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TOPIC: Lateral spreading and lambda parameter

Lateral spreading and lambda parameter 8 years 1 month ago #61

I miss a clear explanation about the formulation applied to the RAMMS module currently in force. On one hand, in the manual there is no explanation, just a practical user-guide for the software tools; on the other hand, reference articles I understand that are mixing approaches that are not still applied to commercial version in use. So, which is the more detailed formulation of the operational current RAMMS?

In particular I am concerned about the governing formulation for the lateral spreading of the flow.

From what I understand of the workshop documentation and articles, is that the only motors for the flow in y direction are evidently transverse component of gravity, according to the terrain slope, and the inertia in the sense of conservation of momentum.

If it was only so, on a perfect inclined plane terrain with flow parallel to the maximum slope direction, there would be no dispersion. I understand that there should be spreading also for this case, due the gradient of h in transverse direction (y) [δh/δy] and the hydrostatic pressure of the snow uncompensated laterally (affected by parameter lambda). Is not like this?

In Bartelt + Salm + Gruber (1999) is included the pressure effect of the snow (and the parameter lambda in active/passive sense), but only in the longitudinal direction. How is it incorporated in RAMMS? It also includes the parameter lambda, is it right? But I find nowhere a clear explanation about how is involved... I understand that this effect should also work in the transverse direction.

I will appreciate a lot if you can tell me where to find the detailed explanations to resolve these doubts.

Thank you!
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Re:Lateral spreading and lambda parameter 8 years 4 weeks ago #62

  • Marc
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Dear Marc,

Thanks for your questions.

First, there is a lot going on in avalanche dynamics and you are right that we have two RAMMS versions: the commercial version with the standard Voellmy model with active/passive pressures and the research version accounting for granular turbulence, entrainment and temperature. Basically we strongly recommend the use of the Voellmy model because it gives more conservative results for a wider range of hazard scenarios. Also, we want to provide continuity with the Swiss guidelines and existing model results.

We have tested RAMMS with different active-passive lambda coefficients. The active-passive stresses were applied in both the longitudinal and transverse directions (the x and the y). However, we found that introducing active-passive stresses did not improve the simulation results, especially for snow avalanches. We want to emphasize that we have never "measured" an active-passive coefficient independently: it is simply a model parameter that can be selected to give hopefully better agreement to observations. Therefore, we do not allow the user to change lambda in the commercial version. Of course, we do account for longitudinal and transverse pressures from hydrostatic pressure gradients. On a flat plane, the avalanche will spread in all directions.

Today we are finding that active-passive pressure mimics other processes that occur in the avalanche, including the dissipation of turbulent energy by cohesion. This strongly effects the spreading.

I hope we answered your questions! If not, give us a shout.

All the best

Perry and Marc
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