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

首次对到达深海的强大浊流进行高分辨率直接测量

• 批准号: NE/L009358/2
• 金额: $ 3.28万
• 依托单位: NATIONAL OCEANOGRAPHY CENTRE
• 依托单位国家: 英国
• 项目类别: Training Grant
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FL009358%2F2
• 关键词: First; resolution; direct; measurements; powerful

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.

海底浊流可以说是在我们的星球上移动沉积物的体积最重要的过程。它们形成了地球上最大的沉积物堆积（海底扇），单次水流就可以输送世界上所有河流年流量的十倍。然而，浊流最显著的特征是，从这些水流中直接测量的数据很少，因为它们在实际中很难监测。这与其他主要的沉积物输运过程形成鲜明对比，例如我们有数千次直接测量的河流。强大的长期径流浊流特别难以监测，但正是这些水流形成了海底扇。这些流量非常重要，因为它们破坏了承载全球95%以上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。到达大陆坡以外的浊流速度以前只在五个地点测量过，主要是从只记录平均前缘速度的电缆断裂处测量的。它们的沉积物浓度从未被直接测量过。这位博士生将分析一个了不起的数据集，其中包括第一个同步的速度和浓度剖面的浊流以外的大陆坡，收集在成本超过100万美元的案例合作伙伴雪佛龙和同事在刚果峡谷（从2009年至2013年）。这是首次测量大陆坡以外浊流的高时间分辨率（>1/min）速度和浓度同步剖面。它们也是迄今为止仪器测量到的最快的浊流（2.5米/秒）。这些数据是为一条需要穿越刚果峡谷的主要石油和天然气管道收集的。这是一个具有挑战性的项目，因为以前的电缆断裂表明峡谷经常被强大的水流冲刷。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，该仪器测量速度和声学后向散射。后向散射部分取决于颗粒大小，但也记录了沉积物浓度的变化。最初的结果令人惊讶，原因有二。首先，水流的持续时间长达数天，平均速度约为1米/秒。有趣的是，据观察，较大的流量总是有一个类似的持续时间约6天。这似乎表明在峡谷的前150 km建立了平衡流结构。已经提出了几个假设来解释这种行为，然而，没有一个被证实。第二，测得的湍流强度随着流速的增加而降低，这种反直觉的关系表明湍流的阻尼由悬浮泥沙浓度升高。虽然它已被推测以前的湍流阻尼沉积物可能是根本的重要性，这种阻尼以前从未被记录在现场的全尺寸流的直接观察。因此，湍流阻尼与泥沙浓度的关系尚需在真实的海底水流中得到验证，需要建立数值模型来检验平衡流态产生多日流的假设，并探讨湍流阻尼对海底水流的影响。这种数值模型将受益于刚果峡谷的良好水深测量（来自雪佛龙公司和法国海洋所过去的出版物）。在本提案中，我们将使用最先进的全三维数值模型，该模型由项目合作伙伴挪威复杂流设计AS（CFD）在过去八年中开发。该模型是独特的，因为它的三维方法和它的能力，将泥沙浓度的影响，其湍流模型。