The effects of slide cohesion on impulse waves generated by landslides

When gravity-driven mass flows, such as avalanches and landslides, enter bodies of water, they can generate large impulse waves whose effects can be devastating. Experimental studies of impulse wave formation have mostly used rigid blocks or granular materials to mimic landslides at the laboratory scale. These studies have deduced that material deformability plays a key part in wave formation: the more rigid the sliding mass, the higher the impulse wave. It is, however, still unclear whether higher wave amplitudes arise solely from lower deformability. Indeed, blocks are not only rigid, but they are also cohesive, whereas granular media are deformable and cohesionless. To shed light on this issue, we ran experiments using two deformable materials of equal density, one exhibiting no cohesion (soft 15-mm diameter balls) and the other exhibiting cohesion (a viscoplastic polymeric gel called Carbopol Ultrez 10). A finite volume of material was released at the top of a chute, penetrated a body of water and generated impulse waves. We monitored how the mass slid and interacted with the water volume. Using high-speed cameras, we measured maximum wave heights, amplitudes and lengths of the leading wave. We used dimensionless groups to reduce the dimension of the parameter space, making it possible to carry out a regression analysis. Viscoplastic slides generated larger wave amplitudes but shorter wave lengths than granular materials. Surprisingly, the wave features did not depend on the polymer concentration. In other terms, impulse wave features were not found to be dependent on the cohesion of the deformable material landslides causing them, within the range of concentrations tested.

Christophe Ancey, Zhenzhu Meng

Swiss Academy of Science, SCNAT2019

Experimental study of impulse waves generated by viscoplastic fluid

Zhenzhu Meng

Landslide-generated waves, also called impulse waves, occur as a result of the intrusion of landslides (such as rock falls, debris flows, and avalanches) into bodies of water (such as lakes, reservoirs, and seas). The objective of this thesis was to study the momentum transfer from the slide material to the body of water, in order to develop a better understanding of how the slide material's properties affect the wave generation and to discover alternative modeling approaches to existing empirical equations. Previous experimental studies have usually used blocks and granular materials to mimic natural landslides. However, many landslides in real worlds have been idealized as viscoplastic fluids in theoretical and numerical studies. No studies have used viscoplastic material in experimental studies of landslide-generated waves. The originality of this thesis lies in the use of a viscoplastic material called Carbopol Ultrez 10, an artificial aqueous micro-gel whose rheological behavior can be described using the Herschel-Bulkley model. Carbopol's cohesive and deformable properties are different to both block and granular slides. Further, Carbopol is transparent and can easily be seeded with micro-seeding particles, so its velocity field can be measured using particle image velocimetry (PIV). As a comparison of Carbopol, I also used a granular material named polymer-water balls whose density is close to that of Carbopol. The investigations of this thesis are as follows: I conducted two series of experiments. First, I observed waves generated by Carbopol, water balls, and mixtures of them using high-speed cameras, to investigate the role of slide material's properties in wave prediction. Second, I conducted PIV experiments with Carbopol to investigate the internal dynamic of slide-water interaction. (1) I developed a theoretical model that combined the momentum conservation of twophase flow in a control volume (Zitti et al., 2016) and viscoplastic theory (Ancey et al., 2012). With the experimental results obtained from PIV measurements, I analyzed the drag force and hydrostatic force that act on stopping the sliding mass, and validated the theoretical model. (2) I developed empirical equations using the dimensionless groups that emerged from the governing equations to quantify the wave characteristics (for example, maximum wave amplitude and height) as functions of the slide parameters. Using empirical equations, I compared the characteristics of waves generated by cohesive Carbopol and cohesionless water balls, and discussed the effect of slide cohesion on wave generation. (3) Taking advantage of a purely data-driven approach that strictly relies on the dataset and does not need any physical constraints in advance, I applied an artificial neural network method to predict wave characteristics under complex configurations, such as dealing with an experimental dataset with several different slide materials (Carbopol, water balls, and mixtures of them). (4) Using a panel data model called random coefficient model, I predicted the time series data of wave characteristics from the time series data of slide parameters on impact. Given the slide parameters on impact by the viscoplastic theory, the temporal wave characteristics were quantified from the parameters of the slide material at the initial stage (at rest on the slope and then starting to move).

EPFL2020

Experimental study of impulse waves generated by viscoplastic fluid

Zhenzhu Meng

EPFL2020