First high resolution direct measurements for powerful turbidity currents that reach the deep ocean

海底浊流可以说是在地球上移动沉积物的体积上最重要的过程。它们形成了地球上最大的沉积物堆积(海底扇)，单次流动就可以从世界上所有的河流输送十倍于年流量的流量。然而，浑浊流最显著的特征是从这些流中获得的直接测量非常少，因为它们的实际监测是出了名的困难。这与其他主要的泥沙输送过程形成了鲜明的对比，例如我们有数千次直接测量的河流。强大的长时间流出的浑浊流特别难以监测，然而正是这些水流建造了海底扇子。这种流动之所以重要，是因为它们打破了承载全球95%数据流量的海底电缆，包括支撑日常生活的互联网和金融市场。此前只在五个地点测量了到达大陆斜坡以外的浑浊流的速度，主要是从电缆断裂处测量的，这些电缆只记录了平均前沿速度。它们的泥沙浓度从未被直接测量过。这位博士生将分析一个引人注目的数据集，该数据集包含大陆坡外浊流的第一个同步速度和浓度剖面，由Case合作伙伴雪佛龙及其在刚果峡谷的同事(从2009年至2013年)花费100万英镑收集。这是首次测量到大陆斜坡以外的浊流具有高时间分辨率(&gt；1/min)的速度和浓度的同步剖面。它们也是迄今仪器测得的最快的浑浊流(2.5m/S)。这些数据是为一条主要的石油和天然气管道收集的，该管道将需要穿越刚果峡谷。这是一个具有挑战性的项目，因为之前的电缆中断表明，峡谷经常被强大的水流席卷。这些数据来自具有向下指向的声学多普勒速度剖面仪(ADCP)的系泊设备，这些ADCP测量速度和声波后向散射。后向散射部分依赖于颗粒大小，但也记录了泥沙浓度的变化。最初的结果令人惊讶，原因有两个。首先，气流的持续时间令人惊讶，平均速度为~1m/S。有趣的是，观察到较大的气流总是有类似的持续时间，约为6天。这似乎表明在峡谷的前150公里上建立了一种平衡的流动结构。人们提出了几个假说来解释这一行为，但这些假说都没有得到证实。其次，测量到的湍流强度随着流速的增加而减小，这一与直觉相反的关系表明，悬沙浓度升高对湍流有抑制作用。尽管以前有人推测，泥沙对湍流的抑制可能具有根本的重要性，但这种抑制以前从未在现场全尺度流动的直接观测中被记录下来。因此，湍流阻尼力与含沙量之间的关系在实际的海底水流中还有待验证。需要一个数值模型来检验平衡流动构型产生多日流动的假设，并探索湍流阻尼力对海底流动的影响。这样的数值模型将受益于刚果峡谷的良好水深测量(来自雪佛龙和IFREMER过去的出版物)。在这份提案中，我们将使用挪威项目合作伙伴Complex Flow Design AS(CFD)在过去八年中开发的最先进的全三维数值模型。该模型是独一无二的，因为它的三维方法和它的能力，以考虑泥沙浓度效应到其湍流模型。

Submarine turbidity currents are arguably the volumetrically most important process for moving sediment across our planet. They form the largest sediment accumulations (submarine fans) on earth, and single flows can transport ten times the annual flux from all of the world's rivers. However, the most remarkable feature of turbidity currents is how few direct measurements are available from these flows, as they are notoriously difficult to monitor in action. This is a stark contrast to other major sediment transport processes, such as rivers for which we have many thousands of direct measurements.Powerful long run-out turbidity currents are especially difficult to monitor, yet it is these flows that build submarine fans. Such flows are important because they break sea-floor cables that carry > 95% of global data traffic, including internet and financial markets that underpin daily lives. The velocity of turbidity currents that reach beyond the continental slope had previously been measured in just five locations, primarily from cable breaks that only record averaged front velocities. Their sediment concentration had never been measured directly. This globally important sediment transport process is therefore poorly understood, and laboratory or numerical models for such flows are poorly validated.This PhD student will analyse a remarkable dataset comprising the first synchronous velocity and concentration profiles for turbidity currents beyond the continental slope, collected at a cost of > $1M by CASE partner Chevron and co-workers in the Congo Canyon (from 2009-2013). This is the first time that high temporal resolution (>1/min) synchronous profiles of both velocity and concentration have been measured for turbidity currents beyond the continental slope. They are also the fastest (2.5 m/s) turbidity currents yet measured by instruments. The data were collected for a major oil and gas pipeline that will need to cross the Congo Canyon. This is a challenging project as previous cable breaks show the canyon is regularly swept by powerful flows. The data comes from moorings with downward pointing Acoustic Doppler Velocity Profilers (ADCPs) that measure velocity and acoustic backscatter. Backscatter is partly dependent on grain size, but also records changes in sediment concentration. Initial results were surprising for two reasons. First, flows had surprising durations of several days, with average speeds of ~1 m/s. Interestingly, it was observed that the larger flows always had a similar duration of ~6 days. This seems to indicate the establishment of an equilibrium flow configuration over the first 150 km of the canyon. Several hypothesis have been put forward to explain this behaviour, however, none of them have yet been validated. Second, the measured turbulence intensity decreased as flow speeds increased, this counter-intuitive relation suggests damping of turbulence by elevated suspended sediment concentrations. Although it has been speculated previously that turbulence damping by sediment may be of fundamental importance, such damping has never previously been documented in direct observations from full scale flows in the field. Thus, the relation between turbulence damping and sediment concentration remains to be validated in real submarine flows.A numerical model is needed to test hypotheses that equilibrium flow configurations produce multi-day flows, and to explore how turbulence dampening may affect submarine flows. Such numerical model will benefit from the well-mapped bathymetry of the Congo canyon (from Chevron and past publications by IFREMER). In this proposal we will use a state-ofthe-art fully three-dimensional numerical model that has been developed over the last eight years by project partner Complex Flow Design AS (CFD), Norway. This model is unique due to its 3D approach and its capability to introducesediment concentration effects into its turbulence model.