Rapid adjustments to catchment sediment yield following a catastrophic rock-ice avalanche and debris flow, Uttarakhand, India

2021年2月7日，印度喜马拉雅山北阿坎德查莫利地区的一个山脊发生了一场巨大的岩石-冰雪崩，转变为一场快速移动的灾难性泥石流，沿着沿着Rishiganga，Dhauliganga和Alaknanda河流动。洪水造成数百人死亡，摧毁或损坏了成熟和在建的水电项目，并对河道和更广泛的谷底景观造成严重影响，包括陡峭山谷两侧的不稳定。洪水消退后，迅速的事后分析显示，泥石流沉积的沉积物在某些地方超过20米厚，而且泥石流能够运送直径超过20米的巨石。接下来的12个月是这条河流系统的关键时期，因为我们预测这段时间新沉积的沉积物将被大量侵蚀和搬运，我们认为大部分活动将发生在距离雪崩源约50公里的范围内，其中包括四个水电设施和滨江定居点和基础设施。洪水留下的沉积物的“重新激活”对当地水电运营商有影响，他们需要预测这些升高的沉积物负荷并相应地制定计划，以减少大坝出口和隧道堵塞的风险，避免降低排放能力和损坏机械设备。此外，随着新的沉积物覆盖层被移除，被初始水流底切的河岸变得更加不稳定，或者在新的区域开始底切，进一步造成河谷侧翼不稳定的风险很高。我们还预计，当沉积物与谷底能源和交通基础设施相交时，沉积物沉积也可能带来危险。为了紧急预测洪水后河道改造的速率和模式，我们将使用能够模拟河流流量和侵蚀的计算机模型沉积物的运输和沉积。我们将在一年的初始期内运行该模型（包括夏季季风，这会带来河流流量的数量级增加），我们将生成关键的摘要数据集，这些数据集可以快速传达给国内的最终用户。我们已经可以访问设置和运行模型所需的大部分数据，并且项目合作伙伴可以很好地提供我们执行初始运行和定期检查模型性能所需的缺失数据。这项工作将由一个国际小组开展，该小组由极端洪水和洪水数值建模、高山景观水文学和社区适应（快速）环境变化方面的专家组成。该团队包括来自英国，印度，加拿大和美国的研究人员，他们在提供高质量的科学知识以指导现实世界的决策方面拥有集体记录。后续工作将扩大工作范围，以研究更大面积的沉积物迁移和沉积：卫星图像分析表明，洪水产生的初始沉积物羽流在24小时内移动了150公里以上，我们预计每年洪水沉积物的重新激活将对这一极端事件造成的危害产生重大影响。

On 7th February 2021 a massive rock-ice avalanche originating from a mountain ridge in Chamoli District, Uttarakhand, Indian Himalaya, transformed into a fast-moving and catastrophic debris flow which travelled along the Rishiganga, Dhauliganga, and Alaknanda rivers. The flow killed hundreds of people, destroyed or damaged mature and under-construction hydropower projects, and caused severe modification to the channel and wider valley floor landscape, including the destabilising of steep valley sides. Once the flood subsided, rapid post-event analysis revealed that sediments deposited by the debris flow were more than 20 m thick in places, and that the flow was capable of transporting boulders exceeding 20 m in diameter. The next 12 months are a crucial period for this river system because this is when we predict that newly deposited sediments will be eroded and transported in vast quantities, and we believe that most of this activity will occur within a distance of around 50 km from the avalanche source, which includes four hydropower facilities and riverside settlements and infrastructure. This 're-activation' of sediments left behind by the flood has implications for local hydropower operators, who need to anticipate these elevated sediment loads and plan accordingly to reduce the risk of blockage to dam outlets and tunnels, avoid reduced discharge capacity, and damage to mechanical equipment. In addition, there is a high risk of further valley flank instability as this new drape of sediment is removed and banks that were undercut by the initial flow become more unstable, or undercutting is initiated in new areas. We also anticipate that sediment deposition could also present a hazard where these deposits intersect with valley floor energy and transport infrastructure.To urgently predict rates and patterns of post-flood channel modification we will use a computer model that is capable of simulating river flow and the erosion, transport, and deposition of sediment. We will run this model for an initial period of one year (including the summer monsoon, which brings an order-of-magnitude increase in river discharge) and we will generate critical summary datasets that can be rapidly communicated to in-country end users. We already have access to most of the data that we require to set up and run the model, and project partners are well-placed to provide missing data that we need to perform initial runs and perform regular checks on model performance. The work will be carried out by an international team comprised of experts in extreme floods and numerical flood modelling, the hydrology of high mountain landscapes, and community adaptation to (rapid) environmental change. The team includes researchers from the UK, India, Canada and the USA with a collective track record of delivering high quality science to inform real-world decision-making. Follow-on work will broaden the scope of the work to look at sediment transport and deposition over a much larger area: analysis of satellite imagery shows that the initial sediment plume generated by the flood travelled >150 km in ~24 h and we anticipate that annual re-activation of flood sediment will have significant impacts on the hazard posed by this extreme event.