On the run-out distance of geophysical gravitational flows: Insight from fluidized granular collapse experiments

We present the results of laboratory experiments on the emplacement of gravitational granular flows generated from axisymmetrical release of columns of fine (~ 75 μm) or coarse (~ 330 μm) particles initially fluidized with air. Internal friction is first negligible in the granular columns and then increases as pore pressure diffuses within the propagating flows, which are thus characterized by a mean friction lower than that of dry (i.e., non fluidized) flows. For columns of height-to-radius ratios a ≈ 0.2–30, we identify the modes of flow propagation and the scaling laws that characterize the morphology of the resulting deposits. Here we show that the normalized run-out distance of the initially fluidized flows scales as a power law of a (i.e., λan), thus demonstrating that this scaling law is not only typical of dry granular flows, as claimed in the literature. Fluidization reduces contacts between the grains and thus effective energy dissipation. Its effect increases the coefficient λ compared to dry flows but it has no influence on the exponent n that decreases from 1 to 1/2 at increasing a, mainly due to axisymmetrical spreading as shown by earlier works on dry coarse particles, except for the initially dry flows of fine particles at a > ~ 2 as it decreases to ~ 2/3. In this latter case the flows could experience (partial) auto fluidization as their normalized flow run-out is equal to that of their initially fluidized counterparts at a > ~ 4. The auto fluidization mechanism, supported by other recent experimental works, is particularly appealing to account for the long run-out distance of natural dense gas–particle mixtures such as pyroclastic flows. At high a, fluidization also affects the generation of surface waves with clear signatures on the deposits. We compare our experimental results with published data on Valles Marineris landslides (Mars) whose emplacement mechanisms are controversial. These natural events are characterized by values of λ higher than that of the laboratory flows, including those with low friction. This shows that some mechanism and/or scale effects promoted energy dissipation for the VM landslides that was significantly smaller than for typical dry frictional granular materials, as suggested by Lucas and Mangeney (2007).