EGU2020-12698
https://doi.org/10.5194/egusphere-egu2020-12698
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Solifluction patterns arising from competition between gravity and cohesion

Rachel Glade, Mulu Fratkin, Joel Rowland, and Mara Nutt
Rachel Glade et al.
  • Los Alamos National Lab, Los Alamos, USA

Arctic soil movement, accumulation and stability exert a first order control on the fate of permafrost carbon in the shallow subsurface and landscape response to climate change. A major component of periglacial soil motion is solifluction, in which soil moves as a result of frost heave and flow-like “gelifluction”. Because soliflucting soil is a complex granular-fluid-ice mixture, its rheology and other material properties are largely unknown. However, solifluction commonly produces distinctive spatial patterns of terraces and lobes that have yet to be explained, but may help constrain solifluction processes. Here we take a closer look at these patterns in an effort to better understand material and climatic controls on solifluction. We find that the patterns are analogous to classic instabilities found at the interface between fluids and air—for example, paint dripping down a wall or icing flowing down a cake. Inspired by classic fluid mechanics theory, we hypothesize that solifluction patterns develop due to competition between gravitational and cohesive forces, where grain-scale soil cohesion and vegetation result in a bulk effective surface tension of the soil. We show that, to first order, calculations of lobe wavelengths based on these assumptions accurately predict solifluction wavelengths in the field. We also present high resolution DEM-derived data of solifluction wavelengths and morphology from dozens of highly patterned hillslopes in Norway to explore similarities and differences between solifluction lobes and their simpler fluid counterparts. This work leads us toward quantitative predictions of the presence or absence of solifluction patterns and their response to variation in material properties (e.g., vegetation, rock type, grain size) and climatic conditions (e.g., water content, active layer depth, variability in snow cover).

How to cite: Glade, R., Fratkin, M., Rowland, J., and Nutt, M.: Solifluction patterns arising from competition between gravity and cohesion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12698, https://doi.org/10.5194/egusphere-egu2020-12698, 2020

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  • CC1: Comment on EGU2020-12698, Douglas Jerolmack, 07 May 2020

    Beautiful presentation! I am happy to see how far this has come, and I think you are likely correct about the instability - and that finding the effective surface tension is key. 

    I know this is not a major point of what you present: but I am curious about the use of something like Herschel-Bulkley for modeling rheology of these kinds of 'flows'. For granular materials or suspensions of course one can use it, but this is for flows ABOVE yield. For this problem, I am imagining that the material is BELOW yield - at least the classic yield in which it fluidizes to make an inertial flow. In other words, it's more in a creeping regime beneath yield. The rheology of creeping materials is much less studied, but so-called "Elastoplastic models" that are being developed to describe creep in amorphous solids might be more appropriate. Here's a nice review: Nicolas, A., Ferrero, E. E., Martens, K. & Barrat, J.-L. Deformation and flow of amorphous solids:Insights from elastoplastic models.Reviews of Modern Physics90,45006 (2018)

    • AC1: Reply to CC1, Rachel C. Glade, 07 May 2020

      Hi Doug,

      Thanks for the comments! I mostly agree with you about Herschel bulkley. For one, in numerical modeling using a yield stress fluid I am having difficulty recreating the slow velocities observed in the field for any reasonable value of soil viscosity found in the literature. However, no one has measured the instantaneous velocities- only averaged over weeks or months. In addition, some vertical displacement profiles in the field are exponential- much more creep like- while some look more like traditional fluid flow. From what we can tell solifluction occurs sporadically, with a creep-like phase followed by a fluid-like phase (gelifluction) in which the soil is almost fully saturated. Experiments and field studies have shown this fluid-like regime to be thixotropic. Luckily, most of the fluid experiments and theory have shown that the rheology does not matter as much to pattern wavelengths as the capillary number. Though of course this number includes an "effective viscosity," which needs to be defined carefully. I will check out that review, because I think you're right that it could be much more representative of what's going on rheologically and may lead to a better model!