Tracking sediment waves through Himalayan fluvial cascades following extreme mass flows

Mountain landscapes experience sudden and violent geohazards, such as landslides, lake outburst floods, and debris flows. The size and frequency of such events is anticipated to increase due to climate change, enhancing landscape instability. These landscapes are also experiencing rapid population growth, directly exposing people and assets to geohazards, but also exposing them to legacy impacts which manifest after an event and are commonly overlooked and unquantified. A legacy impact of many mountain geohazards is enhanced coarse sediment transport in rivers. This is a problem because sediment travelling as 'bedload' is the primary driver of river channel adjustment. These adjustments affect: 1) flood hazard, by modifying channel bed elevation; 2) the integrity of riparian infrastructure, e.g. hydropower, by blocking intakes and rapidly filling reservoirs, and 3) fluvial ecology, by reorganising channel substrate. It is therefore vital to generate well-constrained knowledge of the pace and manner in which the bedload transport regime evolves in mountain rivers after extreme disturbances. However, due to technical limitations and challenges associated with working in unstable, post-flood landscapes, we have little first-hand information on the behaviour of such systems, which this project aims to address.

This new project will consolidate a new international partnership of leading researchers from the UK and India. The team is led by the University of Plymouth, working in close collaboration with the Indian Institute of Technology Roorkee (IITR) and the Wadia Institute of Himalayan Geology (WIHG), the University of Exeter, and Newcastle University. The diverse team bring complementary expertise in geomorphology, hydrology, and environmental sensor networks, and the work would not be possible without the regional knowledge, technical competencies, and field experience of the international partners. The project also features prominent early- and early-to-mid-career researchers in leading roles. Working together we will apply a suite of innovative environmental monitoring and modelling tools to characterise the hydrological and bedload transport regime of the Alaknanda river, Uttarakhand, India, which experienced an extreme debris flow in February 2021 which killed >200 people and triggered enhanced sediment transport as a legacy impact, evidenced through pilot work.

To achieve our aim, we will: 1) Develop a new hydrological model of the Alaknanda catchment, enabling us to identify and disentangle the key components of flow (e.g. snowmelt, rainfall). This information will be used to better understand the hydrological drivers of sediment transport; 2) Quantify the grain size characteristics of channel bars using drone- and satellite-based observations and modelling. This information will allow us to explore downstream transitions in grain size through time and examine the influence of the Chamoli event; 3) Deploy innovative, low-cost 'smart' tags to track the motion of cobbles and boulders travelling as bedload. We will supplement these data with measurements of the timing and relative magnitude of bedload transport using low-cost passive seismics.

We will effect skills and knowledge transfer in-person via joint fieldwork and discussions at IITR and WIHG), and a regular series of virtual project meetings and seminars. We will publish results in peer-reviewed open-access journals and will produce a technical summary report which we will disseminate to local stakeholders. Project success will lead to future joint funding bids which will appraise the role of hydropower as a disruptor to coarse sediment transport in mountain rivers and explore operational practices that can mitigate the immediate and legacy impacts of extreme floods. In doing so we will further consolidate a wider research network involving regional academics and practitioners, whilst supporting the development of early career researchers in both countries.