The Swiss National Science Foundation has funded a new project that will involve two PhD students and a technician to work on the behaviour of the areas that form in front of glaciers (proglacial forcefields) as they retreat. They are dramatically increasing in size due to rapid climate warming and glacier recession. Impacts of climate change upon glacier recession are well-established over the timescale of years to decades. Geomorphic response of proglacial margins, of which forefields are one component, has also received significant attention (e.g. consequences of glacial debuttressing; role of vegetation as an ecosystem engineer in stabilising deglaciated terrain). Forefields themselves, especially in Alpine settings, have received less attention notably (1) how they are forced by their upstream boundary condition, glacier runoff and sediment export; and (2) how they filter this signal to influence downstream sediment yield. Quantifying these processes fully and continually at scales from the sub-daily (due to rapid discharge variation following from snow/ice melt) to the seasonal has not yet been attempted but is necessary if we are to understand how changing glacier sediment export translates into downstream sediment delivery. It is of practical importance (e.g. for hydropower management) and scientific interest (e.g. whether measurements of sediment yield can be used to infer glacial erosion rates; how forefield morphodynamics create the habitats upon which new postglacial ecosystems can develop).
The core aim of this project is to undertake the first, coupled study of the relationship between subglacial sediment export, forefield morphodynamics and downstream sediment flux for a retreating Alpine glacier. The project is structured around two broad sets of hypotheses. The first addresses subglacial sediment export, a critical boundary condition that will drive forefield morphodynamics. It seeks to quantify how and over what timescales the marginal zone of a glacier regulates the export of glacially-eroded sediment to its forefield, for both bedload and suspended load. The second focuses upon how the forefield responds to glacial sediment export in terms of morphodynamics and how these morphodynamics in turn filter glacier-exported sediment to drive downstream sediment yield.
The project uses both field data collection and computational modelling. The former focuses upon a representative temperate Alpine valley glacier forefield, Otemma, in Switzerland. In a technician-led work package (WP1) new opportunities for monitoring bedload continuously using acoustic pipe samplers will be combined with standard stage and turbidity monitoring to produce, after calibration, the first season-scale, continuous records of discharge, suspended load and bedload. WP2 will use gauging station data to quantify glacial sediment export and its timescales of variability. It will be supported by ground penetrating radar survey of the subglacial channel in the snout marginal zone and the first attempt to introduce into crevasses and moulins tagged gravel/cobble particles and to track their emergence at the snout. Existing one-dimensional models will be adapted to allow us to simulate how sediment moves through the snout marginal zone under different forcing conditions. WP3 focuses on the forefield, quantifying sub-daily morphodynamics and grain size patterns using UAV systems. These will allow us to quantify how the forefield filters glacier-exported sediment and to relate it to morphodynamics. The HSTAR numerical model will be developed for multiple grain sizes and applied to simulate how different boundary conditions, including the size of the proglacial area, filter the signal of glacier sediment export. WP4 will bring together results from WPs 1-3 to answer the critical science questions: what are the timescales over which glacier sediment export can be used to infer glacial erosion rates? how do proglacial margins filter glacial sediment export to determine basin sediment yield? and how do proglacial morphodynamics evolve as glaciers retreat, so impacting frequencies of forefield disturbance.