Debrisflow FAQ ¶

Problem: My debris flow simulation takes ages to calculate. I use a 0.5m resolution. Do you have any suggestions on how to speed up my simulation? And sometimes I get this error message:

Error-Messages:
% Unable to allocate memory: to make array.
  Not enough space
% Execution halted at:  RAMMS_OPENBINARYOUTPUT 2237
%                       RAMMS_OPENOUTPUTFILE 10730
%                       RAMMS_OPENOUTPUTFILEEVENT 10590
%                       IDLRTMAIN         183
%                       $MAIN$          

Answer:

There are several important issues that affect your simulation speed and also your (RAMMS) memory management:

1. Dump Step: By increasing the dump step (when starting a simulation, e.g. 10s instead of 5s), you decrease the amount of memory needed to open a simulation file.
2. Calculation domain: Do you use a narrow calculation domain? If not, please have a look in the manual (section 3.5.4) on how to define an ideal calculation domain.
3. Simulation resolution: A very good (=small) simulation resolution increases the amount of memory needed to open a simulation file.
4. Do you use a large ortho-image that you overlay? This could also be a memory problem. Reduce the resolution of your ortho-image (do not forget to change the .tfw file too) and try again.

Answer: Erosion is explained in this article.

Answer:

Debris flow

The “best” constitutive model for debris flows is still a very open question in the scientific community. We recommend using the Voellmy model until a better model is found. Voellmy basically has only two parameters and after some calibration a useful solution can usually be found. With Voellmy one can control the flow velocity (parameter xi) and runout distance (mu).

One reason Voellmy is useful is that it only requires two parameters to calibrate. The turbulent term dominates the frictional behavior when the flow is moving rapidly and the Coulomb term is dominant when the flow is moving slowly, allowing the model to be approximately calibrated to observations of flow velocity and the stopping location of the flow front.

Finding the “right” debris flow model is more difficult than finding the “right” snow avalanche model because debris flows are two component systems (fluid, solid). Much of the behaviour of a debris flow — including the stopping process — involves the interaction between the fluid-solid components. Thus, without a two component model, it will be unlikely that we are able to model all aspects of debris flows. The Voellmy model mixes the two components and therefore models the debris flow when the components volumes are constant and well mixed. This assumes, of course, that the relative portions of solid and fluid remain the same, from head to tail of the event. This is hardly true

Answer:

There a several good reasons.

Firstly, hazard mitigation experts are often interested in the flow behaviour only near the fan. Calculating the movement of the debris flow in the torrent is a time consuming and often useless task. Therefore using a hydrograph can often cut calculation times dramatically.

Another reason is that it is impossible to describe the initial conditions of debris flows as a "block release". There are cases where block release is a good approximation of reality (e.g. dam breaks), but, in general, it does not accurately reflect the starting conditions of flows from intense precipitation.

Read this publication for more information on this issue:
Deubelbeiss, Y.; Graf, C., 2013: Two different starting conditions in numerical debris-flow models – case study at Dorfbach, Randa (Valais, Switzerland)