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.

Sea levels around the world are currently rising, threatening populations living near the coast with flooding and increased coastal erosion. Evaluating the future threat requires a better understanding of the physical processes responsible for driving changes in the Earth's ice sheets. Recent observations show that in some key locations around the ice sheets' margins, rapid thinning is currently contributing 1.3 mm/yr to global sea level rise, and that that number has risen dramatically in recent years. Most of the attention has been focussed on the Greenland and West Antarctic ice sheets, where the thinning is most widespread and rapid. It is generally assumed that the culprit is a warming of the ocean waters that come into contact with the ice sheet. Increased melting of the floating ice shelves and tidewater glaciers has caused them to thin, forcing the grounding line or calving front to retreat and allowing the inland ice to flow faster towards the coast. Although thinning of the East Antarctic Ice Sheet (EAIS) is currently much less widespread and dramatic than that observed in West Antarctica, a large sector of the EAIS is grounded below sea level and is thus potentially vulnerable to the same process of ice shelf thinning, grounding line retreat and ice stream acceleration. In addition, analogous ocean forcing to that in West Antarctica could influence the marine-based sector of the EAIS. In both regions the Antarctic Circumpolar Current brings warm Circumpolar Deep Water (CDW) close to the continental slope. While CDW may already be influencing Totten Glacier, which now shows the strongest thinning signature over the entire EAIS, other glaciers in the region, most notably Mertz Glacier, may be protected by the formation of dense, cold Shelf Water in local polynyas. However, our knowledge of the oceanography of the continental shelf and of the waters that circulate beneath and interact with the floating ice shelves is presently insufficient to understand what processes are driving the change on Totten Glacier and how vulnerable its near neighbours such as Mertz Glacier might be. Our ability to project the future behaviour of these outlet glacier systems is severely limited as a result. To address this deficiency, this project will make observations of the critical processes that take place beneath the floating ice shelves, to determine how the topography beneath the ice and the oceanographic forcing from beyond the cavity control the rate at which the ice shelves melt. The key tool with which the necessary observations will be made is an Autonomous Underwater Vehicle (Autosub3), configured and run in a manner analogous to that used for an earlier, highly successful campaign in which it completed 500 km of along-track observations beneath the 60-km long floating tongue of Pine Island Glacier in West Antarctica. We will use these data to validate a numerical model of ocean circulation beneath the ice shelves and use the computed melt rates to force a numerical model of ice flow, in order to investigate the response of the glaciers to a range of climate forcing. A detailed understanding of ocean circulation and melting beneath Totten and Mertz glaciers will generate insight into ocean-ice interactions that will be relevant to many other sites in Greenland and Antarctica, and will advance our developing knowledge of ice sheet discharge and its future effect on sea-level rise. This work forms part of an intensive observational campaign focused on ocean-ice shelf interactions in East Antarctica. The collaborative, interdisciplinary effort consists of coordinated ocean and glacier studies conducted by groups at Australian, French, UK and US institutions.