Impulse waves generated by snow avalanches: Momentum and energy transfer to a water body

When a snow avalanche enters a body of water, it creates an impulse wave whose effects may be catastrophic. Assessing the risk posed by such events requires estimates of the wave’s features. Empirical equations have been developed for this purpose in the context of landslides and rock avalanches. Despite the density difference between snow and rock, these equations are also used in avalanche protection engineering. We developed a theoretical model which describes the momentum transfers between the particle and water phases of such events. Scaling analysis showed that these momentum transfers were controlled by a number of dimensionless parameters. Approximate solutions could be worked out by aggregating the dimensionless numbers into a single dimensionless group, which then made it possible to reduce the system’s degree of freedom. We carried out experiments that mimicked a snow avalanche striking a reservoir. A lightweight granular material was used as a substitute for snow. The setup was devised so as to satisfy the Froude similarity criterion between the real-world and laboratory scenarios. Our experiments in a water channel showed that the numerical solutions underestimated wave amplitude by a factor of 2 on average. We also compared our experimental data with those obtained by Heller and Hager (2010), who used the same relative particle density as in our runs, but at higher slide Froude numbers.

Viscoplastic fluid impacting a body of water: predicting wave features from the fluid properties

Christophe Ancey, Yating Hu, Zhenzhu Meng

We investigate viscoplastic fluids (described using a Herschel–Bulkley model) striking a body of water: a fixed volume of a viscoplastic material (a polymeric gel called Carbopol ultrez 10) initially contained in a box is released on a slope and flows down under gravity. When it enters flume filled with water, it generates an impulse wave. The problem is to determine impulse waves’ features from the viscoplastic fluid properties. First, we use momentum and mass conservation in a control volume for modeling the interaction between the viscoplastic flow and water [1]. To close the resulting equations, we calculate the flow depth and average velocity of the viscoplastic fluid (at the shoreline) using an approximate model (based on lubrication theory and matched asymptotic expansions) [2]. We then solve the mass and momentum balance equations numerically. The scale analysis of these equations allows us to define 5 dimensionless groups, which are used to correlate wave features and fluid properties. Second, we compare the theoretical model with experimental data specifically acquired for this purpose. Figure 1 shows the experimental facility. Using PIV techniques, we measure the velocity profile of the viscoplastic fluid near the shoreline and its velocity field after immersion. This makes it possible to determine the water velocity field and the interface between the solid, water, and air phases. Good agreement is found between the measured and predicted wave characteristics (e.g. amplitude and height).

2019

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 viscoplastic avalanches striking a water body using PIV techniques.

Zhenzhu Meng

A recurrent problem in avalanche engineering is related to impulse waves induced by snow avalanches striking a water basin. We took a first step toward the avalanche dynamics when falling into water by using a viscoplastic material that mimics avalanches. The objective was to understand how momentum is transferred from the viscoplastic material to the water body. A series of experiments were conducted using PIV techniques and transparent materials (water and Carbopol). To that end, we proceeded images taken at high frequency. We tracked fluorescent seeds in the viscoplastic material and water illuminated by a laser, which made it possible to visualize the velocity field inside the materials. The images obtained were also used to track the progression of the interface between the viscoplastic material and the water body. Our experiments provided preliminary insights into the interaction between the two materials. In particular, momentum evolution of the two phases was analyzed. In addition, how the viscoplastic material deformed after entering the water body was considered.

2018

Experimental study of impulse waves generated by viscoplastic fluid

Zhenzhu Meng

EPFL2020