。该研究结论的适用范围为初期潜入点,不适用于稳定潜入点和初期潜入点与稳定潜入点之间的过渡阶段。但高浓度浊流和泥石流的异重流初期潜入点Fr与其稳定潜入点Fr很接近,粘性泥石流的异重流更接近。     

小浪底水库“腾库迎洪”期异重流形成分析

小浪底水库异重流的形成与运行规律是关系黄河调水调沙方案拟定的重要指标。基于现场调研、理论探讨与预测分析结合等手段,明晰了2018年"腾库迎洪"期小浪底水库来水来沙过程,根据水库调度与库区淤积形态等边界条件,观测了异重流的形成与运行,采用基于流速分布的水库异重流潜入点预测公式进行了计算分析。结果表明:2018年7月库区产生的剧烈冲刷,在潜入点处发生大量淤积,水深大幅减小,引起潜入点下移;潜入点位置随入库水沙增大、坝前水位降低及库区淤积的推进向坝前移动,计算与实测结果反映了该变化过程。     

小浪底水库“腾库迎洪”期异重流形成分析

小浪底水库异重流的形成与运行规律是关系黄河调水调沙方案拟定的重要指标。基于现场调研、理论探讨与预测分析结合等手段,明晰了2018年"腾库迎洪"期小浪底水库来水来沙过程,根据水库调度与库区淤积形态等边界条件,观测了异重流的形成与运行,采用基于流速分布的水库异重流潜入点预测公式进行了计算分析。结果表明:2018年7月库区产生的剧烈冲刷,在潜入点处发生大量淤积,水深大幅减小,引起潜入点下移;潜入点位置随入库水沙增大、坝前水位降低及库区淤积的推进向坝前移动,计算与实测结果反映了该变化过程。     

Trapping of sustained turbidity currents by intraslope minibasins

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.     

Sedimentary processes in the Selvage sediment-wave field, NE Atlantic: new insights into the formation of sediment waves by turbidity currents

An integrated geophysical and sedimentological investigation of the Selvage sediment-wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise and display wavelengths of up to 1 km and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil-based dating of the sediments indicates a bulk sedimentation rate of 2·4 cm 1000 years^–1, and the waves are migrating upslope at a rate of 0·28 m 1000 years^–1. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south. Investigation of existing models for sediment-wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0·5–1·9, which is very close to the antidune existence limits. Depositional flow velocities range from <6 to 125 cm^–1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected, as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable with results obtained for other unconfined turbidity currents undergoing flow expansion.     

浊流沉积研究的新进展:鲍马序列、海底扇的重新审视

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。     

泥沙异重流与环境物质交换经验式对比

采用异重流层平均水沙耦合数学模型,模拟开闸式和恒定入流式泥沙异重流水槽实验,对比分析异重流与环境之间物质交换经验式的适应性和不确定性。考虑4个水卷吸经验式:e_w59、e_w86、e_w87和e_w01,5个泥沙侵蚀经验式:E_s77、E_s86、E_s87、E_s93和E_s04。数值研究表明:水卷吸对于水槽异重流影响较小,应用综合考虑底床摩擦和剪切不稳定的e_w经验式时模拟结果较好;开闸式异重流对床面侵蚀能力有限。对于恒定入流式异重流,应用E_s87和E_s93侵蚀经验式计算所得淤积厚度与实测值吻合较好,可能是率定时综合考虑了异重流实验数据。     

The influence of basin setting and turbidity current properties on the dimensions of submarine lobe elements

Submarine lobes have been identified within various deep-water settings, including the basin-floor, the base of slope and the continental slope. Their dimensions and geometries are postulated to be controlled by the topographic configuration of the seabed, sediment supply system and slope gradient. Ten experiments were conducted in a three-dimensional-flume to study the depositional characteristics of submarine lobes associated with: (i) different basin floor gradients (0 to 4°); (ii) different sediment concentrations of the parent turbidity current (11 to 19% vol); and (iii) varying discharge (25 to 40 m³ h⁻¹). Most runs produced lobate deposits that onlapped onto the lower slope. Deposit length was proportional to basin-floor angle and sediment volume concentration. A higher amount of bypass is observed in the proximal area as the basin-floor angles get steeper and sediment concentrations higher. Deposits of runs with lower discharge could be traced higher upslope while runs with higher discharge produced an area of low deposition behind the channel mouth, i.e. discharge controlled whether lobe deposits were attached or detached from their channel-levée systems. A particle-advection-length scale analysis suggests that this approach can be used as a first order estimation of lobe element length. However, the estimations strongly depend on the average grain size used for calculations (for example, silt is still actively transported after all sand has been deposited) and the method cannot be used to locate the main depocentre. Furthermore, attempted reconstructions of turbidity current velocities from natural systems suggest that the method is not appropriate for use in inversions from more complex composite bodies such as lobes.     

On the ignition of geostrophically rotating turbidity currents

Two models of a geostrophically rotating turbidity current are examined to compare predictions for ignition with the catastrophic state. Both models describe the current as a tube of sediment-laden water traversing along and down a uniform slope. The first (four-equation) model neglects the energy required to lift the sediment from the seabed into suspension. The second (five-equation) model rectifies this shortcoming by introducing a turbulent kinetic energy equation and coupling the bottom stress to turbulence in the plume. These models can be used to predict the ignition, path and sediment deposition of a geostrophically rotating turbidity current. The criteria for ignition in the four-equation model can be described by a surface in three-dimensional phase space (for a non-entraining current). This surface lies near the geostrophic equilibrium state. For a turbidity current occurring in the Greenland Sea, velocities above 0·053 m s^–1 or volumetric concentrations of sediment above 2·7 × 10^–5 lead to ignition. In general, if the tube is started pointing downslope, then ignition is more likely than if it is initially directed alongslope. However, there exists a set of initial conditions in which the current ignites if started along or downslope, but deposits if started at an intermediate angle. The five-equation model requires a larger initial velocity (greater than 1·6 m s^–1) to ignite than does the four-equation model. Ignition is determined qualitatively by the geostrophic state and the initial normal Froude number. Solutions show a tendency to travel further alongslope during ignition, reflecting the restriction that the energy budget places on the sediment load. A qualitative difference to phase space in the five-equation model is the existence of a region in which the tube has insufficient energy to support the sediment. Turbulence dies rapidly in this region, and so the sediment is deposited almost immediately.     

浊流沉积研究综述和展望

浊流理论的建立具有划时代的意义。浊流沉积的研究已经进行了半个多世纪,从理论到实践都取得了巨大的进展。本文首先讨论了浊流及其相关的几个概念,同时概述了浊流沉积的国内外研究历史和进展,重点介绍了浊积岩的识别标志及其沉积序列,指出下一步研究的重点应放在陆相湖泊浊流沉积及其含矿性上。