Geometry and dynamics of subglacial channels

The geometry and dynamics of subglacial channels underneath Alpine temperate glaciers

PhD candidate Pascal Egli

Supervised by Prof. Stuart Lane, co-supervised by Dr. James Irving

Sediment exported from partially glacierized alpine catchments has diverse impacts on the downstream reaches of mountain streams. It fills water storage dams and hydropower intakes (Gabbud et al., 2016), leading to diminished energy production. It may affect the morphology and ecology of mountain streams and river valleys (Gabbud & Lane, 2016). It may contribute to natural hazards such as debris flows and landslides (Chiarle et al., 2007; Deline et al., 2004).

Figure 1: (a) A highly turbid stream overflowing the water intake ‘Otemma’ despite previous sedimentation. (b) UAV-based DEM and Orthoimage of the glacier margin of ‘Glacier d’Otemma’.

This project aims at determining the structure and dynamics of the subglacial hydrological network of an alpine temperate glacier with the help of Ground Penetrating Radar (GPR), channel exploration and hydraulic modelling. The objective is to observe the structure of subglacial channels, to investigate the erosion of these channels into sediment, to observe their evolution over the course of an ablation season and to determine the rate of sediment erosion and transport on a sub-daily timescale. We attempt to demonstrate the development of efficient, fast-flowing subglacial channels over the course of a melt season, as it has been previously suggested from tracer experiments (Nienow et al., 1998), and to show the erosion of subglacial channels into the sediment.

The main field site is the ‘Glacier d’Otemma’, situated at the end of the Haut Val de Bagnes in the South West of the Swiss Alps, at an elevation of 2450 – 3800 m.a.s.l. This glacier has retreated by approximately 2 km since 1964. Its length is of 8 km, the Equilibrium Line Altitude is situated at approximately 3000 m and the ice thickness goes up to 260 m (Gabbi et al., 2012). The meltwater flows into the Mauvoisin hydropower lake, with a water intake operated by the company ‘Forces Motrices de Mauvoisin’.

Figure 2: (a) dGPS lines registered during an April 2017 GPR survey of the Glacier d’stemma; (b) data collection during this survey.

Spatially dense GPR survey grids on a glacier (Figure 2) are repeated over the course of an ablation season in order to detect the location and size of channels. Precise mapping of one subglacial channel with a laser scanner (Mankoff et al., 2017) will serve as ground truth for the GPR surveys and provide high-resolution geometry as a boundary condition for a detailed Large Eddy Simulation (Lane et al., 2004) of water flow inside the conduit. Additional methods to assess daily meltrate and glacier surface movement include Light Detection and Ranging (LiDAR) surveys, UAV photogrammetric imagery and timelapse photography. Meteorological data is used to quantify the influence of variables such as temperature and precipitation on discharge and sediment transport.

References

Chiarle, M., Iannotti, S., Mortara, G., & Deline, P. (2007). Recent debris flow occurrences associated with glaciers in the Alps. Global and Planetary Change, 56(1–2), 123–136. https://doi.org/10.1016/j.gloplacha.2006.07.003

Deline, P., Chiarle, M., & Mortara, G. (2004). The July 2003 Frebouge debris flows (Mont Blanc Massif, Valley of Acosta, Italy): Water pocket outburst flood and ice avalanche damming. Geografia Fisica E Dinamica Quaternaria, 27(2), 107–111.

Gabbi, J., Farinotti, D., Bauder, A., & Maurer, H. (2012). Ice volume distribution and implications on runoff projections in a glacierized catchment. Hydrology and Earth System Sciences, 16(12), 4543–4556. https://doi.org/10.5194/hess-16-4543-2012

Gabbud, C., & Lane, S. N. (2016). Ecosystem impacts of Alpine water intakes for hydropower: the challenge of sediment management. Wiley Interdisciplinary Reviews: Water, 3(February), 41–61. https://doi.org/10.1002/wat2.1124

Gabbud, C., Micheletti, N., & Lane, S. N. (2016). Response of a temperate alpine valley glacier to climate change at the decadal scale. Geografiska Annaler: Series A, Physical Geography, 98(1), 81-95.

Lane, S. N., Hardy, R. J., Elliott, L., & Ingham, D. B. (2004). Numerical modeling of flow processes over gravelly surfaces using structured grids and a numerical porosity treatment, 40. https://doi.org/10.1029/2002WR001934

Mankoff, K. D., Gulley, J. D., Tulaczyk, S. M., Covington, M. D., Liu, X., Chen, Y., … & G?owacki, P. S. (2017). Roughness of a subglacial conduit under Hansbreen, Svalbard. Journal of Glaciology, 63(239), 423-435.

Messerli, A., & Grinsted, A. (2015). Image georectification and feature tracking toolbox: ImGRAFT. Geoscientific Instrumentation, Methods and Data Systems, 4(1), 23.

Nienow, P., Sharp, M., & Willis, I. (1998). Seasonal changes in the morphology of the subglacial drainage system, Haut Glacier d’Arolla, Switzerland. Earth Surface Processes and Landforms, 23(9), 825–843. https://doi.org/10.1002/(SICI)1096-9837(199809)23:9<825::AID-ESP893>3.0.CO;2-2