How was a thousand kilometre cable-breaking submarine flow triggered by an exceptional Congo River flood?

这项提议旨在了解刚果河沿岸50年来最大的洪水是如何在2020年1月14日引发1250公里长的海底沉积物雪崩(浑浊流)的。这股海底水流破坏了支撑西非数据通信的两条海底电信电缆，导致从尼日利亚到南非的互联网速度放缓。这些海底电缆在过去20年中从未断过。这股水流还导致一系列海洋系泊设施浮出水面，这些系泊设施由国家海洋研究中心的一个项目(NE/R001952)沿着刚果海底峡谷布置。缆索断裂和水面系泊表明，这股惊人的水流沿着峡谷轴线测量，持续了1200多公里。此外，水流不断自我加速，达到8米/S的前沿速度，距离刚果河入海口源头约1,150至1,250公里。这是迄今监测到的最长的超限浑浊流，也是唯一监测到的持续自加速超过1000公里的水流。重要的是要了解如何触发如此强大和超长的浑浊洋流，特别是对战略海底电缆的危害，包括计划在2020-21年间在西非海域铺设的电缆路线。1月14日至16日的海底水流与地震无关，它发生在波高较低的时期。然而，这与2019年12月沿刚果河观测到的8万M3s-1极端洪水不谋而合。因此，还必须确定未来刚果盆地的气候和水文变化将如何影响海底流动的频率。在这里，我们通过对刚果河河口进行详细调查，试图了解这场异常的河流洪水是如何引发1000公里长的海底水流的。我们将利用河流-海底-峡谷过渡的地貌来了解河流洪水是如何触发近海水流的，例如，通过绘制山体滑坡伤痕图，或测试河流推移质被驱动过单一陡峭的雪崩面的假设。这是一项紧急拨款，因为2020年1月水流如何被触发的证据(例如海底崩塌陡峭)将被刚果河在2020年10月的下一次洪峰流量所掩埋或抹去。对流经数百至数千公里的深海的最强大的浑浊水流的直接测量非常少，而以前可用的少数测量结果改变了人们的理解。事实上，以前只有一次直接测量到的这种规模的浑浊洋流，那就是1929年的大浅滩事件，它打破了横跨北大西洋的所有~20条电缆。大银行的比赛跑了800多公里，但从19米/S减速到3米/S，而不是像2020年1月的比赛那样持续加速。此外，2020年1月的活动已经有了关于离岸系泊时间的更详细的测量，进一步的数据将通过回收这些系泊和12个OBS(带有水听器和地震检波器)在NERC邮轮上提供。因此，2020年1月的这一事件是一个难得的、极其宝贵的机会，可以了解大范围水流的作用距离，与具有更长(50-100年)重复间隔的特殊河流洪水有关。我们对2020年1月事件的主要理解差距是在河口发生的事情，这是预测水流频率和与气候变化联系的关键。目前尚不清楚这条河从峡谷头过渡到河面的地貌。例如，UKHO水深图主要使用19世纪90年代收集的数据。在这里，我们将使用小水线面多波束超声测深仪以更高的分辨率和三维的方式测量河流到峡谷的过渡，从而以前所未有的细节记录其地貌。过去的工作表明，一次单一的多波束测深测量可以对河口触发的浊流产生重大洞察。

This proposal seeks to understand how a prodigious 1,250 km runout submarine sediment avalanche (turbidity current) was triggered on 14th January 2020, by the largest flood in 50 years along the Congo River. This submarine flow broke two seabed telecommunication cables that underpin data traffic to West Africa causing the internet to slow from Nigeria to South Africa. These submarine cables had not previously broken in the last 20 years. This flow also caused a series of oceanographic moorings to surface, placed along Congo Submarine Canyon by a NERC project (NE/R001952). Cable breaks and surfaced moorings show that this remarkable flow ran out for over 1,200 km, as measured along the canyon axis. Moreover, the flow continuously self-accelerated, such that it reached front speeds of >8 m/s, some 1,150 to 1,250 km from its source at the mouth of the Congo River. This is the longest runout turbidity current yet monitored in action, and the only monitored flow to continuously self-accelerate for over a thousand kilometres. It is important to understand how such powerful and very long runout turbidity currents are triggered, especially for hazards to strategic seabed cables, including cable routes that are planned for 2020-21 off West Africa. The January 14-16th submarine flow is not associated with an earthquake, and it occurred during a period of low wave heights. However, it does coincide with an extreme flood of 80,000 m3s-1 observed in December 2019 along the Congo River. It is thus also important to determine how the frequency of submarine flows will be effected by future climate and hydrological changes in the Congo Basin. Here we seek to understand how this exceptional river flood triggered a thousand kilometre submarine flow, by conducting a detailed survey of the Congo River mouth. We will use the geomorphology of that river-to-submarine-canyon transition to understand how the offshore flow was triggered by the river flood, for example by mapping landslide scars, or testing a hypothesis that river bedload was driven over a single steep avalanche face. This is an urgency grant because evidence of how the Jan 2020 flow was triggered (e.g. seabed failure scarps) will be buried or wiped-out by the next peak discharge of the Congo River in Oct 2020.There are extremely few direct measurements of the most powerful turbidity currents that run out for hundreds to thousands kilometres to the deep ocean, and the few measurements available previously produced step changes in understanding. Indeed, there has only been one previously directly-measured turbidity current on this scale, which is the Grand Banks event in 1929 that broke all ~20 cables across the N. Atlantic. The Grand Banks event ran out for over 800km, but decelerated from 19 m/s to 3 m/s, rather than continuously accelerating as in the Jan 2020 event. Moreover, the Jan 2020 event already has much more detailed measurements from the timing of offshore moorings, with further data to come via recovery of these moorings and 12 OBS (with hydrophones and geophones) on a NERC cruise. This Jan 2020 event is thus a rare and extremely valuable opportunity to understand how far large-scale flows operate, linked to exceptional river floods with much longer (50-100 year) recurrence intervals. The main gap in our understanding of the Jan 2020 event is what happened at the river mouth, and this is key for predicting flow frequency and links to climate change. The geomorphology of the river to canyon-head transition is currently unknown. For example, UKHO bathymetric charts mainly use data collected in the 1890s. Here we will use swath multibeam echosounder systems to survey the river to canyon transition at much higher resolution and in three dimensions, thereby documenting its geomorphology in unprecedented detail. Past work shows how a single multibeam bathymetric survey can produce major insights into turbidity currents triggering at river mouths.