QUANTITATIVE CLAST SHAPE ANALYSIS APPLIED TO INTERNAL DYNAMICS OF DEBRIS FLOWS

Debris flows are highly concentrated mixtures of water and sediments. They represent a constant threat because, in volcanic environments, they can occur without being associated with volcanic activity, and they can reach as far as 100 km from the source. It is known that debris flows internal behavior is governed by particle interactions and interstitial fluid.

In this work, results of analytical experiments related to these aspects are studied via the morphological evolution of constituent particles. Different mixtures of water and sediment, with compositions resembling debris flows, were prepared and put into a rotating drum for different time intervals. At each interval, shape analysis was performed using three morphological coefficients. These coefficients point to the fact that changes in shape between granulometric classes are gradual and follow a power law. The rate at which these changes occur is related to the type of fine sediment present in the matrix. Coarse granulometric classes acquire more circular and smooth profiles while small particles maintain their irregular profiles. This behavior is the result of clast interaction processes within mixtures. Clast collisions between particles produce comminution of the coarsest fractions, while finer particles fracture along their entire surface.

Besides, in all mixtures it was observed that at a certain critical point, consistent with the vanishing of the certain granulometric classes and comminution of gravel-sized particles, circularity increases rapidly for mixtures rich in clay. In natural debris flows this sharp transition could correspond to the initial stages of movement.

This type of experiment provides important physical information for understanding the phenomena that occur inside debris flows, enabling the study of a variety of textural parameters and their changes at regular time intervals. The experimental patterns observed offer new insights into clast-clast interaction and comminuting theory, with important implications for understanding rheological behavior and kinematics of flows and related hazards.