Flow dynamics and sedimentation in an active submarine channel: a process-product approach

海底通道是壮观的特征，可以在海底延伸数千公里，通常有几公里宽，深达数百米。它们是由密度流形成的；沙、泥和水的水下流动，比海水密度大，因此沿着海底流动。这些通道非常重要，因为它们是将沉积物运送到深海的主要运输途径，并形成了地球上最大的沉积层。这些矿床是天然气和石油储量的重要宿主，并掌握着过去气候变化和造山活动的关键信息。这种流动很难研究，通常是不频繁和高度破坏性的；它们对海底工程（如电缆和管道）构成重大危害，并经常破坏科学测量设备。因此，我们对这种水流的了解主要来自实验室实验，而对它们沉积的了解则来自对现在暴露在陆地上的古代样本的研究。因此，没有对这些自然河道中的水流进行详细的研究，也没有将流量测量与所产生的沉积物联系起来的研究。地球上几乎没有其他环境我们不知道流动过程是如何与沉积沉积物联系在一起的，而且这是地球上最大的沉积物！因此，迫切需要提高我们对活动海底通道内流动、形态和沉积之间相互作用的理解。然而，除了与监测这些密度驱动流相关的技术问题外，大多数海底通道实际上是在海平面比现在低得多的时候形成的；现在的水流通常比形成河道/沉积物的水流要小得多，这使得研究相互作用变得不可能。此外，需要创新的技术来测量这些通道内的详细流动模式。大约在6000年前，海平面接近现在的水平，密集的含盐流体（10-15米厚）从地中海开始流经博斯普鲁斯海峡（经过伊斯坦布尔）进入黑海，形成了一个几乎持续活跃的海底通道网络。这个星系及其沉积沉积物的第一张壮观图像直到2005年才获得。这为首次研究活跃海底通道中的流动和沉积提供了独特的机会，并利用这些知识来制定和测试预测模型。我们召集了一个由英国和国际科学家组成的世界领先小组来应对这一挑战。我们将使用Autosub3 NERC最新的最先进的自主潜艇（当然是黄色！）将我们的测量设备“飞行”到底部上方，与传统的船舶方法相比，我们可以以前所未有的细节绘制通道形态和三维（3D）流动。流量数据将与海床特性的测量、较小的形态特征（如河床）和地震数据联系起来，这些数据通过沉积物成像来揭示沉积物的内部结构。这些数据将用作海底通道流动和沉积的创新计算机模拟模型的输入条件。数值模拟和现场数据将结合起来，使我们能够：i)根据形成河道的水流来评估河道中的河床形态，从而首次从保存在较老岩石中的河床形态中重建过去的水流；Ii)建立弯曲流模型，以预测沉积物的沉积模式；iii)对水流和形态与长期沉积物沉积的关系有了新的认识。这些数据将彻底改变我们对海底环境中流动和沉积的理解，主要应用于：i)地质灾害分析，ii)海底工程设计标准，以及iii)沉积沉积类型和分布预测。

Submarine channels are spectacular features that can extend for thousands of kilometres across the seafloor, are often kilometres wide and up to hundreds of metres deep. They are formed by density currents; underwater flows of sand, mud and water that are denser than sea water and therefore flow along the seafloor. These channels are very important as they are the major transport pathway for moving sediments to the deep sea and form the largest sedimentary deposits on Earth. These deposits are significant hosts for gas and oil reserves and hold key information on past climate change and mountain building episodes. Such flows are difficult to study, typically being infrequent and highly destructive; they pose a major hazard to sea-floor engineering such as cables and pipelines, and have often destroyed scientific measurement equipment. Consequently, our knowledge of such flows comes mainly from laboratory experiments, and understanding of their deposits from studies of ancient examples now exposed on land. As a consequence there are no detailed studies of these flows in natural channels, and no studies that link flow measurements to the deposits that are produced. There is almost no other environment on Earth where we do not have any knowledge of how flow processes are linked to their sedimentary deposits, and this in the largest deposits on Earth! Consequently, there is an urgent need to improve our understanding of the interactions between flow, morphology and deposits within an active submarine channel. However, in addition to the technical problems associated with monitoring these density-driven flows, most submarine channels actually formed when sea-levels were much lower than today; present-day flows are typically much smaller than those that formed the channels/deposits, making study of interactions impossible. Furthermore, innovative techniques are required to measure detailed flow patterns within these channels. Around 6,000 years ago sea-level approached its present level, and dense salty fluid (10-15 m thick) from the Mediterranean started flowing through the Bosphorus Strait (past Istanbul) into the Black Sea, forming an almost constantly active sea-floor channel network. The first spectacular images of this system and its sedimentary deposits were only obtained in 2005. This provides a unique opportunity to study both flows and deposits in an active sea-floor channel for the very first time and to use this knowledge to formulate and test predictive models. We have assembled a world-leading group of UK and international scientists to tackle this challenge. We will use Autosub3 NERC's new state-of-the-art autonomous submarine (yellow, of course!) to 'fly' our measurement equipment just above the bottom, allowing us to map channel morphology and the three dimensional (3D) flows in unprecedented detail compared to what is possible from traditional ship-based methods. The flow data will be linked to measurements of sea-bed properties, smaller morphological features such as bedforms, and seismic data that images through sediment to reveal the internal structure of the deposits. These data will be used as input conditions for an innovative computer simulation model of flow and deposition in submarine channels. The numerical modelling and field-data will be combined to enable us to: i) assess bedforms in the channels with respect to the flows forming them, allowing reconstruction of past flows from preserved bedforms in older rocks for the very first time; ii) model bend flow to enable sediment patterns in the deposits to be predicted, and, iii) develop a new understanding of how flow and morphology is linked to long-term sediment deposition. These data will revolutionise our understanding of both flows and deposits in submarine environments, with key applications to: i) geohazard analysis, ii) design criteria for seafloor engineering, and, iii) prediction of sedimentary deposit types and distributions.