Experimental study of viscoplastic avalanches striking a water body using PIV techniques.

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.

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

Christophe Ancey

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.

Wiley-Blackwell2016

The effect of natural surfactants on air-water momentum exchange under light wind conditions in Lake Geneva

David Andrew Barry, Mehrshad Foroughan, Ulrich Lemmin

Randomly distributed patches of smooth or rough/rippled surfaces are readily observed on most water bodies. Smooth surface patches are called natural slicks and typically form under low wind conditions (< 6 m s-1) when biogenic surfactants in the surface microlayer accumulate above a certain threshold. They may have spatial scales from tens of meters to kilometers. Slicks suppress the formation of wind-induced Gravity-Capillary Waves (GCW), leading to altered surface reflectance of light and microwaves and can also affect near-surface turbulent motions. Therefore, it is of interest to determine the effect slicks can have on the air-water exchange of momentum, heat, and gas, which can influence the biogeochemical dynamics in the near-surface layer of lakes and oceans. We investigated the spatiotemporal variability in momentum flux caused by slicks in Lake Geneva during several field campaigns using eddy covariance instrument setups mounted on an autonomous catamaran. The measurements were combined with aerial and shore-based imagery (both RGB and thermal). In addition, surface microlayer sampling was conducted from an accompanying boat to determine whether visually-identified smooth patches were associated with higher enrichment of fluorescent dissolved organic matter, a proxy for natural surfactants. Wavelet analysis was used to explore short-time (~1 min) averaged air-water exchange variations related to the transition from smooth slicks to rough surface areas that cannot be captured by the conventional eddy covariance analysis method. Our results suggest that under light wind conditions and in the absence of short GCW on the surface of slicks, wind stress cannot be effectively transferred to the water, leading to reduced momentum exchange inside slicks compared to the surrounding non-slick areas. This results in lateral gradients in vertical mixing that can affect air-water exchange processes and contribute to spatial variability in surface temperature and near-surface heat content.

2022

The effect of natural surfactants on air-water momentum exchange under light wind conditions in Lake Geneva

David Andrew Barry, Mehrshad Foroughan, Ulrich Lemmin

2022