30 Apr 2024
 | 30 Apr 2024
Status: this preprint is open for discussion.

Modeling the formation of toma hills based on fluid dynamics with a modified Voellmy rheology

Stefan Hergarten

Abstract. Toma hills are the perhaps most enigmatic morphological feature found in rock avalanche deposits. While it was already proposed that toma hills might emerge from the fluid-like behavior of rock avalanches, there still seems to be no consistent explanation for their occurrence. This paper presents numerical results based on a modified version of Voellmy's rheology, which was recently developed for explaining the long runout of rock avalanches. In contrast to the widely used original version, the modified Voellmy rheology defines distinct regimes of Coulomb friction at low velocities and velocity-dependent friction at high velocities. When movement slows down, falling back to Coulomb friction may cause a sudden increase in friction. Material accumulates in the region upstream of a point where this happens. In turn, high velocities may persist for some time in the downstream and lateral range, resulting in a thin deposit layer finally. In combination, both processes generate more or less isolated hills with shapes and sizes similar to toma hills found in real rock avalanche deposits. So the modified Voellmy rheology suggests a simple mechanism for the formation of toma hills.

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Stefan Hergarten

Status: open (until 20 Jun 2024)

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Stefan Hergarten

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Formation of toma hills Stefan Hergarten

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Formation of toma hills Stefan Hergarten

Stefan Hergarten


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Short summary
Toma hills are more or less isolated hills in the deposits of rock avalanches and their origin is still enigmatic. This paper presents results of numerical simulations based on a modified version of a friction law that was originally introduced for snow avalanches. The model produces more or less isolated hills on the valley floor, which look much like toma hills. The results presented here provide the perhaps first explanation for the occurrence of toma hills based on a numerical model.