Meltwater channels over ice: Tipping points for planform morphology and evolution

Overview
Melting icecaps and glaciers dictate sea-level rise on Earth. The largest contributions to meltwater discharge and sea-level rise occur in the summer months when surface ablation is highest. Depending on scale and seasonal temperature variations, some meltwater channels are ephemeral and some are perennial. Most channels tend to form due to the flow of water released during snowmelt and ice melt in the spring. During surface ablation, meltwater flow sheets gradually concentrate along longitudinal lines of structural weakness in the ice or become channelized due to local factors that favour melting. Channel geometry and planform morphology is mainly driven by thermal erosion (melting), a process that is enhanced by solar radiation through the water column.

Greenlandic melt water channel (Wikimedia Commons).

Compared with the cases of alluvial and bedrock channels, the study of meltwater channels on ice has received little attention [1], and experimental work reported to date has focused on small-scale meandering channels only [2]. Moreover, production, storage, and transport of meltwater over ice is one of the least-studied processes on Earth [3] and meltwater like channels have also been recognized on the South Pole of Titan (one of Saturn’s moons), where rivers of liquid methane carve their way through frozen landscapes [4]. Understanding, the dynamics of meltwater channels is therefore crucial to predict the rate of sea level rise in response to climate change and to understand environmental conditions on extra-terrestrial bodies. Meltwater channels over ice show many features also observed in alluvial and bedrock rivers, such as terraces/bars, overhangs, knickpoints (waterfalls), features analogous to scroll bars due to channel bend migration, and bend cut-offs. Meltwater channels also have certain features that are unique to them. For example, in these channels flows may increase downstream even in the absence of tributaries or overland flow due to thermal melting of the channel bed and walls and the role of direct solar radiation through the flowing water. This and other phenomena dependent on temperature gradients and flow characteristics (e.g. depth, velocity), among other variables, create channels with different planform morphologies. This PhD project will focus on identifying the tipping points responsible for them.
In spite of the advances in numerical models and remote sensing capabilities [1, 5-6], new laboratory experiments are key to motivate and validate processes understanding of supraglacial meltwater channel hydrodynamics and morphodynamics.

Laboratory meltwater channel over ice (R. Fernández).


Aims and Objectives
The aim of this PhD project is to characterize the formation and evolution of meltwater channels over ice and identify tipping points responsible for different planform morphologies. To achieve the aim of the PhD, research will focus on the following objectives:
1) Develop novel experiments of surface meltwater channel formation as a function of different slopes, water-ice temperature gradients and flow velocities.
2) Conduct remote monitoring of polar and extra-terrestrial surface meltwater channels to quantify real-world planform geometries, using data from ESA Sentinel, Nasa Landsat and higher resolution satellite data.
3) Develop empirical and theoretical models of meltwater flux from channel planform geometry and slope, quantifying both water-ice temperature gradients and meltwater contribution to sea-level rise.
Experiments will be conducted in a new temperature controlled 6 m long and 1.5 m wide flow facility, built to study meltwater channel hydrodynamics. New acoustic and optical techniques will be developed to resolve flow dynamics. Experiments will be documented using planform photography, for comparison against remote monitoring data, and through 3D moulds.

Impact
Despite the importance of meltwater, as a contributor to sea level rise, the formation of channels and thus the net flux of freshwater into the ocean is poorly understood. The PhD will develop new quasi-empirical models to accurately constrain surface meltwater contribution to sea level rise. Further, integration of the experimental work with satellite monitoring will enable the PhD to develop a new technique to detail maps of water-ice temperature gradients, and thus ice sheet melting, remotely.


References
[1] Pitcher and Smith (2019) doi:10.1146/annurev-earth-053018-060212;
[2] Fernández (2018) http://hdl.handle.net/2142/101494;
[3] Smith et al. (2015) doi:10.1073/pnas.1413024112
[4] Lorenz et al. (2008) doi: 10.1016/j.pss.2008.02.009.
[5] Karlstrom et al. (2013) doi: 10.1002/jgrf.20135
[6] Rippin et al. (2015) doi:10.1002/esp.3719, 2015.