New Grant to Start : Glacier recession, glacial sediment export and the morphodynamics of proglacial forefields

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.

New Paper Published : Alpine glacier shrinkage drives shift in dissolved organic carbon export from quasi-chemostasis to transport-limitation

Boix Canadell, M. Escoffier, N., Ulseth, A.J., Lane, S.N. and Battin, T.J., 2019. Alpine glacier shrinkage drives shift in dissolved organic carbon export from quasi-chemostasis to transport-limitation. Geophysical Research Letters, 46, 8872-81

The export of dissolved organic carbon (DOC) from catchments is considered as an important energy flux through streams and a major connection between terrestrial and aquatic systems. However, the impact that predicted hydrological changes due to glacier retreat and reduction in snow cover changes will have on DOC export from high?mountain streams remains unclear. In this study, we measured daily runoff and DOC yield during 1 year in Alpine streams draining catchments with different levels of glacier coverage. DOC yield showed a varied response to runoff across the catchments and varied seasonally as a function of the degree of glaciation and vegetation cover. Using space?for?time substitution, our results indicate that the controls on DOC yield from Alpine catchments change from chemostasis to transport limitation as glaciers shrink.

 

New Paper Published : Revisiting the morphological method in two?dimensions to quantify bed?material transport in braided rivers

Antoniazza, G., Bakker, M., and Lane, S. N. ( 2019) Revisiting the morphological method in two?dimensions to quantify bed?material transport in braided rivers. Earth Surf. Process. Landforms, 44: 22512267

Research in the 1990s showed that bed?material transport rates could be estimated at the reach scale in both one?dimension and, over small spatial scales (10s of m), in two?dimensions. The limit on the latter was the spatial scale over which it was possible to obtain distributed data on morphological change. Here, we revisit the morphological method given progress in both topographical data acquisition and hydraulic modelling. The bed?material transport needed to conserve mass is calculated in both one and two dimensions for a 1600?m?×?300?m Alpine braided river “laboratory”. High?resolution topographical data were acquired by laser scanning to quantify Digital Elevation Models (DEMs), and morphological changes caused by the flushing of the water intake were derived from repeated surveys. Based on DEMs of differences, 1D bed?material transport rates were calculated using the morphological method. Then, a 2D hydraulic model was combined with a topographic correction to route sediment through the network of braided channels and to obtain a spatially variable estimate of transport in both downstream and cross?stream directions. Monte Carlo simulation was applied to the routing model parameters, allowing identification of the most probable parameter values needed to minimize negative transport. The results show that within?section spatial compensation of erosion and deposition using the 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to braiding intensity. Even though the 2D application showed that a large proportion of the total transport was actually concentrated into one main channel during the studied low flow event, the proportion of transport in secondary anabranches is substantial when the river starts braiding. Investigations of the effects of DEM resolution, competent flow duration and survey frequency related to ‘travelling bedload’ and sequential erosion?deposition emphasized the critical importance of careful data collection in the application of the morphological method.

A copy is available here.