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

刚果河特大洪水是如何引发数千公里电缆断裂的海底水流的？

• 批准号: NE/V004387/1
• 负责人: Daniel Parsons
• 金额: $ 37.81万
• 依托单位: University of Hull
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FV004387%2F1
• 关键词: How; was; thousand; kilometre; cable

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.

该提案旨在了解 2020 年 1 月 14 日刚果河沿岸 50 年来最严重的洪水如何引发长达 1,250 公里的巨大海底沉积物雪崩（浊流）。这次海底水流破坏了支撑西非数据流量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减慢。这些海底电缆在过去 20 年里从未发生过断裂。这种水流还导致一系列海洋系泊装置浮出水面，这些系泊装置由 NERC 项目 (NE/R001952) 沿刚果海底峡谷放置。电缆断裂和浮出水面的系泊装置表明，沿着峡谷轴线测量，这股非凡的水流流淌了 1,200 多公里。此外，水流不断自我加速，前端速度达到>8 m/s，距刚果河河口源头约1,150至1,250公里。这是迄今为止监测到的运行中最长的跳动浊流，也是唯一监测到的连续自加速超过一千公里的流。了解如此强大且非常长的跳动浊流是如何触发的非常重要，特别是对于战略海底电缆的危害，包括计划于 2020-21 年在西非附近的电缆路线。 1月14日至16日的海底气流与地震无关，它发生在低波高时期。然而，它确实与 2019 年 12 月沿刚果河观察到的 80,000 m3s-1 的极端洪水同时发生。因此，确定刚果盆地未来气候和水文变化如何影响海底水流的频率也很重要。在这里，我们通过对刚果河河口进行详细调查，试图了解这次特殊的河流洪水是如何引发数千公里的海底水流的。我们将利用河流到海底峡谷过渡的地貌来了解河流洪水是如何触发离岸流的，例如通过绘制山体滑坡疤痕，或测试河床荷载被驱动到单个陡峭雪崩面的假设。这是一笔紧急赠款，因为 2020 年 1 月的水流如何触发的证据（例如海底溃决陡坡）将被 2020 年 10 月刚果河的下一个流量高峰掩埋或消除。强大的浊流延伸数百至数千公里到达深海，而之前可用的少数测量结果使人们的认识发生了阶跃变化。事实上，以前只直接测量过一次如此规模的浊流，那就是 1929 年发生的大浅滩事件，该事件导致横跨北大西洋的 20 条电缆全部断裂。 Grand Banks 赛事跑了 800 多公里，但速度从 19 m/s 减速至 3 m/s，而不是像 2020 年 1 月的赛事那样持续加速。此外，2020 年 1 月的活动已经对离岸系泊点的时间进行了更详细的测量，进一步的数据将通过 NERC 巡航上这些系泊点和 12 个 OBS（带有水听器和地震检波器）的恢复来获得。因此，2020 年 1 月的这次事件是一个难得且极其宝贵的机会，可以了解大规模水流的运行范围，以及与更长（50-100 年）复发间隔的异常河流洪水相关。我们对 2020 年 1 月事件理解的主要差距是河口发生的事情，这对于预测水流频率及其与气候变化的联系至关重要。河流到峡谷头过渡的地貌目前尚不清楚。例如，UKHO 测深图主要使用 1890 年代收集的数据。在这里，我们将使用测绘带多波束回声测深仪系统以更高的分辨率和三个维度来调查河流到峡谷的过渡，从而以前所未有的细节记录其地貌。过去的工作表明，单次多波束测深测量如何能够对河口触发的浊流产生重要的见解。

项目成果

Carbon and sediment fluxes inhibited in the submarine Congo Canyon by landslide-damming

• DOI: 10.1038/s41561-022-01017-x
• 发表时间: 2022-09
• 期刊: Nature Geoscience
• 影响因子: 18.3
• 作者: E. Pope;M. Heijnen;P. Talling;R. Jacinto;A. Gaillot;Megan L. Baker;S. Hage;M. Hasenhündl;C. Heerema;C. McGhee;Sean C. Ruffell;S. Simmons;M. Cartigny;M. Clare;B. Dennielou;D. Parsons;C. Peirce;M. Urlaub