Self-assembly in stratified fluids and extreme depth fluctuations in high-altitude Himalayan lakes

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• Richard Mclaughlin, University of North Carolina
• Monday 09 October 2023, 13:00-14:00
• MR5, CMS.

If you have a question about this talk, please contact Prof. John R. Taylor.

We first discuss experiments and modeling of a new phenomenon we discovered in our fluids lab in which particles suspended at similar depths in stratified water experience an effective attractive force which arises from diffusion induced flows.  These flows lead to particles forming molecule-like assemblages which seem to solve jig-saw like puzzles on their way to forming a large scale, compact aggregate disc.  This led us to seek such phenomena in the environment through a field campaign in pristine, high altitude stratified Himalayan lakes.  This resulted in an unexpected finding concerning seasonal depth fluctuations of the Gokyo lake system.  Himalayan lakes represent critical water resources, culturally important waterbodies, and potential hazards. We found that some of these lakes experience dramatic water-level changes, responding to seasonal monsoon rains and post-monsoonal draining. To address the paucity of direct observations of hydrology in retreating mountain glacial systems, we will review our field program focused on a series of high altitude lakes in Sagarmatha National Park, adjacent to Ngozumba, the largest glacier in Nepal. In situ observations find extreme (>12 m) seasonal water-level changes in a 60-m deep lateral-moraine-dammed lake (lacking surface outflow), during a 16-month period (May 2018-Oct 2019), equivalent to a 5 million cubic meter volume change annually. The water column thermal structure was also monitored over the same period. A hydraulic model is constructed, validated against observed water levels, and used to estimate hydraulic conductivities of the moraine soils damming the lake and improves our understanding of this complex hydrological system. Our findings indicate that lake level compared to the damming glacier surface height is the key criterion for large lake fluctuations, while lakes lying below the glacier surface, regulated by surface outflow, possess only minor seasonal water-level fluctuations. Lakes adjacent to glaciers may thus  exhibit very different filling/draining dynamics based on presence/absence of surface outflows and elevation relative to retreating glaciers, and consequently may have very different fates in the next few decades as the climate warms.  Drone videos of the rugged terrain will be shown, and we will preview our most recent dataset (Oct 2019-May 2022).

https://www.nature.com/articles/s41467-019-13643-y

https://www.nature.com/articles/s41598-023-37667-z

This talk is part of the Geophysical and Environmental Processes series.

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